Which of the follwing is the example of `S_(N)2` reaction .

Options

A. `CH_(3)Br+OH^(-)rarr CH_(3)OH+Br^(-)`
B. `CH_(3)underset(Br)underset(|)(C )HCH_(3)+OH^(-)rarr CH_(3)underset(OH)underset(|)(C )HCH_(3)+Br^(-)`
C. `CH_(3)CH_(2)OH overset(-H_(2)O)rarr CH_(2)=CH_(2)`
D. `CH_(3)-overset(CH_(3))overset("| ")underset(Br)underset("| ")("C ")H-CH_(3)+OH^(-)rarr CH_(3)-overset(CH_(3))overset("| ")underset(Br)underset("| ")("C ")H-CH_(3)+Br^(-)`

Answer

A

Explanation

For `S_(N^(2))` reaction to occur, the attack of reagent and removal of leaving groups takes place simultaneously. For this to happen the attack must take place from back-side nd hence no bulky group or sterically hindered site would prefer back-side attack, as approach of incoming group becomes difficult. Also, more numberof R groups on reactive centres stabilises the carbonium ion which may be formed when reaction proceeds through `S_(N^(1))` reaction. Thus, reaction (b) and (d) may proceed through either of the reaction. Reaction (c ) is an example of elimination reaction. Reaction (a) has `CH_(3)Br` subtract which neither back-side attack nor the `CH_(3)^(+)` carbocation is stabilised. Thus, reaction (a) would only follow `S_(N^(2))` mechanism.