The high reactivity of alkyl halides can be explained in tems of nature of C-X bond which is highly polarised covalent bond due to large difference in the electronegativities of carbon and halogen atom. This polarity is responsible for the nucleophilic substitution reaction of alkyl halides which mostly occur by `S_(N^(1))` and `S_(N^(2))` mechanisms. `S_(N^(1))` reaction is a two step process and in the first step R-X ionises to give carbocation (slow process). In the second step the nucleophilic attacks the carbocation from either side to form the prodcut (fast process) . In `S_(N^(1))` reaction there can be reacemization and inversion . `S_(N^(1))` reaction is favoured by heavy (bulky) groups on the carbon atom attached to halogens. i.e., `R_(3)C-Xgt R_(2)CH-Xgt R_CH_(2)X gt CH_(3)X. " In " S_(N^(2))` reaction the strong nucleophilie `OH^(-)` attacks from the opposite side of the chlorine atom to give an inyermediate (transition state). which breaks to yield the product (alcohol) and leaving `(X^(-))` group. The alcohol has a configuration opposite to that of the bromide and is said to proceed with inversion of configuration. `S_(N^(2))` reaction is favoured by small groups on the carbon atom attached to halogen i.e., `CH_(3)-X gt R-CH_(2)X gt R_(2) CHX gt R_(3) C-X` An `S_(N^(2))` reaction at an asymmetric carbon of a compound always gives:A. an enantiomer of the substrateB. a product with opposite optical rotationC. a mixture of diastereomersD. a single stereoisomer