A Guidebook to Mechanism in Organic Chemistry

• The E2 Mechanism

HO

I I

C—C—Hal

slow

H.O+ ^C-L^Hal - C=C +Hale

/ \

(II)

The formation of a carbanion such as (II) with so little possibility of

stabilisation (cf. p. 211) seems inherently unlikely and evidence

against the a^ial participation of carbanions is provided by a study

of the reaction of/3-phenylethyl bromide (III) with eOEt in EtOD.

Carbanion formation would, a priori, be expected to be particularly

easy with this halide because of the stabilisation that can occur by

delocalisation of the negative charge via the n orbitals of the benzene

nucleus (llla):

CHaCH 2 Br CH=CH^2

"OEt

(i)

CH CH 2 Br CH CH,Br

(llla)

-Br

(IV)

Carrying out the reaction in EtOD should leadTo the formation of

Ph-CHD-CH 2 Br by reversal of (i) and this in its turn should yield

some Ph-CD=CH 2 as well as Ph-CH=CH 2 in the final product.

If, however, the reaction in EtOD is stopped short of completion, i.e.

while some bromide is still left, it is found that neither this nor the

styrene (IV) formed contain any deuterium. Thus a carbanion is not

formed as an intermediate even in this especially favourable case,

and it seems likely that in such E2 eliminations, abstraction of

proton, formation of the double bond and elimination of the halide

ion or other nucleophile normally occur simultaneously as a concerted

process.

Though this is generally true there is, in the highly special case of

the elimination reactions of trichloro- and some dihalo-ethylenes,

some evidence that carbanions are involved:

HO* H Cl H,0 Cl

C=C

\

Cl (I Cl

ci—c—C—Cl

61

of proton by base followed by the faster, non rate-determining elim­

ination of halide ion from the resultant carbanion, as a separate step,

would still conform to the above rate law: