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Alkynes

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hydrocarbons that contain C-C triple bonds

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acetylenes

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another name for alkynes because they are derivatives of acetylene(H-C triple bonded to C-H), the simplest alkyne.

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what kinds of reactions do alkynes undergo most?

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the same reactions as alkenes, especially additions and oxidations, as well as some alkyne specific reactions

question

how do triple bonds affect the number of hydrogens?

answer

triple bonds give an alkyne with 4 fewer hydrogenous than the corresponding alkane

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how do ou calculate the number of hydrogen sin an alkyne?

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C2H(2n-2)

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how many elements of unsaturation do triple bonds have?

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2

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what are alkynes most used for in nature?

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plants use alkynes to protect themselves from disease/predators (toxins)

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How are IUPAC names assigned to alkynes?

answer

in a manner similar to that of alkenes: find the longest carbon chain that includes the triple bond, change the ending of the parent chain to -yne, number the chain from the end closest to the triple bond, designating the triple bond by its lower numbered carbon. substituents are assigned numbers to indicate their locations

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How do you name the alkyne when additional functional groups are present?

answer

the suffixes are combined to produce compound names, alkynes are usually lower priority, so the functional group is assigned the lower numbered carbon. ex) t.b + alcohol = alkynols, or d.b + t.b = alkenynes.

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how are alkynes assigned common names?

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as a molecule of acetylene with one or more alkyl substituents. see pg 389

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what do many of an alines chemical properties depend on?

answer

whether the triple bond comes at the end of the carbon chain or not

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terminal alyne/acetylene

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an alkyne at the end of the carbon chain

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internal alkyne/acetylene

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when the the triple bond is located somewhere other than the end of the carbon chain

question

what are the physical properties of alkynes similar to? Describe them (polarity, solubility, boiling points, odor, state)

answer

those of alkanes and alkenes of similar molecular weights. They are relatively polar and nearly insoluble in water, nearly insoluble in water, mostly soluble in organic solvents (like acetone, ether, methylene chloride, chloroform, and alcohols), many have mildly offensive odors, gases at room temp, boiling points are almost the same as alkenes and alkanes with similar carbon skeletons

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why are alkynes commercially important?

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acetylene is an important industrial feedstock, but its largest use is fuel for welding.

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what is acetylene made of?

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coal or natural gas

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describe the electronic structure of a triple bond in acetylene

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there are 3 pairs of electrons between the carbon nuclei. each carbon is bonded to two other atoms and there are no nonbonding valence electrons. each C atom needs 2 hybrid orbitals to form the sigma bond framework, which are formed by hybridization of the s orbital with one p orbital. these two sp orbitals overlap with each other and the hydrogen s orbitals to give the sigma bond framework = 180 degree structure. 2 pi bonds resulting from overlap of the 2 remaining unhybridized p orbitals overlap at right angles to each other with electron cloud density in front and in back of the sigma bond, forming a cylinder of electron density encircling the sigma bond between the 2 carbon atoms

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why is a triple bond relatively short?

answer

because of the attractive overlap of 3 pairs of electrons and the high s character of the sp hybrid orbitals

question

are terminal alkynes acidic? Why or why not?

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they are much more acidic than other hydrocarbons, due to the nature of the sp hybrid triple bonded carbon bonded to an H

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what happens when an acetylene proton is removed?

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an acetylide ion is formed, which plays a central role in alkyne chemistry

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how is acidity related to hybridization?

answer

cidity (of a C-H bond) increases with the increasing s character of the orbitals

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what affect do very strong bases have on acetylenes?

answer

very strong bases deprotonate terminal acetylenes to form carbanions called acetylide ions/alkynide ions

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why don't internal alkynes react?

answer

they don't have acetylene protons

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what is one of the best methods for synthesizing substituted alkynes and why?

answer

Because acetylide ions are strong nucleophiles, one of the best methods for synthesizing substituted alkynes is a nucleophilic attack by an acetylide ion on an unhindered alkyl halide

question

how many approaches are commonly used for synthesis of alkynes?

