Elimination Reactions—C-^C—C—Y «-» — C=C—C—Y
II IIi °°\ i
NJ-C—C-Y «- N=C—C-Y
3
o eQbut it is not certain that such intermediates are formaias a matter of
course in all these reactions, however. An interesting example above
is the way in which the)>C=0 group makes possible a base-induced
elimination of water from the aldol (XXX), whereas the elimination
of water from a compound not so activated is nearly always acid-
induced (cf. p. 174).
It has been suggested that a good deal of the driving force for the
elimination reactions of suitably substituted carbonyl compounds
is due to the product being conjugated and so able to stabilise itself
by delocalisation (XXXI):MeCH-j-CH—6H==0Me • CH—CH=CH— 6
tMeCH=^CBicH=^0(XXXI)Me CH-CH—CH-O
IBBThat this is not the most important feature, however, is revealed by
the difference in behaviour exhibited by 1- and 2-halogenoketones
(XXXII and XXXIII, respectively). Both could eliminate hydrogen
halide to yield the same olefine (see p. 205) as the product of reaction,
so if its stability were the prime driving force little difference would
be expected ip their rates of elimination. In fact (XXXIII) eliminates
very much more rapidly than ^XXXII) suggesting that the main
effect of the carbonyl substituent is in increasing^the acidity of the
hydrogens on the adjacent carbon atom: this is the one that loses
proton in (XXXIII) but not in (XXXII). It is indeed found to be
generally true that the elimination-pronioting effect of a particular