Hydride is a poor leaving group so a hydride shift should not take place and instead the attacking hydroxyl should leave the carbonyl group. I don’t understand why this doesn’t happen?
$\begingroup$
$\endgroup$
3
-
1$\begingroup$ Well if the hydroxyl group just falls off then you have no reaction, right? $\endgroup$– orthocresolCommented Jan 14, 2017 at 11:20
-
$\begingroup$ Right but what about the poor leaving hydride group $\endgroup$– Sahil GuptaCommented Jan 14, 2017 at 12:08
-
3$\begingroup$ A discrete hydride ion may not form. An intermolecular hydride transfer takes place instead. Just compare it with normal hydride reagent reduction (like LiAlH4). $\endgroup$– EJCCommented Jan 14, 2017 at 13:17
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
2
You are correct that a hydride ion is a terrible leaving group and that it can under no circumstances be eliminated in e.g. an $\mathrm{S_N2}$ reaction. However, there are quite a few reactions where hydride does leave one atom and subsequently attaches to another; Wagner-Meerwein rearrangements jump to mind as one of the most prominent examples.
The feature of a Cannizzaro reaction is that it is not actually a leaving hydride. Rather, the reactivity can be explained in a cyclic transition state as shown in scheme 1 below.
Scheme 1: Transition state and products of a Cannizzaro reaction.
Instead of seeing the reaction as a displaced hydride that subsequently attacks another atom, we can see it as a type of pseudo-electrocyclic reaction. Breaking and forming the bonds will happen almost simultaneously. At no point in the mechanism do we actually have a ‘free’ hydride anywhere.
Note, by the way, that the Cannizzaro reaction is not the only one that features a hydride shift in a cyclic transition state; you may, for example, also be interested in the Evans-Tishchenko asymmetric reduction.
-
1$\begingroup$ I guess, going by their formal definitions, it would not be an electrocyclic reaction (goldbook.iupac.org/E01948.html), but rather a group transfer reaction. Not that it matters. $\endgroup$ Commented Jan 14, 2017 at 20:30
-