Other Nucleophilic Displacement ReactionsIt will be noticed from these examples that the attacking nucleophile
need not of necessity be an anion with a full-blown negative charge
(e.g. HO®, Br®, (Et0 2 C) 2 HC®) but it must at least have unshared
electron pairs available (e.g. R 3 N:, R 2 S:) with which to attack a
positive carbon or other atom. Equally the species that is attacked may
e
be a cation with a full-blown positive charge (e.g. R 3 N:R), but more
commonly it is a neutral molecule (e.g. RBr). It must, of course, also
be remembered that what is a nucleophilic attack from the point of
view of one participant will be an electrophilic attack from the point
of view of the other. Our attitude, and hence normal classification of
reactions, tends to be formed by somewhat arbitrary preconceptions
about what constitutes a reagent as opposed to a substrate (cf. p. 27).
Overall, the most common nucleophile of preparative significance is
probably HO® or, producing essentially the same result, H 2 0,
especially when the latter is the solvent and therefore present in
extremely high concentration.
Hardly surprisingly, not all displacement reactions proceed so as to
yield nothing but the desired product. Side reactions may take place
yielding both unexpected and unwanted products, particularly elimin
ation reactions to yield unsaturated compounds; the origin of these is
discussed subsequently (p. 189). •