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For the following acids compare the rate of decarboxylation on heating

I $\ce{Ph-CO-CH2COOH}$

II $\ce{Ph-CO-COOH}$

III $\ce{CH3-CH2-COOH}$

IV $\ce{HOOC-CH2-COOH}$

I tried this question by forming carbanions and comparing their stability. But I am stuck at comparing I and IV.

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  • $\begingroup$ Is the answer 4? $\endgroup$
    – Tony
    Apr 6, 2020 at 19:17
  • $\begingroup$ @Tony Yeah, (4) $\endgroup$ Apr 6, 2020 at 20:01
  • $\begingroup$ Yes the ans is 4 but please tell me how did you compare between I and IV ? $\endgroup$ Apr 6, 2020 at 20:04
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    $\begingroup$ In the mechanism for intramolecular decarboxylation,if you recall,the EWG bature of the carbonyl is responsible for withdrawing electrons from the -COOH part. It won't be able to do that very efficiently in IV due to cross conjugation presented by the OH ,which will decrease it's withdrawing nature $\endgroup$ Apr 6, 2020 at 23:20
  • $\begingroup$ For comparing the rate of decarboxylation just compare the stability of the carbanion formed. It's an easy jee concept. $\endgroup$
    – user91688
    Apr 7, 2020 at 4:11

1 Answer 1

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First thing to see is to see carbanion stability.

Now the question arises that between carbonyl and COOH groups which can stabilize better. For that you can look at the COOH 's already possible resonance,within its molecule, which decreases it electron withdrawal tendency. Hence order would be: I>IV>III>II.

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  • $\begingroup$ Compound 1 undergoes decarboxylation via vinologous intermediate. The intermediate is a vinyl alcohol. Not a carbanion. The comparision is akin to bananas and apples. $\endgroup$ May 9, 2020 at 17:04
  • $\begingroup$ So i still haven't got the concept! Call u provide a link from where I can read it? $\endgroup$ May 11, 2020 at 12:01

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