A Guidebook to Mechanism in Organic Chemistry

Orientation in E2 Eliminations

of Hofmann product produced, and the same result is observed
when branching is introduced into the structure of a compound
(with halides as well as onium salts) that might be expected to lead
to increasing crowding in the E2 transition state. Perhaps most cogent
of all, an increase in the proportion of Hofmann product is seen when
the size of the initiating base is increased. Thus in the dehydro-
bromination of Me-CH 2 CMe 2 Br the change from Me-CH 2 -Oe
-^Me 8 C-O^0 ->EtMe 2 C-O^0 ->Et 3 C-Oe leads to formation of the
Hofmann product, MeCH 2 C(Me)=CH2) in yields of 29, 72, 78
and 89 per qgat respectively. The importance that steric factors can
play in deciding which type of elimination will result can, perhaps,
best be seen by comparing the transition states (XXIII) and (XXIV)
for the two modes of E2 elimination from R • CH, • CMe 2 X:

HO^H Me R Me

R^cic-Me - C=C Saytzeff

H t>

H Me

Y ©

(XXIII)

H 20

HO*H _ CH 2 R H CH 2 R

-C-Me -> C==C Hofmann

H Me

h-cXq- M«

Y ©

(XXIV)

It can be seen that if Y is large, and especially if R is large as well,

transition state (XXIV) will be favoured over (XXIII) as, in the

former, R is much better able to get out of Y's way; as indeed will be

the case if QOH is replaced byabulkier base, Ybeing the same distance

away in both cases but R being much less of a hindrance in (XXIV)

than in (XXIII). *

The classical Ijofmann elimination reaction has been of the utmost

value in structure elucidation, particularly in the alkaloid field. Any

basic nitrogen atom present is converted to the quaternary salt by

exhaustive methylation and the corresponding quaternary hydroxide