CHEMISTRY EDEXCEL Topic 6: Organic Chemistry – Flashcards
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Hydrocarbon
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Hydrocarbon is a compound consisting of hydrogen and carbon only
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Saturated
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Contain single carbon-carbon bonds only
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Unsaturated
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Contains a C=C double bond
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Molecular formula
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The formula which shows the actual number of each type of atom
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Empirical formula
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shows the simplest whole number ratio of atoms of each element in the compound
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General formula
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algebraic formula for a homologous series e.g. CnH2n
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Structural formula
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shows the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3(CH2)2CH3,
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Displayed formula
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show all the covalent bonds present in a molecule
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Skeletal formula
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shows the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional Groups.
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Homologous series
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families of organic compounds with the same functional group and same general formula. •They show a gradual change in physical properties (e.g. boiling point). • Each member differs by CH2 from the last. • same chemical properties.
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Functional group
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is an atom or group of atoms which when present in different molecules causes them to have similar chemical properties
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Alkane Structure/Prefix
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-ane
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Alkenes Structure/Prefix
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-ene
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Alcohols Structure/Prefix
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-ol
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Halogenoalkanes Structure/Prefix
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chloro- bromo- iodo-
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Aldehydes Structure/Prefix
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-al
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Ketones Structure/Prefix
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-one
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carboxylic acids Structure/Prefix
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-oic acid
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Esters Structure/Prefix
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-yl -oate
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General rules for naming carbon chains
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- Count the longest carbon chain and name appropriately - Find any branched chains and count how many carbons they contain - Add the appropriate prefix for each branch chain E.G 3,5-dimethylheptane
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Basic rules for naming functional groups
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- When using a suffix, add in the following way : If the suffix starts with a vowel- remove the -e from the stem alkane name e.g. Propan-1-ol, butan-1-amine, ethanoic acid, ethanoylchloride, butanamide - If the suffix starts with a consonant or there are two or more of a functional group meaning di, or tri needs to be used then do not remove the the -e from the stem alkane name e.g. Propanenitrile, ethane-1,2-diol, propanedioic acid, propane-1,2,3-triol, Pentane-2,4-dione.
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Isomers
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- Structural isomers: same molecular formula different structures (or structural formulae) - Chain isomers: Compounds with the same molecular formula but different structures of the carbon skeleton - Position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton - Functional group isomers: Compounds with the same molecular formula but with atoms arranged to give different functional groups
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Stereoisomerism
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Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms. E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the double bond. E-Z isomers exist due to restricted rotation about the C=C bond Priority Group: The atom with the bigger atomic number is classed as the priority atom
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Refining crude oil
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Fractional Distillation: • Oil is pre-heated • then passed into column. • The fractions condense at different heights • The temperature of column decreases upwards • The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the London forces • Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures • and big molecules condense at the bottom at higher temperatures.
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Cracking
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Cracking: conversion of large hydrocarbons to smaller molecules by breakage of C-C bonds
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Economic reasons for catalytic cracking
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• The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more useful and valuable than the starting materials (e.g. ethene used to make poly(ethene) and ethane-1,2-diol, and ethanol) The smaller alkanes are used for motor fuels which burn more efficiently.
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Reforming
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Turns straight chain alkanes into branched and cyclic alkanes and Aromatic hydrocarbons
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Complete Combustion
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In excess oxygen alkanes will burn with complete combustion The products of complete combustion are CO2 and H2O. C8H18(g) + 12.5 O2(g) --> 8CO2(g) + 9 H2O(l)
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Incomplete Combustion
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If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame) CH4(g) + 3/2 O2(g) --> CO(g) + 2 H2O(l) CH4(g) + O2(g) --> C(s) + 2 H2O(l)
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Pollution from Combustion
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Sulphur containing impurities are found in petroleum fractions which produce SO2 when they are burned. S+ O2 --> SO2 CH3SH+ 3O2 --> SO2 + CO2 + 2H2O
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Pollutant and Environmental consequence
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- Nitrogen oxides (formed when N2 in the air reacts at the high temperatures and spark in the engine): NO is toxic and can form smog NO2 is toxic and acidic and forms acid rain - Carbon monoxide: toxic - Carbon dioxide: Contributes towards global warming - Unburnt hydrocarbons (not all fuel burns in the engine): Contributes towards formation of smog - Soot/particulates: Global dimming and respiratory problems
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Catalytic converters
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These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into 'harmless' CO2, N2 and H2O.
