Which of the following `beta`-keto carboxylic acid does not undergo decarboxylation on heating ?

Options

A.
B.
C.
D. None of these

Answer

A

Explanation

(a) `beta`-Keto acids undergo decarboxylation through the keto form via a cyclic T.S. which results in the formation of the enol form of the ketone product . The enol then charges to the more stable keto form . If decarboxylation of the given `beta`-Keto acid could follow this route , then :

If we draw the acid as shown in(I) , it can be seen that the methylene group forms a bridge , which is perpendicular to the cyclohexane ring .

The enol product would have a double bond in the 1,2-position . Thus C-1 is `sp^(2)` -hybridised and therefore the bond leading to the methylene bridge (1,3) would have to be coplanar with the groups joined to C-2 in the enol , i.e., the bridge would have to flattern. This would require the normal valency angles (`~ 109.5^(@)`) to change to `~120^(@)` . Even if a compromise situation occured , i.e., all bond angles involved `(angle s1,3 "and" 4)` underwent change , the resulting strain would be very large , too large to be expected for it to occur. Hence , it is reasonable to argue that since decarboxylation does occur , it does so through a route requiring a much higher activation energy than that required for the 'usual' route . Hence the difficulty of decarboxylation . (Bredt's rule )