Effect of Entering and Leaving Groupshalide, for this reagent does not take part in the rate-determining step
of the reaction. With an Sjy2 displacement, however, the more strongly
nucleophilic the reagent the more the reaction will be promoted. The
' nucleophilicity' of the reagent can broadly be equated with the extent
to which it has available electrons, i.e. with its basic strength. This
parallel, though useful, is by no means exact, however, for in a
displacement reaction an ion such as Ye is usually showing nucleo
philicity for carbon while when acting as a base it is exhibiting nucleo
philicity for hydrogen and nucleophilic attack on carbon is usually
much more subject to stericynfluences than is coordination with a
proton. The parallel can however be used as a general guide with fair
success, particularly if the attacking atom of the nucleophiles con
sidered is the same in each case. Thus strong bases such as EtOe and
HO® are more strongly nucleophilic agents than weak bases such as
Me • C0 2 ®. From what has already been said about the effect of change
of reagent on the two types of mechanism, it follows that in the dis
placement of any particular atom or group, the more powerfully
nucleophilic the reagent employed the greater is the chance of the
reaction proceeding by the S^2 route. Thus as the series H 2 0,
Me-COa®, PhO®, HO®, EtO® is traversed, it may well be that a
displacement reaction of R - Hal which started bvjjeing SN\ with H 20
or Me • COa® has changed over to SJV2 by the time EtO® is reached.
So far as change of attacking atom in a nucleophile is concerffPd, it is
broadly true, within a single group or subgroup of the periodic table,
that the larger the atom the greater its nucleophilic reactivity; thus
decreasing reactivities Is > Br® > CI® > F® and RS® > RO® are
observed. This is probably due to the fact that as the atom increases
in size, the hold the nucleus has on the peripheral electrons decreases,
with the result that they become more readily polarisable leading to
bonding interaction at greater internuclear distances. Also the larger
the ion or group the less its solvation energy, which means the less
energy that has to be supplied to it in order to remove, in whole or in
part, its envelope of solvent molecules so as to get it into a condition
in which it will attack a carbon atom. It is a combination of these two
factors which makes the large I® a better nucleophile than the small
F®, despite the fact that the latter is a considerably stronger base than
the former.
So far as the leaving group, i.e. the one expelled or displaced, in an
SN 2 reaction is concerned, the more easily the C-leaving group bond
can be distorted the more readily the transition state will be formed,