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Biofuels
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Most fossil fuels come from crude oil, which is a non- renewable resource. Fossil fuel reserves will eventually run out Alternative fuels have been developed from renewable resources. Alcohols and biodiesel, which can both be made from plants, are two examples of renewable plant- based fuels
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Advantages of using Biofuels
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Reduction of use of fossil fuels which are finite resources biofuels are renewable Use of biodiesel is more carbon-neutral Allows fossil fuels to be used as a feedstock for organic compounds No risk of large scale pollution from exploitation of fossil fuels
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Disadvantages of Biofuels
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Less food crops may be grown Land not used to grow food crops Rain forests have to be cut down to provide land Shortage of fertile soils
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Mechanisms
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To understand how the reaction proceeds we must first understand how bonds are broken in organic mechanisms There are two ways to break a covalent bond: 1. HOMOLYTIC FISSION: each atom gets one electron from the covalent bond 2. HETEROLYTIC FISSION: (one atom gets both electrons) It proceeds via a series of steps: STEP ONE: Initiation STEP TWO: Propagation STEP THREE: Termination
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Free Radical Substitution Reactions of Alkanes
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Overall Reaction CH4 +Cl2 --> CH3Cl+HCl methane --> chloromethane This is the overall reaction, but a more complex mixture of products is actually formed The MECHANISM for this reaction is called a FREE RADICAL SUBSTITUTION
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STEP ONE: Initiation
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Essential condition: UV light The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others as it is the weakest. UV light does not have enough energy to break the C-H bond. The bond has broken in a process called homolytic fission. When a bond breaks by homolytic fission it forms Free Radicals. Free Radicals do not have a charge and are represented by a (black dot). A Free Radical is a reactive species which possess an unpaired electron.
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STEP TWO: Propagation
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CH4 + Cl. --> HCl +.CH3 .CH3 + Cl2 --> CH3Cl + Cl. The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical. The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical. All propagation steps have a free radical in the reactants and in the products. As the Cl free radical is regenerated, it can react with several more alkane molecules in a CHAIN REACTION.
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STEP THREE: Termination
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.CH3 + Cl. --> CH3Cl .CH3 + .CH3 --> CH3CH3 Cl. + Cl. --> Cl2 Collision of two free radicals does not generate further free radicals: the chain is TERMINATED. Minor step leading to impurities of ethane in product. Write this step using structural formulae and don't use molecular formulae.
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Alkenes
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C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which 'like' electrons: these species are called electrophiles.
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Formation of σ bond
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One sp2 orbital from each carbon overlap to form a single C-C bond called a sigma σ bond. Rotation can occur around a sigma bond
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Formation of π bond
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The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond. The pi bond leads to resultant high electron density above and below the line between the two nuclei
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Reaction of Alkenes with Hydrogen
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Change in functional group: alkene --> alkane Reagent: hydrogen Conditions: Nickel Catalyst Type of reaction: Addition/Reduction Electrophilic Addition: Reactions of Alkenes A π bond is weaker than a σ bond so less energy is needed to break π bond The π bonds in alkenes are areas with high electron density. This is more accessible to electrophilic attack by electrophiles. Alkenes undergo addition reactions.
