Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 9
discovered that some solvents, which do not participate in the chemical reaction, are capable
of slowing down the process. 6 In 1890, Menschutkin, in a now classical study on the reaction
of the trialkylamines with haloalcans in 23 solvents, made it clear how the choice of
one or the other could substantially affect the reaction rate. 7 It was also Menschutkin who
discovered that, in reactions between liquids, one of the reactants could constitute a solvent
inadvisable for that reaction. Thus, in the reaction between aniline and acetic acid to produce
acetanilide, it is better to use an excess of acetic acid than an excess of aniline, since the
latter is a solvent which is not very favorable to this reaction.
The fruits of these experiments with series of solvents were the first rules regarding the
participation of the solvent, such as those discovered by Hughes and Ingold for the rate of
the nucleophilic reactions. 8 Utilizing a simple electrostatic model of the solute - solvent interactions,
Hughes and Ingold concluded that the state of transition is more polar than the
initial state, that an increase of the polarity of the solvent will stabilize the state of transition
with respect to the initial state, thus leading to an increase in the reaction rate. If, on the contrary,
the state of transition is less polar, then the increase of the polarity of the solvent will
lead to a decrease of the velocity of the process. The rules of Hughes-Ingold for the
nucleophilic aliphatic reactions are summarized in Table 2.1.1.
Table 2.1.1. Rules of Hughes-Ingold on the effect of the increase of the polarity of the
solvent on the rate of nucleophilic aliphatic reactions
Mechanism Initial state State of transition Effect on the reaction rate
S N2
S N1
Y - + RX [Y--R--X] -
slight decrease
Y + RX [Y--R--X] large increase
Y - +RX +
[Y--R--X] large decrease
[Y--R--X] +
slight decrease
Y+RX +
RX [R--X] large increase
[R--X] +
slight decrease
RX +
In 1896 the first results about the role of the solvent on chemical equilibria were obtained,
coinciding with the discovery of the keto-enolic tautomerism. 9 Claisen identified the
medium as one of the factors which, together with the temperature and the substituents,
proved to be decisive in this equilibrium. Soon systematic studies began to be done on the
effect of the solvent in the tautomeric equilibria. Wislicenus studied the keto-enolic equilibrium
of ethylformylphenylacetate in eight solvents, concluding that the final proportion between
the keto form and the enol form depended on the polarity of the solvent. 10 This effect
of the solvent also revealed itself in other types of equilibria: acid-base, conformational,
those of isomerization and of electronic transfer. The acid-base equilibrium is of particular
interest. The relative scales of basicity and acidity of different organic compounds and homologous
families were established on the basis of measurements carried out in solution,
fundamentally aqueous. These scales permitted establishing hypotheses regarding the effect
of the substituents on the acidic and basic centers, but without being capable of separating
this from the effect of the solvent. Thus, the scale obtained in solution for the acidity of