A Guidebook to Mechanism in Organic Chemistry

The E\ Mechanism

THE El MECHANISM
This mechanism, like the SNl, envisages the rate of the reaction as
being dependent on the substrate concentration only, the rate-deter­
mining stage involving this species alone. Thus with the halide,
Me 3 CBr
Rate oc [Me 3 CBr]

the reaction rate being measured is that of the formation of the
carbonium ion (I):
Me 3 CBr -> MesC^ + Br^
(I)

Rapid, non rate-determining attack by other species in the system,

for example ®OH or H 2 0, can then take place. If these act as

nucleophiles (i.e. electron pair donors towards carbon) the result is

an overall substitution

©OH

Me,Ce'

\

H.O

Me,C—OH

Me 3 C—OH -> Me 3 C—OH + H®

H

while if they act as bases (electron pair donors towards hydrogen),

the result is removal of a proton from a /J-carbon atom to yield an

olefine:

CH.==CMe 2 + H 20

Me JX

I /©OH

CH 3 —C®

I \H'°

Me ^

CH^CMej + H-,0®

Obviously conditions that promote Sjyl reactions (p. 60) will lead

to El reactions also, for carbonium ion formation is the significant

stage in both. Thus the ratio of unimolecular elimination to substi­

tution has, in a number of cases, been shown to be fairly constant for

a given alkyl group, no matter what the halogen atom or other group

lost as an anion from it. This shows that El and SN\ are not proceed-