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Reaction of Alkenes with bromine/chlorine
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Change in functional group: alkene --> dihalogenoalkane Reagent: Bromine (dissolved in organic solvent) Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic
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Reaction of Hydrogen Bromide with alkenes
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Change in functional group: alkene --> halogenoalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic Addition Type of reagent: Electrophile, H+ Type of Bond Fission: Heterolytic The order of stability for carbocations is tertiary > secondary > primary In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
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Reaction of Potassium Manganate(VII) with Alkenes
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Change in functional group: alkene --> diol Reagent: KMnO4 in an acidified solution Conditions: Room temperature Type of reaction: Oxidation Observation: purple colour of MnO4- ion will decolourise to colourless
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Reaction of Bromine Water with Alkenes
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Reagent: Bromine dissolved in water Conditions: Room temperature Type of reaction: Addition Observation: Orange colour of bromine water will decolourise to colourless
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hydration of alkenes to form alcohols
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Industrially alkenes are converted to alcohols in one step rather than the two in the above sulphuric acid reaction. They are reacted with water in the presence of an acid catalyst. Essential Conditions High temperature 300 to 600°C High pressure 70 atm Catalyst of concentrated H3PO4 This reaction can be called hydration: a reaction where water is added to a molecule. The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out. See equilibrium chapter for more on the industrial conditions for this reaction.
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Addition Polymers
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Addition polymers are formed from alkenes This is called addition polymerisation. Poly(ethene): is used to make plastics bags, buckets, bottles. It is a flexible, easily moulded, waterproof, chemical proof, and low density plastic. Poly(propene) is a stiffer polymer, used in utensils and containers and fibres in rope and carpets.
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Incineration
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Rubbish is burnt and energy produced is used to generate electricity. Some toxins can be released on incineration. (e.g. Combustion of halogenated plastics (ie PVC) can lead to the formation of toxic, acidic waste products such as HCl.) Modern incinerators can burn more efficiently and most toxins and pollutants can be removed. Greenhouse gases will still be emitted though. Volume of rubbish is greatly reduced.
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Recycling
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Saves raw materials- nearly all polymers are formed from compounds sourced/produced from crude oil. Saves precious resources. Polymers need collecting/ sorting- expensive process in terms of energy and manpower. Polymers can only be recycled into the same type - so careful separation needs to be done. Thermoplastic polymers can be melted down and reshaped.
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feedstock for cracking
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Polymers can be cracked into small molecules which can be used to make other chemicals and new polymers- Saves raw materials-
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Naming Halogenoalkanes
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Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.
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Primary halogenoalkane
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One carbon attached to the carbon atom adjoining the halogen
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Secondary halogenoalkane
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Two carbons attached to the carbon atom adjoining the halogen
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Tertiary halogenoalkane
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Three carbons attached to the carbon atom adjoining the halogen
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Reactions of Halogenoalkanes
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Halogenoalkanes undergo either substitution or elimination reactions.
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Nucleophilic substitution reactions
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Substitution: swapping a halogen atom for another atom or groups of atoms. Nucleophile: electron pair donator e.g. :OH-, :NH3, CN-. :Nu represents any nucleophile - they always have a lone pair and act as electron pair donators. The rate of these substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to break and the faster the reaction. The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive
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Comparing the rate of hydrolysis reactions
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Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water. Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction. Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a SILVER HALIDE PRECIPITATE. The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes. The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction. AgI (s) - yellow precipitate (forms fastest) AgBr(s) - cream precipitate AgCl(s) - white precipitate
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Nucleophilic substitution with aqueous hydroxide ions
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Change in functional group: halogenoalkane --> alcohol Reagent: potassium (or sodium) hydroxide Conditions: In aqueous solution; Heat under reflux Mechanism: Nucleophilic Substitution Role of reagent: Nucleophile, OH- The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs
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SN1 nucleophilic substitution mechanism for tertiary halogenoalkanes
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Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is made stabilised by the electron releasing methyl groups around it. (see alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above. Primary halogenoalkanes don't do the SN1 mechanism because they would only form an unstable primary carbocation.
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Nucleophilic substitution with ammonia
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Change in functional group: halogenoalkane --> amine Reagent: NH3 dissolved in ethanol Conditions: Heating under pressure in a sealed tube Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :NH3
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Elimination with alcoholic hydroxide ions
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Change in functional group: halogenoalkane --> alkene Reagents: Potassium (or sodium) hydroxide Conditions: In ethanol ; Heat Mechanism: Elimination Role of reagent: Base, OH- Note the importance of the solvent to the type of reaction here. Aqueous: substitution Alcoholic: elimination The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution Tertiary tends towards elimination
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Uses of halogenoalkanes
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Haloalkanes have been used as refrigerants, fire retardants, pesticides and aerosol propellants chloroalkanes and chlorofluoroalkanes can be used as solvents. Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer.
