Organic Chemistry

Section 11.3 The E1 Reaction 409

Movie:

E1 Elimination

An E1 reaction is a first-order elimination reaction because the rate of the reaction

depends only on the concentration of the alkyl halide.

We know, then, that only the alkyl halide is involved in the rate-determining step of the

reaction. Therefore, there must be at least two steps in the reaction.

The foregoing mechanism shows that an E1 reaction has two steps. In the first step,

the alkyl halide dissociates heterolytically, producing a carbocation. In the second

step, the base forms the elimination product by removing a proton from a carbon that

is adjacent to the positively charged carbon (i.e., from the -carbon). This mecha-

nism agrees with the observed first-order kinetics. The first step of the reaction is the

rate-determining step. Therefore, increasing the concentration of the base—which

comes into play only in the second step of the reaction—has no effect on the rate of

the reaction.

We have seen that the of a compound such as ethane that has hydrogens

attached only to hybridized carbons is 50 (Section 6.9). How, then, can a weak

base like water remove a proton from an hybridized carbon in the second step

of the reaction? First of all, the is greatly reduced by the postively charged

carbon that can accept the electrons left behind when the proton is removed from an

adjacent carbon. Second, the carbon adjacent to the positively charged carbon

shares the positive charge as a result of hyperconjugation, and this electron-

withdrawing positive charge also increases the acidity of the bond.

Recall that hyperconjugation—where the electrons in the bond adjacent to the

positively charged carbon spread into the empty p orbital—is responsible for

the greater stability of a tertiary carbocation, compared with a secondary carbo-

cation (Section 4.2).

H C

H

H

hyperconjugation

the electron-

withdrawing

positive charge

decreases its pKa

δC

δ+ +

H

H

s

C¬H

pKa

sp^3

sp^3

pKa

b

rate = k[alkyl halide]

CH 3 C CH 3 + H 2 OBr+ H 3 O+ + −

Br

CH 3

tert-butyl bromide

2-methylpropene

CH 2 C CH 3

CH 3

3-D Molecules:

t-Butyl chloride;

t-Butyl cation

CH 3 C CH 3 CH 3 H 3 O+

Br

CH 3

slow

C +

+

H

H 2 O

CH 2

CH 3

CH 3

CH 3

CH 2 C

fast

mechanism of the E1 reaction

Br−

+

the alkyl halide

dissociates, forming

a carbocation

the base removes

a proton from a

-carbon

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