The reaction,
proceeds by the........... mechanism.

Options

A. `S_(N^1)`
B. `S_(N^2)`
C. `S_(E^2)`
D. `S_(N^i)`

Answer

A

Explanation

`S_(N^1)` mechanism: retention of configuration: Retention of configuration, means the starting material and product have the same configuration.

The reaction has been shown to follow a second order rate equation rate `= k_2 [ROH][SOCl_2]` , but clearly cannot proceed by the simple `S_(N^2)` mode for this would lead to inversion ofconfiguration in the product, which is not observed.
Carrying out the reaction under milder conditions allows of the isolation of an alkyl chlorosulphite , ROSOCl (31) , and this can be shown to be a true intermediate. The chlorosulphite is formed with retention ofconfiguration, the R-O bond not being broken during the reaction. The rate at which the alkyl chlorosulphite intermediate (31) breaks down to the product, RCl (30a), is found to increase with increasing polarity of the solvent, and also with increasing stability ofthe carbocation `R^(oplus)` : an ion pair, `R^(oplus Theta)` OSOCI (32), is almostcertainlyinvolved. Provided collapse ofthe ion pair to products thenoccurs rapidly, i.e., in the intimate ion pair (33) within a solvent cage, then attackby `Cl^(Theta)` is likely to occur on the same side of `R^(oplus)` from which `""^(Theta) OSOCl` departed, i.e., with retention of configuration:

Whether the breaking ofthe c-oand the S-Clbonds occurs simultaneously, orwhether the former occurs first, is still a matter of debate.
It is interesting that if the `SOCl_2` reaction on ROH (29) is carried out in the presence of pyridine, the product RCI is found now to have undergone inversion of configuration (30b). This occurs because the HCl produced during the formation of (31) from ROH and `SOCl_2` is converted by pyridine into `C_5H_5NH^(Theta)` and `Cl^(Theta)` being an effective nucleophile, attacks (31) from the back. in a normal `S_(N^2)` reaction with inversion of configuration.