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Naming Alcohols
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These have the ending -ol and if necessary the position number for the OH group is added between the name stem and the -ol If the compound has an -OH group in addition to other functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-): If there are two or more -OH groups then di, tri are used. Add the 'e' on to the stem name though
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Bond angles in Alcohols
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The HCH bond has 4 bonded pairs and 0 lone pairs, so the bond angle is 109.5 (tetrahedral). 2. The COH bond has 2 bonded pairs and 2 lone pairs (on the oxygen atom), so the bond angle is 104.5 (V-shaped or bent).
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Different types of alcohols
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- Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen - Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen - Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen
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1. Combustion of Alcohols
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Alcohols combust with a clean flame
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2. Reaction of Alcohols with Sodium
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Sodium reacts with alcohols 2CH CH OH + 2Na --> 2CH3CH2O-Na+ + H2 This reaction can be used as a test for alcohols Observations: • effervescence, • the mixture gets hot, • sodium dissolves, • a white solid is produced.
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3. Substitution reactions of Alcohols to form Halogenoalkanes
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This reaction with PCl5 (phosphorous(v)chloride) can be used as a test for alcohols. You would observe misty fumes of HCl produced. For Br use KBr, 50% concentrated H2SO4 to produce HBr. The reaction of KI and conc H2SO4 cant be used to produce HI because the sulphuric acid with oxidise the hydrogen halides to other products.
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4. Oxidation reactions of the alcohols
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Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. -- Partial Oxidation of Primary Alcohols: Reaction: primary alcohol --> aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms: -- Full Oxidation of Primary Alcohols: Reaction: primary alcohol --> carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished) -- Oxidation of Secondary Alcohols: Reaction: secondary alcohol --> ketone Reagent: potassium dichromate(VI) solution and dilute sulphuric acid. Conditions: heat under reflux
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Distinguishing between Aldehydes and Ketones
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The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for tests that are commonly used to distinguish between aldehydes and ketones. Fehling's (Benedict's) solution Reagent: Fehling's Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling's solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react
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Reaction of Alcohols with Dehydrating Agents
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Reaction: Alcohol --> Alkene Reagents: Concentrated Phosphoric acid Conditions: warm (under reflux) Role of reagent: dehydrating agent/catalyst Type of reaction: acid catalysed elimination Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil
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Distillation
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In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid. Classic AS reaction using distillation Reaction: primary alcohol --> aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: use a limited amount of dichromate and warm gently and distil out the aldehyde as it forms [This prevents further oxidation to the carboxylic acid] CH3CH2CH2OH + [O] --> CH3CH2CHO + H2O Observation: Orange dichromate solution changes to green colour of Cr3+ ions
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Reflux
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Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated. Classic AS reaction using reflux Reaction: primary alcohol --> carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished using distillation set up) CH3CH2CH2OH + 2[O] --> CH3CH2CO2H + H2O Observation: Orange dichromate solution changes to green colour of Cr3+ ions
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Purifying an organic liquid
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• Put the distillate of impure product into a separating funnel • wash product by adding either - sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced. Sodium hydrogencarbonate will neutralise any remaining reactant acid. - Saturated sodium chloride solution. •Allow the layers to separate in the funnel, and then run and discard the aqueous layer. •Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (anhydrous sodium sulphate) to dry the organic liquid. • Carefully decant the liquid into the distillation flask •Distill to collect pure product
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Solvent extraction
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Mix organic solvent and oil-water mixture in a separating funnel then separate the oil layer. Distil to separate oil from organic solvent Add anhydrous CaCl2 to clove oil to dry oil Decant to remove CaCl2
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Measuring boiling point
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Purity of liquid can be determined by measuring a boiling point. This can be done in a distillation set up or by simply boiling a tube of the sample in an heating oil bath. To get a correct measure of boiling point the thermometer should be above the level of the surface of the boiling liquid and be measuring the temperature of the saturated vapour. Pressure should be noted as changing pressure can change the boiling point of a liquid. Measuring boiling point is not the most accurate method of identifying a substance as several substances may have the same boiling point.