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2: an appropriate electrophile undergoes nucleophilic attack by an acetylide ion, or a triple bond is formed by a double dehydrohalognation of a dihalide

question

describe the synthesis of alkynes when an appropriate electrophile undergoes nucleophilic attack by an acetylide ion

answer

the electrophile may be an unhindered primary alkyl halide (undergoes Sn2), or it may be a carbonyl compound (undergoes addition to give an alcohol). Either reaction joins two fragments and gives a product with a lengthened product skeleton

question

describe the synthesis of alkynes when a triple bond is formed by a double dehydrohalognation of a dihalide

answer

this reaction doesn't enlarge the carbon skeleton. Isomerization of the triple bond may occur, so dehydrohalogenation is useful only when the desired product has the triple bond in a thermodynamically favored position.

question

describe the alkylation of acetylide ions (pg 395)

answer

It is an Sn2 reaction. an acetylide ion is a strong base and a powerful nucleophile. It can displace a halide ion from a suitable substrate, giving a substituted acetylene. an alkyl halide must be an excellent substrate to produce good yield, and it must be primary, with no bulky substituents or branches close to the reaction center. If the back-side is hindered, the acetylide ion may abstract a proton, giving elimination by the E2 mechanism

question

what qualities do acetylide ions share with other carbanions

answer

they are strong nucleophiles and strong bases

question

describe the addition of acetylide ions to carbonyl groups

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acetylide ions can add to carbonyl groups. because the O is more e-, the C=O bond is polarized: the O is slightly neg, the C slightly pos. (electrophilic). the nucleophile attacks the C, placing a neg charge on the O, producing an alkoxide (strong base). addition of H2O or a dilute acid protonates the alkoxide to give the alcohol. an acetylide ion can serve as the nucleophile in the is addition to a carbonyl group, when it adds to the carbonyl group to form an akoxide ion. addition of dilute acid protonates the alkoxide to give the alcohol. the acetylide adds to the formaldehyde (H2C=O) to give a primary alcohol with one or more C atom than there was in the acetylide. An acetylide adds to an aldehyde to give a 2ndary alcohol

question

alkoxide ion

answer

the conjugate base of an alcohol, a weak acid

question

how many alkyl groups does a ketone have bonded to its carbonyl carbon atom?

answer

two. addition of an acetylide followed by protonation gives a tertiary alcohol

question

what do you do if a synthesis requires both alkylation of an acetylide and addition to a carbonyl?

answer

add the less reactive group first: alkylate, then add to a carbonyl. In general, you should add reactive functional groups late in a synthesis.

question

describe synthesis of alkynes by elimination reactions

answer

by eliminating HX from a dihalide, a carbon-carbon triple bond can be generated.

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what results from dehydrohalogenation of a geminal or vicinal dihalide

answer

a vinyl halide.

question

under strong basic conditions (fused/molten/alcoholic KOH and heat or sodium amide, NaNH2), what may result from dehydrohalogenation of a geminal or vicinal dihalide?

answer

after the vinyl halide is produced, a second dehydrohalogenation may occur to form an alkyne

question

see pg 400 for a summary of alkyne synthesis reactions

answer

see pg 400 for a summary of alkyne synthesis reactions

question

why are many of the reactions of alkynes similar to the corresponding reactions of alkenes?

answer

because both involve pi bonds between two C atoms, which are electron rich and readily undergo addition reactions. reagents add across triple bonds of alkynes just as they add across the double bond of alkenes, converting a pi bond and a sigma bond into two sigma bonds. This reaction is exothermic because sigma bonds are generally stronger than pi bonds.

question

how many molecules can add across a triple bond? What might happen due to different conditions?

answer

up to two molecules, depending on the reagents and conditions. Some conditions allow for double addition across the triple bond of an alkyne, while others may allow the reaction to stop after a single addition

question

describe catalytic hydrogenation to alkanes: what catalysts are used how many stages occur, is there an intermediate

answer

In the presence of a suitable catalyst, hydrogen adds to an alkyne, reducing it to an alkane. Platinum, palladium, and nickel are commonly used catalysts in this reduction. It occurs in two stages with an alkene intermediate

question

describe catalytic hydrogenation to cis alkenes

answer

hydrogenations of an alkyne can be stopped at the alkene stage by using a partially deactivated/poisoned catalyst by treating a good catalyst with a compound that makes a catalyst less effective. ex. Lindlars catalyst- poisoned palladium catalyst

question

how is catalytic hydrogenation of alkynes similar to hydrogenation of alkenes?

answer

both proceed with syn stereochemistry: the face of a pi bond contacts the solid catalyst, and the catalyst weakens the pi bond, allowing 2 hydrogen atoms to add simultaneously, ensuring sun stereochemistry

question

what is the stereochemistry of a catalitically hydrogenated internal alkyne?

answer

a syn addition gives a cis product

question

describe metal-ammonia reduction to trans alkenes

answer

to form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry. sodium metal in liquid ammonia reduces alkynes with anti stereochemistry, converting alkynes to trans alkenes. proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation (protons from the ammonia solvent or added alcohol) to give a neutral radical. addition of another electron gives the cis product. the anti stereochemistry of the sodium-ammonia reduction appears to result from the greater stability of the vinyl radical in the trans configuration, where the alkyl groups are farther apart. an electron is added to the trans radical to give a trans vinyl anion, which is quickly protonated to the trans alkene.

