Page 99 - Mechanism and Theory in Organic Chemistry

Polarizability is a measure of the ease with which the electrons of a molecule
are distorted. It is the basis for evaluating the nonspecific attraction forces
(London dispersion forces) that arise when two molecules approach each other.
Each molecule distorts the electron cloud of the other and thereby induces an
instantaneous dipole. The induced dipoles then attract each other. Dispersion
forces are weak and are most important for the nonpolar solvents where other
solvation forces are absent. They do, nevertheless, become stronger the larger
the electron cloud, and they may also become important for some of the higher-
molecular-weight polar solvents. Large solute particles such as iodide ion interact
by this mechanism more strongly than do small ones such as fluoride ion.
Furthermore, solvent polarizability may influence rates of certain types of reac-
tions because transition states may be of different polarizability from reactants
and so be differently solvated.

Hydrogen Bonding
Hydrogen bonding probably has a greater influence on solvent-solute interac-
tions than any other single solvent property. Solvents that have 0-H or N-H
bonds are hydrogen bond donors, whereas most hydrogens bound to carbon are
too weakly acidic to form hydrogen bonds. Any site with unshared electrons is a
potential hydrogen bond acceptor, although the more strongly basic and the less
polarizable the acceptor site the stronger will be the hydrogen bond.28
We class as protic those solvents that are good hydrogen bond donors and as
aprotic those that are Since the protic solvents have hydrogen bound to
oxygen or nitrogen, they are also good hydrogen bond acceptors; the aprotic
solvents may or may not be hydrogen bond acceptors.
Because negative ions have extra electrons, they are hydrogen bond accep-
tors and can be expected to be strongly solvated by protic solvents. Many neutral
molecules also contain basic sites that will act as acceptors. Aprotic solvents, on
the other hand, will be less able to solvate negative ions and basic molecules.
Positive ions will ordinarily be solvated by dipolar interactions with the polar
solvents, whether protic or not. Protic solutes will ordinarily interact by hydrogen,
bonding with protic solvents.
Solvent Structure
Dipole-dipole interactions between solvent molecules, and, in the case of protic
solvents, intermolecular hydrogen bonding, lead to a certain amount of structure
in pure solvents. Water, which is both an excellent hydrogen bond donor and
acceptor, is perhaps the foremost example. It exhibits structure very like that in
the ice crystal over rather extended regions, although of course the structure is
a dynamic one in the sense that molecules are continually leaving and joining the
structured regions.
When a solute is introduced into a solvent, the structure of the solvent will
be disturbed in some way, and it will be the energy and entropy changes that
accompany this disturbanse, together with those arising from the new interactions

" For further discussion of these concepts see Section 3.5.
29 Note that aprotic solvents may nevertheless contain hydrogen.