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Hydrocarbon
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Hydrocarbon is a compound consisting of hydrogen and carbon only
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Saturated
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Contain single carbon-carbon bonds only
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Unsaturated
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Contains a C=C double bond
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Molecular formula
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The formula which shows the actual number of each type of atom
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Empirical formula
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shows the simplest whole number ratio of atoms of each element in the compound
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General formula
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algebraic formula for a homologous series e.g. CnH2n
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Structural formula
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shows the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3(CH2)2CH3,
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Displayed formula
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show all the covalent bonds present in a molecule
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Skeletal formula
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shows the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional Groups.
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Homologous series
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families of organic compounds with the same functional group and same general formula. •They show a gradual change in physical properties (e.g. boiling point). • Each member differs by CH2 from the last. • same chemical properties.
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Functional group
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is an atom or group of atoms which when present in different molecules causes them to have similar chemical properties
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Alkane Structure/Prefix
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-ane
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Alkenes Structure/Prefix
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-ene
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Alcohols Structure/Prefix
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-ol
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Halogenoalkanes Structure/Prefix
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chloro- bromo- iodo-
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Aldehydes Structure/Prefix
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-al
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Ketones Structure/Prefix
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-one
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carboxylic acids Structure/Prefix
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-oic acid
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Esters Structure/Prefix
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-yl -oate
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General rules for naming carbon chains
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- Count the longest carbon chain and name appropriately - Find any branched chains and count how many carbons they contain - Add the appropriate prefix for each branch chain E.G 3,5-dimethylheptane
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Basic rules for naming functional groups
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- When using a suffix, add in the following way : If the suffix starts with a vowel- remove the -e from the stem alkane name e.g. Propan-1-ol, butan-1-amine, ethanoic acid, ethanoylchloride, butanamide - If the suffix starts with a consonant or there are two or more of a functional group meaning di, or tri needs to be used then do not remove the the -e from the stem alkane name e.g. Propanenitrile, ethane-1,2-diol, propanedioic acid, propane-1,2,3-triol, Pentane-2,4-dione.
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Isomers
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- Structural isomers: same molecular formula different structures (or structural formulae) - Chain isomers: Compounds with the same molecular formula but different structures of the carbon skeleton - Position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton - Functional group isomers: Compounds with the same molecular formula but with atoms arranged to give different functional groups
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Stereoisomerism
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Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms. E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the double bond. E-Z isomers exist due to restricted rotation about the C=C bond Priority Group: The atom with the bigger atomic number is classed as the priority atom
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Refining crude oil
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Fractional Distillation: • Oil is pre-heated • then passed into column. • The fractions condense at different heights • The temperature of column decreases upwards • The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the London forces • Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures • and big molecules condense at the bottom at higher temperatures.
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Cracking
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Cracking: conversion of large hydrocarbons to smaller molecules by breakage of C-C bonds
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Economic reasons for catalytic cracking
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• The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more useful and valuable than the starting materials (e.g. ethene used to make poly(ethene) and ethane-1,2-diol, and ethanol) The smaller alkanes are used for motor fuels which burn more efficiently.
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Reforming
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Turns straight chain alkanes into branched and cyclic alkanes and Aromatic hydrocarbons
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Complete Combustion
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In excess oxygen alkanes will burn with complete combustion The products of complete combustion are CO2 and H2O. C8H18(g) + 12.5 O2(g) --> 8CO2(g) + 9 H2O(l)
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Incomplete Combustion
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If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame) CH4(g) + 3/2 O2(g) --> CO(g) + 2 H2O(l) CH4(g) + O2(g) --> C(s) + 2 H2O(l)
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Pollution from Combustion
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Sulphur containing impurities are found in petroleum fractions which produce SO2 when they are burned. S+ O2 --> SO2 CH3SH+ 3O2 --> SO2 + CO2 + 2H2O
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Pollutant and Environmental consequence
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- Nitrogen oxides (formed when N2 in the air reacts at the high temperatures and spark in the engine): NO is toxic and can form smog NO2 is toxic and acidic and forms acid rain - Carbon monoxide: toxic - Carbon dioxide: Contributes towards global warming - Unburnt hydrocarbons (not all fuel burns in the engine): Contributes towards formation of smog - Soot/particulates: Global dimming and respiratory problems
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Catalytic converters
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These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into 'harmless' CO2, N2 and H2O.
