Exam 3 ORGANIC 255

Leaving group:
halide sometimes or any other electronegative group. This is the group that is displaced in a substitution of lost in elimination.
Electrophile:
electron loving, Lewis Acid
Nucleophilic substitution:
Any reaction in which one nucleophile (nucleus loving) is substituted for another at the tetravalent carbon atom, Lewis base
β-Elimination:
A reaction in which a molecule such as HCl, HBr, HI, or HOH id split out or eliminated from adjacent carbons
POINTS FOR NUCLEOPHILIC SUBSTITUTION
1.) If the nucleophile is negatively charged, the atom donating the electrons will become neutral
2.) If the atom is neutral, then the atom donating the electron pair will become positively charged
3.) Some nucleophile substitution reactions from new carbon—carbon bonds (like alkynes)
a. Important because it can extent a molecular carbon skeleton
b. See chart on page 331
Protic Solvent:
A solvent that is a hydrogen bond donor, in other words there is partially positively charged hydrogen somewhere in the carbon chain. Most common is –OH groups. H¬2O and low molecular weight alcohols or carboxylic acids.
Aprotic solvents:
A solvent that cannot serve as a hydrogen bond donor, there are not hydrogen atoms bonded to highly electronegative atoms. Common aprotic solvents are Dichloromethane, Diethyl Ether, Dimethyl sulfoxide
Dielectric Constant:
A measure of a solvents ability to insulate opposite charges from one another
What are the 2 limiting mechanisms for Nucleophilic substitutions
Sn1 and Sn2
Sn2:
A bimolecular Nucleophilic substitution reaction, when the bond breaking and the bonding forming occur simultaneously. 1 STEP!
Bimolecular reaction:
A reaction in which 2 species are involved in the rate-determining step
Describe the Sn2 Reaction mechanism
o Its called bimolecular because both the haloalkane and the nucleophile are involved in the rate-determining step
• Attacks from the BACK SIDE
o Because the polarized C—Br bond the Carbon atom had a δ+ charge and that attracts the electron rich nucleophile.
o Also goes into the antibonding orbital
o The electrodensity of the nucleophile entering from the backside assists in breaking the C—Br bond thus helping the Br leave
• Sn2 Only has one Energy Barrier (ONE STEP) because 5 groups are interacting with the central carbon, its unstable, because the transition compound has a higher internal energy, or standard relative to the reactant/product.  think rubber balls
Sn1:
a unimolecular Nucleophilic substitution reaction
unimolecular reaction:
A reaction in which only one species is involved in the rate-determining step, the bond breaking between the Carbon and leaving group is completed BEFORE the bond forming with the nucleophile begins.
What molecule is involved in the rate-determining step for Sn1 reactions?
ONLY the Halo Alkane
solvolysis:
A Nucleophilic substitution in which the solvent is also the nucleophile
Some Common Polar Protic Solvents are….
1.) Water (79)
2.) Formic Acid (59)
3.) Methanol (33)
4.) Ethanol (24)
5.) Acetic Acid (6) Moderately polar protic
rate is dependent on what?
CONCENTRATION!!!
What order is Sn1? Sn2?
sn1= first order
Sn2= first order
Sn1 is governed by ___?
Electronic factors (relative instability)
Sn2 is governed by _____?
Steric Factors
Steric Hindrances:
the ability of groups because of their size to hinder the access to a reaction site within a molecule
Allylic Carbocations:
a carbocation in which an allylic carbon bears the positive charge. A way for 10 haloalkanes to undergo an Sn1 reaction, they have a double bond next to the electron deficient carbon, more stable then alkyl’s because of the resonance ability.
allylic:
next to the carbon—carbon double bond
Benzylic carbocation:
a carbocation in which a carbon attached to a benzene ring bears the positive charge
What is the Dielectric constant for Polar, nonpolar and in-between-ers?
polar= 15+
Nonpolar = less than 5
in-between= 5-15
Some Common aprotic solvents are… ?
Dichloromethane, Diethyl Ether, Dimethyl sulfoxide
Solvents:
dissolve reactants and provide the medium in which a reaction takes place
Dielectric constant:
A measure of a solvents ability to insulate opposite charges from one another
Protic solvent:
a solvent that is a hydrogen bond donor
Aprotic Solvent:
a solvent that cannot serve as a h-bond donor, nowhere in the molecule is there a hydrogen bonded to an atom of low electronegativity.
Nucleophilicity:
a kinetic property measured by the rate at which a nucleophile causes nucleophilic substitution on a reference compound under a standardized set of experimental conditions
Basicity:
An equilibrium property measure by the position of the reaction, for example the acid—base reaction for example the A—B reaction between ammonia and water
Strong nucleophile has…. what defining character?
pka higher than 11
moderate nucleophile has what defining character
pka of about 11
weak nucleophile has what characteristic?
pka lower than 11
Relative Ions in polar aprotic solvents
(only the halogens)
F- > Cl- > Br- > I-
Relative ions in polar PROTIC solvents [only halogens]
I- > Br- > Cl- > F-
β-Elimination reaction:
A reaction in which a molecule such as HCl, HBr, HI or HOH is split out or eliminated from adjacent carbons
Dehydrohalogenation:
Removal of –H and –X from adjacent carbons, a type of β-Elimination reaction
Zaitsev’s Rule:
(or undergo Zaitsev’s elimination) a rule that the major product of a β-Elimination reaction is the most stable alkene. That is with the highest amount of substituents on the carbon carbon double bond
E1:
A Unimolecular β-Elimination Reaction or when a bond breaks the carbon-leaving group bond to give a carbocation before a reaction with a base to lose a hydrogen and from the carbon—carbon double bond. 1st order kinetics, product is a more stable alkene substituted with carbon—carbon double bonds.
E2:
A Bimolecular β-Elimination reaction where the bond break and make is at the same time. Consider only when hydroxides, alkoxides and acetylides and amides anions are used. 2nd order kinetics has major products

