Question

What is the product of the thermal decarboxylation of dimethylpropanedioic acid (dimethylmalonic acid)?

Answer 1

First off, since it is a propanedioic acid, we should be able to tell that having two methyl groups on the same carbon indicates that the only position they can be on is in between two carbonyls.

So, the structure is:

Now it should be kind of obvious what the decarboxylation leads to. Just by reading the word, decarboxylation is literally the removal of a carboxylic acid group to expunge `"CO"_2` from the reaction.

So, we can expect the bond between the `alpha`-carbon and a carbonyl carbon `beta` from an other carbonyl carbon to break.

Thermal decarboxylation in acid follows this model mechanism, using a `\mathbf(beta)`-keto acid:

Just replace the left ketone portion with a carboxyl group and insert two methyl groups onto the `alpha`-carbon. Hence, the final product is dimethylacetic acid.

CHALLENGE: Try doing this with an `alpha`-keto acid. You should find that it is not favorable and shouldn't happen outside of bioorganic reactions (via decarboxylase enzymes).