A Guidebook to Mechanism in Organic Chemistry

* Effect of Activating Groups

«* EFFECT OF ACTIVATING GROUPS

Thus far we have only considered the effect of alkyl, and occasionally

aryl, substituents in influencing elimination reactions, but a far

more potent influence is exerted by strongly electron-withdrawing

groups such as -N0 8 , )>SOa, -CN, ^>C=0, -COzEt, etc., in

facilitating eliminations. Their influence is primarily on increasing

the acidity of the j3-hydrogen atoms:

BPH B:H

—r-C-^C-^C-^Br -> —C—C=C< H»Br^cic-LJIr G

II I I II I \

o o

O B-^H O B:H®

\f-«-C^C-^Br -* \j—C=c/+Br©

/•I.I /e I X

B-^H B:H®

—C-(fC^C-^OH -> — C—C=c/+OH°

II I I II I X

o o

(XXX)

The reactions can proceed by a^ne- or a two-stage mechanism

depending on wh|ther the removal of proton and the other leaving

group is concerted or whether an intermediate anion is actually

formed. An added effect of substituents like the above is, of course,

to stabilise such an anion by delocalisation

their basicity and nucleophilicity, are in general too weak bases to be
of much value in inducing elimination reactions.
Careful investigation has shown that where substitution and
elimination reactions compete in a given system, elimination normally
has the higher activation energy and is thus the more favoured of the
two by rise in temperature: a fact that has long been recognised in
preparative chemistry.