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Biofuels
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Most fossil fuels come from crude oil, which is a non- renewable resource. Fossil fuel reserves will eventually run out Alternative fuels have been developed from renewable resources. Alcohols and biodiesel, which can both be made from plants, are two examples of renewable plant- based fuels
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Advantages of using Biofuels
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Reduction of use of fossil fuels which are finite resources biofuels are renewable Use of biodiesel is more carbon-neutral Allows fossil fuels to be used as a feedstock for organic compounds No risk of large scale pollution from exploitation of fossil fuels
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Disadvantages of Biofuels
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Less food crops may be grown Land not used to grow food crops Rain forests have to be cut down to provide land Shortage of fertile soils
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Mechanisms
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To understand how the reaction proceeds we must first understand how bonds are broken in organic mechanisms There are two ways to break a covalent bond: 1. HOMOLYTIC FISSION: each atom gets one electron from the covalent bond 2. HETEROLYTIC FISSION: (one atom gets both electrons) It proceeds via a series of steps: STEP ONE: Initiation STEP TWO: Propagation STEP THREE: Termination
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Free Radical Substitution Reactions of Alkanes
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Overall Reaction CH4 +Cl2 --> CH3Cl+HCl methane --> chloromethane This is the overall reaction, but a more complex mixture of products is actually formed The MECHANISM for this reaction is called a FREE RADICAL SUBSTITUTION
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STEP ONE: Initiation
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Essential condition: UV light The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others as it is the weakest. UV light does not have enough energy to break the C-H bond. The bond has broken in a process called homolytic fission. When a bond breaks by homolytic fission it forms Free Radicals. Free Radicals do not have a charge and are represented by a (black dot). A Free Radical is a reactive species which possess an unpaired electron.
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STEP TWO: Propagation
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CH4 + Cl. --> HCl +.CH3 .CH3 + Cl2 --> CH3Cl + Cl. The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical. The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical. All propagation steps have a free radical in the reactants and in the products. As the Cl free radical is regenerated, it can react with several more alkane molecules in a CHAIN REACTION.
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STEP THREE: Termination
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.CH3 + Cl. --> CH3Cl .CH3 + .CH3 --> CH3CH3 Cl. + Cl. --> Cl2 Collision of two free radicals does not generate further free radicals: the chain is TERMINATED. Minor step leading to impurities of ethane in product. Write this step using structural formulae and don't use molecular formulae.
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Alkenes
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C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which 'like' electrons: these species are called electrophiles.
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Formation of σ bond
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One sp2 orbital from each carbon overlap to form a single C-C bond called a sigma σ bond. Rotation can occur around a sigma bond
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Formation of π bond
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The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond. The pi bond leads to resultant high electron density above and below the line between the two nuclei
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Reaction of Alkenes with Hydrogen
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Change in functional group: alkene --> alkane Reagent: hydrogen Conditions: Nickel Catalyst Type of reaction: Addition/Reduction Electrophilic Addition: Reactions of Alkenes A π bond is weaker than a σ bond so less energy is needed to break π bond The π bonds in alkenes are areas with high electron density. This is more accessible to electrophilic attack by electrophiles. Alkenes undergo addition reactions.
