copyright 2002 © mark brandt, ph.d. addition reactions

Copyright 2002 © Mark Brandt, Ph.D.

Addition Reactions


Exam III

• Nucleophilic substitution– Reaction mechanisms

• SN1

• SN2

– Stereochemistry– Leaving groups– Nucleophiles– Substrates– Solvent effects– Products


Exam III

• Elimination reactions– Reaction mechanisms

• E1• E2• Dehydration

– Stereochemistry– Leaving groups– Nucleophiles– Substrates– Solvent effects– Products


Exam III

• Factors controlling competition between reaction types– E2 vs SN2 vs SN1 vs E1

• Rearrangement reactions– Reaction mechanisms

• Methyl and alkyl shifts• Hydride shifts

– Products• Dihalide reactions

– Synthesizing dihalides– Synthesizing alkenes and alkynes


Exam III

• Nomenclature• Spartan problem?• Mechanism• Energy profile diagram• Fill in the blank

• Relative carbocation stability• Relative alkene stability• Review session Wednesday 7 PM

CH3

Br KOC(CH3)3

CH3 CH3

+

Major Minor


Hydrogenation(Alkene Reduction)

• Platinum or Palladium catalysts allow conversion of alkenes or alkynes to alkanes.

• The catalyst has a large surface area. Under high pressure the H2 will dissociate to atomic hydrogen adsorbed to catalyst surface.

• The H will add to the same face of the alkene (syn addition).

CH3

CH3

H2/PtCH3

CH3

H

H


Addition Reactions

• Alkenes are substrates for reactions in which groups add across the double bond.

• An example is the addition of a halogen molecule (e.g. Br2) to form a vicinal dibromide.

R

CR

C

R

R

A B R C C R

R

B

R

A

+


Reactivity of Alkenes

• The bond of an alkene is weaker than typical bonds. Replacing a bond with a bond usually results in a lower energy state.

• The orbitals of a double bond are exposed to attack. Steric hindrance is minimal.

• The orbitals contain relatively loosely associated electrons, and are subject to electrophilic attack.

• Electrophile = “electron-loving”


General Mechanism

• An alkene can act as a Lewis base.

• Carbocations are Lewis acids.

R

CR

C

R

R

A B

R

CR C+

R

RA

B

R C C R

R

B

R

A


Hydrogen Halide Addition

• Markovnikov’s rule – the hydrogen adds to the carbon that has more hydrogens.

• Markovnikov’s rule – the addition occurs to yield the more stable carbocation intermediate.

• This is an example of regioselectivity – the predictable preference of one product over others.

H

CH

C

CH3

CH3

H Br

H

CH C+

CH3

CH3H

Br

H C C CH3

CH3

Br

H

H


Hydrogen Halide Addition

• Hydrogen halide addition may result in the creation of a stereocenter.

• The products form as a racemic mixture.

H

CH

C

H

CH2 CH3

H I

H C C CH2

H

I

H

H

CH3C C

+H

CH2 CH3H

H

H


Sulfuric Acid Addition

• Although hydrogen sulfate ion is a poor nucleophile, in the absence of other nucleophiles, it will attack carbocations.

H

CH

C

H

CH2 CH3

H C C CH2

H

OSO3H

H

H

CH3C C

+H

CH2 CH3H

H

H

S

O

OO

O

HH S

O

OO

O

H OH2

H2SO4

H C C CH2

H

OH

H

H

CH3

+_

Conc.H2SO4


Alcohol Synthesis

• Alcohols may be synthesized directly.

• The reaction is the reverse of the dehydration reaction.

• Rearrangement is a problem, whenever a more stable carbocation can be formed.

H

CH

C

H

CH2 CH3

H C C CH2

H

OH

H

H

CH3 OH C CH3

CH3

CH3

OH2

H2SO4 +


Alcohol Synthesis

• Because the reaction is reversible, it is difficult to get the reaction to go to completion; dilute acid is used.

• The dehydration reaction is more useful, because the product can be removed by distillation during the reaction.

• The conjugate base of the acid must be a poor nucleophile (e.g. sulfate or phosphate) to prevent SN1 and other reactions.


Alkenes and Halogens

• Alkenes react readily with halogen molecules.

• The reaction must be performed in a non-polar solvent to prevent other nucleophilic reactions.

CCCH3

H H

CH3

Br2CCCH3

H H

CH3

BrBrCCl4


Halohydrin Formation

• If a halogen molecule reaction with an alkene in water, the product is a halohydrin.

CCCH3

H CH3

CH3

Br2

OH2

Br Br

CCCH3

H CH3

CH3

Br+

Bromonium ion

O

H

H

CCCH3

H

CH3

CH3

Br

O+

H

H

O

H

H

CCCH3

H

CH3

CH3

Br

OH


Halonium Ions

• The properties of the cyclic halonium ion have two consequences:– The second attack occurs anti to the first.

– The second attack occurs at the more substituted carbon (unless the more substituted carbon is too sterically hindered).

CCCH3

H CH3

CH3

Br+

O

H

H

+

Cl2

OH2

Cl+ Cl

OH2

+

Cl

OH


Alkyne Addition

• Halogens add across triple bonds in the same way as across double bonds.

CH3 C CHCCl4

Br2CH3 C CH

Br Br

CCl4

Br2CH3 C CH

Br Br

Br Br


Alkyne Addition

• Hydrohalogen addition across triple bonds is also similar to addition across double bonds. Addition follows Markovnikov’s rule.

CH3 C CHCCl4

BrHCH3 C CH

Br H

CCl4

BrHCH3 C CH

Br H

Br H


Alkene Oxidation

MnO O

O O

H H

OH OH

coldKMnO4

OH2

OH

MnO O

O O


Alkene Oxidation

MnO O

O O

heatKMnO4

OH2

OHOHOH

O O


Carbenes

• A carbene is a highly unstable compound containing a carbon with two bonds.

CH2 N+

N N N:CH2 +heat

CHCl3KOC(CH3)3

C Cl

Cl

Cl Cl:CCl2 +

CH2I2 Zn(Cu) ICH2ZnI+


Carbene Reactions

• Carbenes react readily with double bonds, by forming a three membered ring.

ICH2ZnI

H H

+

copyright 2002 © mark brandt, ph.d. addition reactions

Documents

mark brandt

pm slide

addition reactions alkenes

dehydration reaction

electronloving slide

alkene syn addition

sulfuric acid addition

reaction types e2 vs