Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 33
of secondary and tertiary
α-acetylenic alcohols to carbonylic
αβ , -unsaturated compounds
(Figure 2.1.16).
Its mechanism consists of
three steps (Figure 2.1.17). The
first one is the protonation of the
oxygen atom. The second, which
determines the reaction rate, is
that in which the 1.3 shift from the
protonated hydroxyl group is produced
through the triple bond to
give way to the structure of alenol.
The last stage corresponds to the
deprotonation of the alenol, producing
a keto-enolic tautomerism
which displaces towards the
ketonic form.
For the step which limits the
reaction rate (rate limiting step),
three mechanisms have been proposed,
two of which are intramolecular - denominated intramolecular, as such, and
solvolytic - and the other intermolecular (Figure 2.1.18). The first of these implies a covalent
bond between water and the atoms of carbon during the whole of the transposition. In
the solvolytic mechanism there is an initial rupture from the O-C1 bond, followed by a
nucleophilic attack of the H2O on the C3. Whilst the intermolecular mechanism corresponds
to a nucleophilic attack of H2O on the terminal carbon C3 and the loss of the hydroxyl group
protonated of the C1. The analysis of the first two mechanisms showed 72 Figure 2.1.17. Steps of the reaction of Meyer-Schuster.
the solvolytic mechanism as the
most favorable localizing itself during the reaction path to an alquinylic carbocation interacting
electrostatically with a molecule of water. This fact has been supported by the experimental
detection of alquinylic carbocations in solvolytic conditions. Things being like that,
two alternatives remain for the slow stage of the Meyer-Schuster reaction, the solvolytic
and the intermolecular mechanism, and it seems that the solvent has a lot to say in this.
Although both mechanisms evolve in two steps, these are notably different. In the
intermolecular mechanism, the first transition state can be described as an almost pure electrostatic
interaction of the entrant molecule of water with the C3, whilst the C1 remains
united covalently to the protonated hydroxyl group. This first transition state leads to a intermediate
in which the two molecules of water are covalently bonded to the C1 and C3 atoms.
The step from the intermediate to the product takes place through a second transition
state, in which the C3 is covalently bonded to the molecule of entrant water and there is an
electrostatic interaction of the other water molecule. In the mechanism which we call
solvolytic, the first transition state corresponds to the pass from covalent to electrostatic interaction
of the H2O united to the C1, that is to say, to a process of solvolysis, so, the water
molecule remains interacting electrostatically with the carbons C1 and C3.