# Internal Cannizzaro reaction of phenylglyoxal

Phenylglyoxal $(\ce{PhCOCHO})$ can undergo both internal Cannizzaro reaction and between two molecules.

The internal Cannizzaro reaction is like this:

Which reaction will be preferred? Internal Cannizzaro or Cannizzaro reaction between two molecules?

• Just a guess. The internal mechanism should be faster because there is a hydride transfer and this transfer should be faster if it happens in the same molecule. – Papul May 29 '16 at 14:53

I agree with the other answers that in general intramolecular reactions are faster than intermolecular reactions. Let's look at why this intramolecular reaction might be fast.

We can describe the Cannizzaro reaction as the delivery of a hydride from an anionic hydrate to a carbonyl group. The reaction shown certainly meets those criteria. I see this reaction as a carbocation rearrangement or a (semi-)pinacol rearrangement. In the latter class of reactions, a group undergoes a 1,2-migration to quench a carbocation and generate a carbonyl.

In the phenylglyoxyal rearrangement, if the ketone is considered as its ionic resonance structure (2), then the ketone carbon is a carbocation. We know that hydrogens can and will undergo 1,2-migration if the resulting carbocation is more stable. In this case (3->4), the resulting carbocation is very stable, because it's really a carboxylic acid (5). Furthermore, the carboxylic acid will be deprotonated rapidly under basic conditions (5->6).

As a rule, intermolecular reactions are slower than intramolecular reactions because you need two or more different molecules to collide with the right energy and the correct orientation. In an intramolecular reaction, you only need the right energy as (1) there is no need for collision and (2) the right orientation is often easier to achieve.

The reaction you describe is also documented in the paper A Highly Efficient and Enantioselective Intramolecular Cannizzaro Reaction under TOX/Cu(II) Catalysis but you'll note that the aldehyde exists as an acetal.

• If you could provide a short summary of the linked information then this would be a very satisfactory answer. Links can become obsolete which leads to answers becoming useless. – bon Jul 5 '16 at 16:47

Intramolecular reactions are generally always faster than intermolecular ones. This can be seen very simply from kinetics.

An intramolecular reaction has only one reactant and hence is a unimolecular reaction. In this case there would be two participating molecules in the intermolecular reaction. Hence this becomes a bimolecular reaction (the intermolecular one).

Since the feasibility of a reaction depends on molecules colliding and reacting, the probability of molecules colliding in the correct orientation and with appropriate energy reduces when there are more reacting species.

Hence intramolecular reactions are faster.