Question

(a) Why do acyl chlorides undergo nucleophilic attack more readily than alkyl chlorides ?
(b) What is hydroxamic acid test and which functional group is determined by this test ?

Answer 1

(a) `RCOCl` is more reactive than `RCl` because :
(i) The electromeric effect of `(C=O)` in `RCOCl` . `C` is more positive and more electrophilic than `C` in `(R-Cl)`.
(ii) The `T.S` (transition state) in acid derivative is tetrahedral intermediate and is less sterically hindered than the `T.S` with pentavalent `C` in the `SN^2` reaction of `RCl`.
(iii) In `RCOCl`, a weaker `pi-bond` is broken while in `RCl`, a `sigma-bond` is partically broken to form alkyl `T.S`.
(b) The test is used to detect ester an anhydride groups. Esters and anhydrides react with hydroxylamine `(NH_2 OH)` to form hydroxamic acid `(RCONHOH)`, which gives red-to-violet coloured complex with `FeCl_3`.