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Reaction of Alkenes with bromine/chlorine
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Change in functional group: alkene --> dihalogenoalkane Reagent: Bromine (dissolved in organic solvent) Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic
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Reaction of Hydrogen Bromide with alkenes
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Change in functional group: alkene --> halogenoalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic Addition Type of reagent: Electrophile, H+ Type of Bond Fission: Heterolytic The order of stability for carbocations is tertiary > secondary > primary In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
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Reaction of Potassium Manganate(VII) with Alkenes
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Change in functional group: alkene --> diol Reagent: KMnO4 in an acidified solution Conditions: Room temperature Type of reaction: Oxidation Observation: purple colour of MnO4- ion will decolourise to colourless
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Reaction of Bromine Water with Alkenes
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Reagent: Bromine dissolved in water Conditions: Room temperature Type of reaction: Addition Observation: Orange colour of bromine water will decolourise to colourless
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hydration of alkenes to form alcohols
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Industrially alkenes are converted to alcohols in one step rather than the two in the above sulphuric acid reaction. They are reacted with water in the presence of an acid catalyst. Essential Conditions High temperature 300 to 600°C High pressure 70 atm Catalyst of concentrated H3PO4 This reaction can be called hydration: a reaction where water is added to a molecule. The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out. See equilibrium chapter for more on the industrial conditions for this reaction.
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Addition Polymers
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Addition polymers are formed from alkenes This is called addition polymerisation. Poly(ethene): is used to make plastics bags, buckets, bottles. It is a flexible, easily moulded, waterproof, chemical proof, and low density plastic. Poly(propene) is a stiffer polymer, used in utensils and containers and fibres in rope and carpets.
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Incineration
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Rubbish is burnt and energy produced is used to generate electricity. Some toxins can be released on incineration. (e.g. Combustion of halogenated plastics (ie PVC) can lead to the formation of toxic, acidic waste products such as HCl.) Modern incinerators can burn more efficiently and most toxins and pollutants can be removed. Greenhouse gases will still be emitted though. Volume of rubbish is greatly reduced.
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Recycling
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Saves raw materials- nearly all polymers are formed from compounds sourced/produced from crude oil. Saves precious resources. Polymers need collecting/ sorting- expensive process in terms of energy and manpower. Polymers can only be recycled into the same type - so careful separation needs to be done. Thermoplastic polymers can be melted down and reshaped.
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feedstock for cracking
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Polymers can be cracked into small molecules which can be used to make other chemicals and new polymers- Saves raw materials-
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Naming Halogenoalkanes
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Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.
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Primary halogenoalkane
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One carbon attached to the carbon atom adjoining the halogen
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Secondary halogenoalkane
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Two carbons attached to the carbon atom adjoining the halogen
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Tertiary halogenoalkane
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Three carbons attached to the carbon atom adjoining the halogen
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Reactions of Halogenoalkanes
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Halogenoalkanes undergo either substitution or elimination reactions.
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Nucleophilic substitution reactions
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Substitution: swapping a halogen atom for another atom or groups of atoms. Nucleophile: electron pair donator e.g. :OH-, :NH3, CN-. :Nu represents any nucleophile - they always have a lone pair and act as electron pair donators. The rate of these substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to break and the faster the reaction. The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive
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Comparing the rate of hydrolysis reactions
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Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water. Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction. Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a SILVER HALIDE PRECIPITATE. The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes. The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction. AgI (s) - yellow precipitate (forms fastest) AgBr(s) - cream precipitate AgCl(s) - white precipitate
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Nucleophilic substitution with aqueous hydroxide ions
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Change in functional group: halogenoalkane --> alcohol Reagent: potassium (or sodium) hydroxide Conditions: In aqueous solution; Heat under reflux Mechanism: Nucleophilic Substitution Role of reagent: Nucleophile, OH- The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs
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SN1 nucleophilic substitution mechanism for tertiary halogenoalkanes
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Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is made stabilised by the electron releasing methyl groups around it. (see alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above. Primary halogenoalkanes don't do the SN1 mechanism because they would only form an unstable primary carbocation.
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Nucleophilic substitution with ammonia
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Change in functional group: halogenoalkane --> amine Reagent: NH3 dissolved in ethanol Conditions: Heating under pressure in a sealed tube Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :NH3
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Elimination with alcoholic hydroxide ions
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Change in functional group: halogenoalkane --> alkene Reagents: Potassium (or sodium) hydroxide Conditions: In ethanol ; Heat Mechanism: Elimination Role of reagent: Base, OH- Note the importance of the solvent to the type of reaction here. Aqueous: substitution Alcoholic: elimination The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution Tertiary tends towards elimination
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Uses of halogenoalkanes
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Haloalkanes have been used as refrigerants, fire retardants, pesticides and aerosol propellants chloroalkanes and chlorofluoroalkanes can be used as solvents. Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer.
