Page 201 - Mechanism and Theory in Organic Chemistry

down the column. For example, the nucleophilicity of the halogens increases in
the order F- < C1- < Br- < I- but the order of the basicity is exactly the
opposite, I- < Br- < C1- < F-.
- -
An explanation that has been frequently given for the observed order of
nucleophilicity in protic solvents (as in Tables 4.4 and 4.5) is that a good nucleo-
phile must be polarizable. But the role of the polarizability and even its direction
have received varying interpretations. The most familiar hypothesis is that, as
the reaction commences, the large electron cloud of the polarizable nucleophile
is distorted toward the substitution site, resulting in appreciable bonding be-
tween the entering reagent and the substrate with little attendant increase in
steric strain at the transition state. More recently, Swain and Scott have suggested
that polarization of the nonbonding electrons on the nucleophile away from the
substrate at the transition state reduces the electrostatic repulsion between the
two negatively charged species-the nucleophile and the leaving group-thus
reducing the energy of the transition state and increasing the rate of reaction.44
Edwards and Pearson have, however, pointed out that if electrostatic repulsion
were diminished in this way, so too would bonding between nucleophile and
substrate in the transition state be diminished and the balance might well not be
favorable. These authors suggest that the electrostatic repulsion considered by
Swain is negligible compared to the much greater repulsion due to the Pauli
exclusion principle between the electrons around the nucleophile and those
around the substrate needing to occupy the same space at the same time. They
conclude that it is the low-lying empty orbitals of polarizable nucleophiles that
make them more reactive. In the transition state the entering group can accom-
modate some of its lone pairs in those of its low-lying empty orbitals that are
directed more away from the substrate than the ground-state orbitals would be-
with a resultant decrease in energy.45
When S,2 reactions are carried out in aprotic solvents, the nucleophilicity
of reagents is dramatically different from that in protic solvents, and the n and
n,,,, values of Tables 4.4 and 4.5 do not apply. The requirement that a base must
be polarizable in order to be a good nucleophile becomes much less important,
and there is a better correlation between proton basicity and nucleophilicity. For
example, SeCN- reacts 4000 times as fast as C1- with methyl iodide in methanol
at O°C, but in dimethylformamide (DMF) also at O°C, C1- reacts twice as fast as
SeCN-.46 Even the order of halide reactivity can be reversed. Bromide reacts 18
times as fast as C1- with methyl iodide in methanol, but in DMF, C1- reacts
twice as fast as Br-.47
The apparent cause for this striking behavior is the difference in degree of
solvation of the small negative ions in the two kinds of solvents-protic and
In
aproti~.~~ protic solvents such as methanol or water, these ions are highly
solvated by hydrogen bonding (see Section 2.4). Thus their effective sizes are very
large and their negative charges dispersed.49 Solvation decreases in the same order

44 See note 41, p. 186.
45 J. 0. Edwards and R. G. Pearson, J. Amer. Chem. Soc., 84, 16 (1962).
4e B. 0. Coniglio, D. E. Giles, W. R. McDonald, and A. J. Parker, J. Chem. Soc. B, 152 (1966).
47 A. J. Parker, J. Chem. Soc. A, 220, (1966).
48 A. J. Parker, Quart. Rev. (London), 16, 163 (1962).
48 See also D. K. Bohme and L. B. Young, J. Amer. Chem. Soc., 92, 7354 (1970).