question_answer

Which compound in the following pairs would react faster in \[{{S}_{N}}2\] displacement reactions? 1-Bromopentane or 2-Bromopentane. 1-Bromo-2-methylbutane or 2-Bromo-2-methylbutane.

Answer:

(i) \[\underset{\text{(Primary)}}{\mathop{\overset{5}{\mathop{C}}\,{{H}_{3}}\overset{4}{\mathop{C}}\,{{H}_{2}}\overset{3}{\mathop{C}}\,{{H}_{2}}\overset{2}{\mathop{C}}\,{{H}_{2}}\overset{1}{\mathop{C}}\,{{H}_{2}}Br}}\,\]                \[\underset{\text{(Secondary)}}{\mathop{C{{H}_{3}}\underset{\begin{smallmatrix}                 | \\  Br \end{smallmatrix}}{\mathop{C}}\,HC{{H}_{2}}C{{H}_{2}}C{{H}_{3}}}}\,\] 1-Bromopentane would react faster since it is a primary alkyl halide and moreover there is less steric hindrance experienced by the attacking nucleophile. (ii)  \[\underset{\text{(Primary)}}{\mathop{\overset{4}{\mathop{C}}\,{{H}_{3}}\overset{3}{\mathop{C}}\,{{H}_{2}}\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{\overset{2}{\mathop{C}}\,}}\,H\overset{1}{\mathop{C}}\,{{H}_{2}}Br}}\,\]                 \[\underset{(Tertiary)}{\mathop{C{{H}_{3}}C{{H}_{2}}\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{3}}}}\,\] 1-Bromo-2-methylbutane would react faster since it is a primary alkyl halide while the other is of tertiary nature.