question_answer

What is the decreasing order of reactivity of the following in \[{{S}_{{{N}^{2}}}}\]reaction? 1-Bromo-2-methylbutane, 1-Bromo-2-2-dimethylpropane, 1-Bromopentane.

Answer:

                All are primary alkyl halides and their structural formulae are: \[\underset{(I)}{\mathop{{{C}_{2}}{{H}_{5}}-\underset{\begin{smallmatrix}  | \\  H \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{2}}Br}}\,\]\[\underset{(II)}{\mathop{C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{2}}Br}}\,\]\[\underset{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(III)}{\mathop{{{C}_{3}}{{H}_{7}}-\underset{\begin{smallmatrix}  | \\  H \end{smallmatrix}}{\overset{\begin{smallmatrix}  H \\  | \end{smallmatrix}}{\mathop{C}}}\,-Br}}\,\] The order of reactivity is: (III) > (I) > (II) Explanation: The\[{{S}_{{{N}^{2}}}}\] reactions are sensitive to steric hindrance. Greater the steric hindrance to the attacking nucleophile, lesser will be the reactivity. In the light of this, the reactivity order is justified.