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Naming Alcohols
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These have the ending -ol and if necessary the position number for the OH group is added between the name stem and the -ol If the compound has an -OH group in addition to other functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-): If there are two or more -OH groups then di, tri are used. Add the 'e' on to the stem name though
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Bond angles in Alcohols
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The HCH bond has 4 bonded pairs and 0 lone pairs, so the bond angle is 109.5 (tetrahedral). 2. The COH bond has 2 bonded pairs and 2 lone pairs (on the oxygen atom), so the bond angle is 104.5 (V-shaped or bent).
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Different types of alcohols
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- Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen - Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen - Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen
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1. Combustion of Alcohols
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Alcohols combust with a clean flame
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2. Reaction of Alcohols with Sodium
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Sodium reacts with alcohols 2CH CH OH + 2Na --> 2CH3CH2O-Na+ + H2 This reaction can be used as a test for alcohols Observations: • effervescence, • the mixture gets hot, • sodium dissolves, • a white solid is produced.
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3. Substitution reactions of Alcohols to form Halogenoalkanes
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This reaction with PCl5 (phosphorous(v)chloride) can be used as a test for alcohols. You would observe misty fumes of HCl produced. For Br use KBr, 50% concentrated H2SO4 to produce HBr. The reaction of KI and conc H2SO4 cant be used to produce HI because the sulphuric acid with oxidise the hydrogen halides to other products.
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4. Oxidation reactions of the alcohols
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Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. -- Partial Oxidation of Primary Alcohols: Reaction: primary alcohol --> aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms: -- Full Oxidation of Primary Alcohols: Reaction: primary alcohol --> carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished) -- Oxidation of Secondary Alcohols: Reaction: secondary alcohol --> ketone Reagent: potassium dichromate(VI) solution and dilute sulphuric acid. Conditions: heat under reflux
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Distinguishing between Aldehydes and Ketones
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The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for tests that are commonly used to distinguish between aldehydes and ketones. Fehling's (Benedict's) solution Reagent: Fehling's Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling's solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react
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Reaction of Alcohols with Dehydrating Agents
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Reaction: Alcohol --> Alkene Reagents: Concentrated Phosphoric acid Conditions: warm (under reflux) Role of reagent: dehydrating agent/catalyst Type of reaction: acid catalysed elimination Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil
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Distillation
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In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid. Classic AS reaction using distillation Reaction: primary alcohol --> aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: use a limited amount of dichromate and warm gently and distil out the aldehyde as it forms [This prevents further oxidation to the carboxylic acid] CH3CH2CH2OH + [O] --> CH3CH2CHO + H2O Observation: Orange dichromate solution changes to green colour of Cr3+ ions
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Reflux
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Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated. Classic AS reaction using reflux Reaction: primary alcohol --> carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished using distillation set up) CH3CH2CH2OH + 2[O] --> CH3CH2CO2H + H2O Observation: Orange dichromate solution changes to green colour of Cr3+ ions
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Purifying an organic liquid
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• Put the distillate of impure product into a separating funnel • wash product by adding either - sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced. Sodium hydrogencarbonate will neutralise any remaining reactant acid. - Saturated sodium chloride solution. •Allow the layers to separate in the funnel, and then run and discard the aqueous layer. •Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (anhydrous sodium sulphate) to dry the organic liquid. • Carefully decant the liquid into the distillation flask •Distill to collect pure product
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Solvent extraction
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Mix organic solvent and oil-water mixture in a separating funnel then separate the oil layer. Distil to separate oil from organic solvent Add anhydrous CaCl2 to clove oil to dry oil Decant to remove CaCl2
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Measuring boiling point
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Purity of liquid can be determined by measuring a boiling point. This can be done in a distillation set up or by simply boiling a tube of the sample in an heating oil bath. To get a correct measure of boiling point the thermometer should be above the level of the surface of the boiling liquid and be measuring the temperature of the saturated vapour. Pressure should be noted as changing pressure can change the boiling point of a liquid. Measuring boiling point is not the most accurate method of identifying a substance as several substances may have the same boiling point.
