Question

Why in Nucleophilic addition-elimination reactions,does the `C=O` bond break when the nucleophile is added and not the`C-CL` bond ?

Answer 1

I take it you refer to nucleophilic attack on an `"acyl halide"`?

Explanation:

`"Reaction: " Nu^(-) + Cl(O=)CRrarr(Nu)(""^(-)O)(Cl)CRrarr Nu(O=)CR +Cl^-`

The carbonyl is presumed to stabilize the negative charge of the nucleophile, presumably by resonance. The ipso carbon is thus 4-coordinate, and we could trap such an intermediate by quenching with, say, `Me_3SiCl` to give `(Nu)(Me_3SiO)(Cl)CR`.

Does this address your query?