Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 29
much less than that of the solid state, is not depreciable. Thus, in a solution, each molecule
of solute finds itself surrounded by a certain number of molecules of solvent which envelope
it forming what has been denominated as the solvent cage. Before being able to escape
from the solvent cage each molecule of solute collides many times with the molecules of
solvent which surround it.
In the case of a dilute solution of two reactants, A and B, their molecules remain for a
certain time in a solvent cage. If the time needed to escape the solvent cage by the molecules
A and B is larger than the time needed to suffer a bimolecular reaction, we can say that this
will not find itself limited by the requirement to overcome an energetic barrier, but that the
reaction is controlled by the diffusion of the reactants. The corresponding reaction rate will,
therefore, have a maximum value, known as diffusion-controlled rate. It can be demonstrated
that the diffusion-limited bimolecular rate constants are of the order of 10 10 -10 11
M -1 s -1 , when A and B are ions with opposite charges. 63 For this reason, if a rate constant is of
this order of magnitude, we must wait for the reaction to be controlled by the diffusion of the
reactants. But, if the rate constant of a reaction is clearly less than the diffusion-limited
value, the corresponding reaction rate is said to be chemically controlled.
Focusing on the chemical aspects of the reactivity, the rupture of bonds which goes
along with a chemical reaction usually occurs in a homolytic manner in the gas phase. For
this reason, the reactions which tend to prevail in this phase are those which do not involve a
separation of electric charge, such as those which take place with the production of radicals.
In solution, the rupture of bonds tends to take place in a heterolytic manner, and the solvent
is one of the factors which determines the velocity with which the process takes place. This
explains that the reactions which involve a separation or a dispersion of the electric charge
can take place in the condensed phase. The effects of the solvent on the reactions which involve
a separation of charge will be very drawn to the polar nature of the state of transition
of the reaction, whether this be a state of dipolar transition, isopolar or of the free-radical
type. The influence of the solvent, based on the electric nature of the substances which are
reacting, will also be essential, and reactions may occur between neutral nonpolar molecules,
between neutral dipolar molecules, between ions and neutral nonpolar molecules, between
ions and neutral polar molecules, ions with ions, etc. Moreover, we should bear in
mind that alongside the non specific solute-solvent interactions (electrostatic, polarization,
dispersion and repulsion), specific interactions may be present, such as the hydrogen bonds.
Table 2.1.3. Relative rate constants of the Menschutkin reaction between
triethylamine and iodoethane in twelve solvents at 50 o C. In 1,1,1-trichloroethane the
rate constant is 1.80×10 -5 l mol -1 s -1 . Data taken from reference 40
Solvent
Relative rate
constant
Solvent
Relative rate
constant
1,1,1-Trichloroethane 1 Acetone 17.61
Chlorocyclohexane 1.72 Cyclohexanone 18.72
Chlorobenzene 5.17 Propionitrile 33.11
Chloroform 8.56 Benzonitrile 42.50
1,2-Dichlorobenzene 10.06