Q 11-11-36 E - Organic Chemistry (Mcmurry)

Question

Methyl esters (${\mathrm{RCO}}_2{\mathrm{CH}}_3$) undergo a cleavage reaction to yield carboxylate ions plus iodomethane on heating with LiI in dimethylformamide:

The following evidence has been obtained: (1) The reaction occurs much faster in DMF than in ethanol. (2) The corresponding ethyl ester (${\mathrm{RCO}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3$) cleaves approximately 10 times more slowly than the methyl ester. Propose a mechanism for the reaction. What other kinds of experimental evidence could you gather to support your hypothesis?

Step-by-Step Solution

Answer

The mechanism for the reaction is,

The two given evidence tells that the reaction proceeds by S_N2 mechanism. In polar aprotic solvents like DMF, these reactions proceed much faster, and the reactivity of methyl esters is faster than ethyl esters.

Reaction of methyl ester with LiI/DMF

The methyl ester reaction with LiI/DMF is ten times faster than ethyl ester because a methyl iodide is formed, which is a gas, and other products are carbon dioxide and ketone. By Krapcho decarboxylation, carbon dioxide is removed by salt in DMF.

The experimental evidence observed in the reaction

The two given evidence tells that the reaction proceeds by  mechanism. In polar aprotic solvents like DMF, these reactions proceed much faster, and the reactivity of methyl esters is faster than ethyl esters. This reaction is an  displacement of methyl ester by iodide ion (I^-).

                                      Cleavage reaction of methyl ester

By varying the concentration of LiI, we can determine whether the reaction is second-order. Also, by varying the nucleophile type, we can distinguish an S_N2 mechanism from an S_N1 mechanism.