A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

tendency of tertiary and secondary, as compared with primary,

halides to undergo unimolecular elimination rather than substitution

reactions whatever the reagent employed:

RCHjCHjHal R CH.CHMe Hal RCH.CMe.Hal

j. < \ < 4.

RCH=CH, R CH=CH Me RCH=CMe,

In bimolecular reactions also it is found that increasing alkyl substi­
tution favours elimination at the expense of substitution for while it
retards S^2 because of overcrowding in the transition state that would
lead to substitution, it promotes E2 because of the hyperconjugative
stabilisation of the incipient olefine in the alternative transition state
that would lead to elimination.
One of the most potent factors influencing the elimination/substi­
tution ratio with a given substrate, however, is change of mechanism
from uni- to bimolecular. The El/Sjyl product ratio will be fixed, as
will the E2/Sjy2 ratio, and, provided the reaction is proceeding by a
purely uni- or bimolecular mechanism, the ratio will thus be inde­
pendent of the concentration of, e.g. ®OH. As the concentration of
eOH is increased, jjowever, there will come a changeover from an

initially unimolecula^to a bimolecular mechanism, a changeover that
takes place quite suddenly with strong bases such as °OH and which
leads to a different, usually higher, proportion of the elimination
product. This reflects the well-known use of high concentrations of
strong bases for the actual preparation of olefines.

It might be expected that the reagent employed would be of great
significance in influencing the relative amounts of E2/Sw2 in a parti­
cular system, for basicity (i.e. electron pair donation to hydrogen)
and nucleophilicity (i.e. electron pair donation to carbon) do not run
wholly in parallel in a series of reagents, Ye or Y:. Thus the use of
tertiary amines, e.g. triethylamine, rather than eOH or^0 OEt for
converting halides to olefines depends on the amines being moderate­
ly strong bases but weak nucleophiles while the latter reagents are
powerful nucleophiles as well as being strong bases. The particular
preparative value of pyridine foi*this purpose, despite its being a
considerably weaker base than simple tertiary amines^R 3 N: (cf. p. 55),
arises in part at least from the stability of the pyridinium cation once
formed; reversal of the abstraction of H® by pyridine is thus unlikely.
Reagents such as eSR which show the widest divergences between