What would be the rate of decarboxylation between the molecules described as- Pyridine molecules having -COOH group at ortho,meta and para position. In which molecule decarboxylation would be fast or slow? Put the -COOH group each at Ortho, meta and para and make 3 different molecule, then determine the rates.

  • $\begingroup$ What are your thoughts on the subject? $\endgroup$ – user55119 Dec 2 '17 at 13:43
  • $\begingroup$ I know that a carbanion is formed in decarboxylation mechanism. So the rate depends upon the stability of the carbanion formed. But I can't decide how to judge the stability of carbanions so formed. $\endgroup$ – Anandit Tripathi Dec 2 '17 at 14:50
  • $\begingroup$ You are dealing with compounds that are both amines and acids. Think zwitterions NOT carbanions. Provide a diagram of your efforts if you have the capability/ $\endgroup$ – user55119 Dec 2 '17 at 15:00

The rate of decarboxylation of picolinic acid 1 is greater than isonicotinic acid 2. Zwitterion 4 has been proposed as the intermediate. Note that the electron pair is in the sigma framework. An electrostatic stabilization is proposed that requires that the charges be proximate. A competitive reaction in base (no zwitterion) demonstrated that 1 decarboxylated >100 times faster than 2 which was recovered after 20 days. Since nicotinic acid 3 is inexpensive and wasn't studied by the researchers, I suspect that it is stable also.

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Dunn, G.E., Lee, G.K.J., Thimm, H., J. Can. Chem., 1972, 50, 3017.


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