question

describe the mechanism of metal ammonia reduction of an alkyne

answer

an electron adds to the alkyne, forming a radical anion the radical anion is protonated to give a radical an electron adds to the radical, forming an anion protonation of the anion gives an alkene

question

Describe the addition of halogens

answer

bromine and chlorine add to alkynes just as they add to alkenes. one mole of alkyne + one mol halogen = dihaloalkene. the stereochem of addition may be either sun or anti, and the products are often mixtures of cis and trans isomers

question

describe the addition of hydrogen halides

answer

hydrogen halides add across the triple bond of an alkyne in much the same way they add across the alkene double bond. The initial product is a vinyl halide. When a hydrogen halide adds to a terminal alkyne, the product has the orientation predicted by Markovnikov's rule. A second molecule of HX can add, usually with the same orientation as the first. the mechanism is similar to the mechanism of hydrogen halide addition to alkenes. The vinyl cation formed in the 1st step is more stable with the positive charge on the more highly substituted carbon atom. when a second mole is used, it adds in the same orientation, leading to a geminal halide

question

hydration of alkynes to ketones and aldehydes

answer

alkynes undergo acid catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. mixture of mercuric sulfate in aq. sulfuric acid is commonly used as the reagent. the hydration of alkynes is similar to the hydration of alkenes, and it also goes with markovnikov orientation. electrophilic addition of mercuric ion gives a vinyl cation, which reacts with water and loses a proton to give organomercurial alcohol. enols tend to be unstable and isomerize to the ketone form. this isomerization involves the shift of a proton and a double bond. The boxed hydroxyl proton is lost, and a proton is regained at the methyl position, while the pi bond shifts from the C=C position to the C=O position. This type of rapid equilibrium is called a tautomerism

question

describe the mechanism of Acid catalyzed Keto-enol tautomerism

answer

proton is added to the methylene group loss of the hydroxyl proton

question

hydroboration-oxidation

answer

water is added across the double bonds of alkenes with anti-markovnikov orientation similar like in with alkenes, but a hindered dialkylboane must be used to prevent addition of two molecules of borane across the triple bond. di(secondary isoamyl)borane, called disiamylborane, adds to the triple bond only once to give a vinylborane. in a terminal alkyne, the boron atom bonds to the terminal carbon atom. oxidation of the vinylborane gives a vinyl alcohol (enol) resulting from anti-Markovnikov addition of water across the triple bond. this enol quickly tautomerizes to its more stable carbonyl (keto) form. In the case of a terminal alkyne, the leto product is an aldehyde. under basi conditions, the leto-enol tautomerism operates by a different mechanism than it does in an acid. In base, the proton is first removed from its old position in the OH group, and then replaced on carbon.

question

describe the mechanism of base catalyzed leto-enol tautomerism

answer

hydroxyl proton is lost reprotonation on the adjacent carbon atom

question

permanganate oxidations

answer

under mild conditions, potassium permanganate oxidizes alkenes to glycols, compounds with 2 -OH groups on adjacent carbon atoms. the oxidation involves adding a hydroxyl group to each end of the double bond (hydroxylation), similar to reactions with alkynes. If an alkyne is treated with cold, aqueous potassium permanganate under nearly neutral conditions, an alpha-diketone results. This is the same as hydroxylating each of the 2 pi bonds of the alkyne, then losing 2 molecules of water to give the diketone. If a reaction mixture becomes warm or too basic, the diketone undergoes oxidative cleavage. the products are the carboxylate salts of carboxylic acids, which can be converting to free acids by adding dilute acid. terminal alkynes are cleaved similarly to give a carboxylate ion and formate ion. formate oxidizes further to carbonate, which becomes CO2 after protonation

question

describe ozonolysis

answer

ozonolysis of an alkyne, followed by hydrolysis, cleaves the triple bond and gives two carboxylic acids. either permanganate cleavage or ozonolysis can be used to determine the position of the triple bond in an unknown alkyne

question

what steps should be followed in multistep synthesis?

answer

Review the functional groups and carbon skeleton of the target compound 2 review the functional groups and carbon skeletons of the starting materials, and see how their skeletons might fit together in the target compound 3. compare methods for assembling the carbon skeleton of the target compound, which ones provide a key intermediate with the correct carbon skeleton and functional groups correctly positioned for conversion to the functionality in the target molecule 4. working backward through as many steps as necessary, compare methods for synthesizing the reactants needed for assembly of the key intermediate with the correct carbon skeleton and functionality 5. summarize the compete synthesis in the forward direction, including all steps and all reagents, and check it for errors and omissions

question

see pg 214-215 for summary of alkyne reactions

answer

see pg 214-215 for summary of alkyne reactions

Organic Chemistry Chapter 9

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