Alkynes:

a carbon—carbon triple bond

Terminal alkynes:

an alkyne in which the triple bond is between carbons 1 and 2

·         Describe naming alkynes:

o   “-yn-” is used to show the presence of a triple bond

o   Number the longest chain that contains the triple bond from end to end giving the triple bond priority to the lowest numbers

o   If the molecule has more than one triple bond use “-adiyn-“, “atriyn-“, and so forth

   Alkylation reaction:

any reaction in which a new carbon—carbon bond to an alkyl group is formed, this is only practical with 1o and methyl halides, this process is common in lab
Dehydrohalogenation:
the removal of the HX from the molecule

Allene:

  Any compound that contains adjacent carbon—carbon double bonds; that is, any molecule that contains a C=C=C functional  group

 Vinylic Carbocation:

a carbocation in which the positive charge is on one of the carbons of a carbon—carbon double bond

Enol:

A compound containing a hydroxyl group bonded to a doubly bonded carbon ato, thinkà alkene (en-) with an alcohol (-ol) resulting in enol

   Tautomers:

constitutional isomers in equilibrium with each other that differ in the location of a hydrogen atoms and a double bond relative to a heteroatom most commonly O, N or S

Keto-Enol Tautomerism:

A type of isomerism involving kept (from ketone) and enol tautomers

·       what are three ways to make and alkyne into an alkene and alkane

o  

Catalytic reduction

 

 

o   Hydroboration-protonolysis

o   Dissolving-metal reduction

1.)    Lindlar catalyst:

Finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. It is particular use is as a catalyst for the reduction of an alkyne to a cis alkene.

1.)    Syn addition: 

the addition of two hydrogen atoms to the carbon—carbon double bond that gives cis alkenes
Organic Synthesis:
a series of reaction by which a set of organic starting materials is converted to a more complicated structure
Retrosynthesis:
A process of reasoning backwards from a target molecule to a suitable set of starting materials

·         what is the STRATEGY for solving organic synthesis?:

 

1.)    Count the carbon atoms of the skeleton of the target molecule. Determine how to build the carbon skeleton from available starting materials is often the most challenging part of a synthesis.

a.       If you must add carbons consider what carbon—carbon bond forming reactions are available to you.

                                                                           i.      Only option that would be given is alkyation of acetylide anions with methyl or primary halides

2.)    Analyze functional groups, how can they change or what needs to be changed.

a.       Keep in mind stereoselectivity and Regiospecificity

3.)    WORK BACKWARDSJ

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