Organic Chemistry

Section 11.2 The Regioselectivity of the E2 Reaction 407

3-D Molecules:

Methyl anion; Ethyl

anion; sec-Propyl anion;

tert-Butyl anion

Carbocation stability:

3 °is more stable than 1°.

Carbanion stability:

1 °is more stable than 3°.

When a hydrogen and a chlorine, bromine, or iodine are eliminated from an alkyl

halide, the halogen starts to leave as soon as the base begins to remove the proton.

Consquently, a negative charge does not build up on the carbon that is losing the pro-

ton. Thus, the transition state resembles an alkene rather than a carbanion

(Section 11.1). The fluoride ion, however, is the strongest base of the halide ions and,

therefore, the poorest leaving group. So when a base begins to remove a proton from

an alkyl fluoride, there is less tendency for the fluoride ion to leave than for the other

halide ions to do so. As a result, negative charge develops on the carbon that is losing

the proton, causing the transition state to resemble a carbanion rather than an alkene.

To determine which of the carbanion-like transition states is more stable, we must de-

termine which carbanion would be more stable.

We have seen that because they are positively charged, carbocations are stabilized

by electron-donating groups. Recall that alkyl groups stabilize carbocations because

alkyl groups are more electron donating than a hydrogen. Thus, tertiary carbocations

are the most stable and methyl cations are the least stable (Section 4.2).

Carbanions, on the other hand, are negatively charged, so they are destabilized by

alkyl groups. Therefore, methyl anions are the most stable and tertiary carbanions are

the least stable.

The developing negative charge in the transition state leading to 1-pentene is on a

primary carbon, which is more stable than the transition state leading to 2-pentene,

relative stabilities of carbanions

most

stable

least

stable <

R

RC

−−

R

tertiary

carbanion

<

R

R

H

secondary

carbanion

<

H

R

H

primary

carbanion

H

H

H

methyl

anion

C

−

C

−

C

most

stable

least

> stable

R

RC+

R

tertiary

carbocation

>

R

RC+

H

secondary

carbocation

>

H

RC+

H

primary

carbocation

H

HC+

H

methyl

cation

relative stabilities of carbocations

CH 2 CHCH 2 CH 2 CH 3

H

OCH 3

F

transition state leading to

1-pentene

more stable

δ−

OCH 3

δ−

δ−

CH 3 CHCHCH 2 CH 3

H

F

transition state leading to

2-pentene

less stable

carbanion-like δ−

transition state

CH 3 CHCH 2 CH 2 CH (^3) CH
3 OH

• CH 3 CH CHCH 2 CH 3 + CH 2 CHCH 2 CH 2 CH 3 + CH 3 OH + F−
  F
  2-fluoropentane
  CH 3 O−
  methoxide
  ion
  1-pentene
  70%
  2-pentene
  30%
  (mixture of E and Z)
  BRUI11-400_436r3 26-03-2003 10:20 AM Page 407