Explain the concept of retention of configuration in a chemical reaction with an example.

(N/A) Retention of configuration is the preservation of the spatial arrangement of bonds around a chiral center during a chemical reaction or transformation.
If the bonds to the chiral center are not broken during the reaction,the product will have the same spatial arrangement of groups around the chiral center as the reactant,and the reaction is said to proceed with retention of configuration.
$(i)$ In such reactions,both the reactant and the product contain a chiral carbon,and the configuration remains unchanged.
(ii) Since none of the four bonds to the chiral carbon are broken,the spatial arrangement around the chiral center in the product is identical to that in the reactant.
(iii) Example: The reaction of $(-)-2-\text{methylbutan}-1-\text{ol}$ with $HCl$ to form $(+)-1-\text{chloro}-2-\text{methylbutane}$.
Note: In this example,the configuration at the chiral center is preserved. The four groups attached to the chiral carbon are $-H$,$-CH_3$,$-CH_2CH_3$,and $-CH_2OH$ (or $-CH_2Cl$). Although the configuration is retained,the sign of optical rotation changes from $(-)$ to $(+)$ because the priority of the groups changes due to the conversion of $-CH_2OH$ to $-CH_2Cl$.