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Wednesday, November 13, 2019

Benzyne Intermediate Mechanism: SN-Ar Reaction:

The mechanisms that accounts for the experimental observations involves formation of a benzene intermediate which has two equivalent carbon atoms to which amino group can be attached. Benzyne has an extra (Pi) bond between two adjacent carbon atoms of benzene and can be formed as

Step-(1): Strong base NH2- removes a proton from the position ortho to halogen:

Step-(2): Anion formed in step (1) eliminates the halide ion, thereby forming Benzyne:

The incoming nucleophile can attack either of the carbons of the “triple bond” of benzyne. Protonation of the resulting anion form the substitution product. The overall reaction is an elimination-addition reaction; benzyne is formed in an elimination reaction and immediately undergoes an addition reaction.

Substitution at the carbon (C-14) that was attached to the leaving group is called direct substitution product (DSP). Substitution at the adjacent labeled carbon (C-14) of is called cine substitution product (CSP).

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