The reaction , 

proceeds by the........... mechanism.

${\mathrm{S}}_{{\mathrm{N}}^1}$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 - k₂ $\left[\mathrm{ROH}\right]\left[{\mathrm{SOCl}}_2\right]$, but clearly cannot proceed by the simple ${\mathrm{S}}_{{\mathrm{N}}^2}$ mode for this would lead to
inversion of configuration in the product, which is not observed.
Carrying out the reaction under milder conditions allows of the isolation of an alkyl chloiosulphite, ROSOCI (31), and this can be shown to be a true intermediate. The chlorosulphite is formed with retention of configuration, the R-O bond nor being broken during the reaction. The rate at which the akyl chlorosulphite intermediate (31) breaks
down to the product, RCI (30a), is found to increase with increasing polarity of the solvent, and also with increasing stability of the carbocation ${\mathrm{R}}^{\oplus }$ : an ion pair, ${\mathrm{R}}^{\oplus \oplus }$ OSOCI (32), is almost certainly involved. Provided collapse of the ion pair to products then occurs rapidly, .e., in the intimate ion pair (33) within a solvent cage, then attack by Cle is likely to occur on the same side of Ro from which${ }^{\ominus }\mathrm{OSOCl}$ departed, i.e., with retention of configuration:

Whether the breaking of the C-O and the SOCl bonds occurs simultaneously, or whether the former occurs first, is still a matter of debate. It is interesting that if the SOCI₂ reaction on ROH (29) is carried our in the presence of prydine, 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 COH and SOCI2 is converted by pyridine into ${\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{NH}}^{\oplus }{\mathrm{Cl}}^{\ominus } \mathrm{and} {\mathrm{Cl}}^{\ominus }$, being an effective nucleophile, attacks (31) from the back' in a normal ${\mathrm{S}}_{{\mathrm{N}}^2}$ reaction with inversion of configuration: