A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

At first sight, therefore, it might be concluded that the Saytzeff

mode of elimination was the normal one and the Hofmann mode

merely an occasional, abnormal departure therefrom. In fact, it is the

latter that predominates with onium salts. Thus on heating (XXII),

it is largely ethylene, rather than propylene, that is obtained despite

the greater stability (due to hyperconjugation) of the latter:

*Me • CHj • CH 2 • NMe 2 + CH 2 =CH 2

Me / Hofmann

I®

Me^CH-CH 2 -N—CHJ-CHJ'

I I I '

H Me H

(XXII) *Me • CH=CH 2 + Me 2 N • CH 2 • CH 3

This can be explained by assuming that, in this case, (a) is of prime
importance: the inductive effect of the methyl group in the n-propyl
substituent causes a lowering of the acidity of the hydrogens
attached to the /J-carbon atom in this group and thus leads to
preferential removal of a proton from the /3-carbon atom of the ethyl
substituent which is not so affected.
Thus it could be claimed that in Satyzeff elimination the hyper-
conjugative effects af alkyl groups are in control while in Hofmann
elimination it is fhsir inductive effects that predominate. But this
leaves unanswered the question as to what causes the shift from the
former mode to the latter. As has already been observed, Hofmann
elimination is more common in the elimination reactions of onium
e e
salts and undoubtedly groups such as R 3 N- and R 2 S- will be much
more potent in promoting acidity in /J-hydrogens by their inductive
effects than will halogen atoms so that the relative acidity of the
jS-hydrogen atoms could well come to be the controlling influence in
the reaction. But this is not the whole of the story: another obvious
difference is that the groups eliminated from onium compounds are
usually considerably larger than those lost in the elimination reac­
tions of halides; so much so that the preferential formation of that
£2 transition state in which there is least crowding becomes imper­
ative, even though this may not be the one favoured by hypercon-
jugative stabilisation. #
The importance of the steric factor has been confirmed in a number
of ways. Thus increase in the size of the leaving group in a compound
of given structure leads to a corresponding increase in the proportion