Acid chlorides and reduction by hydrogenation

Question

An acid chloride being reduced through catalytic hydrogenation is an "unobserved" event in the context of our class, according to my professor. Nonetheless, we are expected to predict that the hydrogenation occurs because acid chlorides are very reactive in general.

Now, this sounds like a case of the right answer with the wrong reason.

Yes, acid chlorides are reactive. We've covered their hydrolysis with water. We've covered their role in Friedel-Crafts acylation. We realize that the $\ce{C-Cl}$ bond in an acid chloride is very weak and that the carbonyl carbon is highly electrophilic. This gives rise to most of the reaction pathways with acid chlorides.

But what does catalytic hydrogenation have to do with the electrophilicity of the carbonyl carbon in an acid chloride? Nothing, in my mind. Hydrogen gas and platinum metal simply work here because the $\ce{C=O}$ part of the acid chloride is flat. I don't think it has anything to do with how reactive the acid chloride is. Right? Hell, even plain jane alkenes can be hydrogenated and they aren't exactly reactive.

Answer 1

Hydrogen gas and platinum metal simply work here because the C=O part of the acid chloride is flat.

It's more than just that. Acid chlorides are significantly more reactive towards catalytic hydrogenation than other carbonyl compounds. They are reactive enough that a poisoned catalyst must be used along with mild reaction conditions to achieve reduction to the corresponding aldehyde, and have the reaction stop there (Rosenmund reaction).

So the question becomes, "why are acid chlorides so reactive towards catalytic hydrogenation." It is thought that because the acid chloride $\ce{C-Cl}$ bond is so reactive that the palladium inserts into this bond to form intermediates such the one pictured below.

Other typical carbonyl compounds can't form such an intermediate; consequently acid chlorides are noticeably more reactive towards catalytic hydrogenation. Anhydrides, where the carbonyl is roughly as reactive as an acid chloride's carbonyl towards nucleophilic attack, have not been reported (to my knowledge) to produce aldehydes under Rosenmund conditions supporting this mechanism involving a palladium intermediate.

Historical Note: Prior to the application of metal hydrides to the reduction of carbonyl groups, the Rosenmund reduction was a major synthetic tool for chemists. It was one of the few methods available for the straightforward transformation of acids, esters, amides, etc, to aldehydes.