r
Structure, Reactivity and Mechanism ^reagent and substrate on their close approach as when bromine adds
to ethylene (p. 137).
In reactions of the first type the reagent is looking for a position
in the substrate to be attacked where electrons are especially readily
available; such reagents are thus referred to as electrophilic reagents or
electrophiles. In reactions of the second type the reagent is looking for
a position where the atomic nucleus is short of its normal complement
of orbital electrons and is anxious to make it up; the reagents employed
are thus referred to as nucleophilic reagents or nucleophiles.
This differentiation can be looked upon as a special case of the
acid/base idea. The classical definition oV acids and bases^s that the
former are proton-donors and the latter proton-acceptors. This was
made more general by Lewis who defined acids as compounds pre
pared to accept electron pairs and bases as substances that could pro
vide such pairs. This would include a number of compounds not pre
viously thought of as acids and bases, e.g. boronJtrifluoride (XXXIX)F Me F Me _
\ / \e •/
F—B + :N—Me ^ F—B:N—Me- / \ */ \
F Me "F Me
(XjftCIX) (XL) j
which acts as an acid by accepting the electron pair on nitrogen in
trimethylamine to form the complex (XL), and is therefore referred
to as a Lewis acid. Electrophiles and nucleophiles in organic reactions^
can be looked upon essentially as acceptors and donors, respectively,
of electron pairs from and to other atoms, most frequently carbon.
Electrophiles and nucleophiles also, of course, bear a relationship to*
oxidising and reducing agents for the former can be looked upon as
electron-acceptors and the latter as electron-donors. A number of the
more common electrophiles and nucleophiles are listed below.Electrophiles
H®, HsO®, HN0 3 , H 2 S0 4 ', HN0 2 (i.e. ®N0 2 , SOs and ®NO respec
tively), PhN 2 ®
BF 3 , AICI3, ZnCl 2 , FeCl 3 , Br 2 ,1—CI, NO—CI, CN—CI
OOO
v II II Ml
>C=0, R—C— CI, R—C—O—C—R, CO,
' * * * *