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