question_answer

Which compound in the following couples, will react faster in \[{{S}_{N}}2\] displacement and why? (a) 1-Bromopentane or 2-bromopentane (b) 1-Bromo-2-methylbutane or 2-bromo-2-methylbutane.

Answer:

                In \[{{S}_{N}}2\] reactions, reactivity depends upon the steric hindrance. (a) In 2-bromopentane, the \[\alpha \]-carbon atom attached to Br atom is further linked to two alkyl groups, i.e., methyl and n-propyl but in 1-bromopentane, it is attached to only one alkyl group, i.e., n-butyl group. Therefore, 2-bromopentane suffers greater steric hindrance to nucleophilic attack than 1-bromopentane and hence 1-bromopentane is more reactive than 2-bromopentane in \[{{S}_{N}}2\] reactions. \[\begin{align}   & \,\,\,\,C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}-\overset{\alpha }{\mathop{C}}\,\,\,{{H}_{2}}-Br \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{1-Bromopentane} \\  & \text{( }\!\!\alpha\!\!\text{ -carbon}\,\text{attached}\,\text{to}\,\text{one}\,\text{alkyl}\,\text{group,} \\  & \text{less}\,\text{steric}\,\text{hindrance}\,\text{to}\,\text{nucleophilic} \\  & \,\,\,\,\,\,\,\,\,\,\,\,\text{attack,}\,\text{more}\,\text{reactive)} \\ \end{align}\] \[\begin{align}   & \,\,\,\,C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}-\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\mathop{\overset{\alpha }{\mathop{C}}\,}}\,H-C{{H}_{3}} \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2\text{-Bromopentane} \\  & \text{( }\!\!\alpha\!\!\text{ -carbon}\,\text{attached}\,\text{to}\,\text{two}\,\text{alkyl}\,\text{groups,} \\  & \text{more}\,\text{steric}\,\text{hindrance}\,\text{to}\,\text{nucleophilic} \\  & \,\,\,\,\,\,\,\,\,\,\,\,\text{attack,}\,\text{less}\,\text{reactive)} \\ \end{align}\] (b) In 1-bromo-2-methylbutane, \[\alpha \]-carbon is attached to one bulky sec. -butyl group while in 2-bromo-2-methylbutane, \[\alpha \]-carbon is attached to three alkyl groups, i.e., two methyl and one ethyl. Therefore, 2-bromo-2-methylbutane suffers greater steric hindrance to nucleophilic attack than 1-bromo-2-methylbutane, and hence 1-bromo-2-methylbutane is more reactive than-2-bromo-2-methylbutane. \[\underset{\begin{smallmatrix}  \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{1-Bromo-2-methylbutane} \\  \text{( }\!\!\alpha\!\!\text{ -carbon}\,\text{attached}\,\text{to}\,\text{one}\,\text{alkyl}\,\text{group,}\,\text{less}\,\text{steric}  \\  \text{hindrance}\,\text{to}\,\text{nucleophilic}\,\text{attack,}\,\text{more}\,\text{reactive)} \end{smallmatrix}}{\mathop{C{{H}_{3}}C{{H}_{2}}-\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}\,H-\overset{\alpha }{\mathop{C}}\,{{H}_{2}}Br}}\,\] \[\underset{\begin{align}   &  \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2\text{-Bromo-2-methylbutane} \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{( }\!\!\alpha\!\!\text{ -carbon}\,\text{attached}\,\text{to}\,\text{three}\,\text{alkyl}\,\text{groups,}\,\text{more}\,\text{steric} \\  & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{hindrance}\,\text{to}\,\text{nucleophilic}\,\text{attack,}\,\text{less}\,\text{reactive)} \\ \end{align}}{\mathop{C{{H}_{3}}C{{H}_{2}}-\overset{\begin{smallmatrix}  C{{H}_{3}} \\  \alpha | \end{smallmatrix}}{\mathop{\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\mathop{C}}\,}}\,-\overset{{}}{\mathop{C}}\,{{H}_{3}}}}\,\]