Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 27
only one (D) joins by hydrogen bond with the O2. Two more molecules of water (C and E)
appear to stabilize electrostatically the hydrogen of the acid (H1). The second snapshot has
been chosen whilst the protonic transfer was happening. In this, the proton (H1) appears
jumping from the acid group to the amine group. However, the description of the first shell
of hydratation around the nitrogen atoms, oxygen (O2) and of the proton in transit remain essentially
unaltered with respect to the first snapshot. In the third snapshot the aminoacid has
now reached the zwiterionic form, although the solvent still has not relaxed to its surroundings.
Now the molecules of water A and B have moved closer to the atom of nitrogen, and a
third molecule of water appears imposed between them. At the same time, two molecules of
water (D and E) are detected united clearly by bridges of hydrogen with the atom of oxygen
(O2). All of these changes could be attributed to the charges which have been placed on the
atoms of nitrogen and oxygen. The molecule of water (C) has followed a proton in its transit
and has fitted in between this and the atom of oxygen (O2). In the fourth snapshot of the Figure
2.1.8 the relaxing of the solvent, after the protonic transfer, is now observed, permitting
the molecule of water (C) to appear better orientated between the proton transferred (H1) and the atom of oxygen (O2), and this now makes this molecule of solvent strongly attached
to the zwitterion inducing into it appreciable geometrical distortions.
From the theoretical analysis carried out it can be inferred that the neutral conformer
of the glycine II, has a brief life in aqueous solution, rapidly evolving to the zwitterionic
species. The process appears to happen through an intramolecular mechanism and comes
accompanied by a soft energetic barrier. For its part, the solvent plays a role which is crucial
both to the stabilization of the zwitterion as well as to the protonic transfer. This latter is favored
by the fluctuations which take place in the surroundings.
2.1.4 THERMODYNAMIC AND KINETIC CHARACTERISTICS OF CHEMICAL
REACTIONS IN SOLUTION
It is very difficult for a chemical equilibrium not to be altered when passing from gas phase
to solution. The free energy standard of reaction, ΔGº, is usually different in the gas phase
compared with the solution, because the solute-solvent interactions usually affect the reactants
and the products with different results. This provokes a displacement of the equilibrium
on passing from the gas phase to the midst of the solution.
In the same way, and as was foreseen in section 2.1.1, the process of dissolution may
alter both the rate and the order of the chemical reaction. For this reason it is possible to use
the solvent as a tool both to speed up and to slow down the development of a chemical process.
Unfortunately, little experimental information is available on how the chemical equilibria
and the kinetics of the reactions become altered on passing from the gas phase to the
solution, since as commented previously, the techniques which enable this kind of analysis
are relatively recent. It is true that there is abundant experimental information, nevertheless,
on how the chemical equilibrium and the velocity of the reaction are altered when one same
process occurs in the midst of different solvents.
2.1.4.1 Solvent effects on chemical equilibria
The presence of the solvent is known to have proven influences in such a variety of chemical
equilibria: acid-base, tautomerism, isomerization, association, dissociation,
conformational, rotational, condensation reactions, phase-transfer processes, etc., 1 that its
detailed analysis is outside the reach of a text such as this. We will limit ourselves to analyz-