The Cannizzaro reaction takes place between carbonyl compounds without a
hydrogen that can be abstracted by a strong base. Common examples are
benzaldehyde and formaldehyde.
The latter is used in the animation below.
The OH- ion acts as a nucleophile and attacks the carbonyl carbon,
to form a tetrahedral intermediate.
The most likely reaction of this intermediate is the reverse process: expulsion
The hydride (H-) is a less likely leaving group. Still,
a hydride shift is observed when there is a second formaldehyde in the
neighbourhood, to act as a hydride acceptor.
The animation shows the subsequent steps, starting with hydroxide attack.
From right to left we have two OH-'s, two formaldehydes and a water molecule.
The products of the hydride shift are formic
acid and the methanolate anion.
Under the reaction conditions these two will
rapidly loose and gain a proton, respectively, making the overall reaction
This way, the reaction will be driven to completion.