Question

Which one is most reactive towards $S_N 1$ reaction?

• A. ${ C6H5CH2Br}$
• B. ${C6H5CH(C6H5)Br }$
• C. ${C6H5CH(CH3)Br }$
• D. ${ C6H5C(CH3)(C6H5)Br}$

Answer 1

Answer: (D)

${ C6H5C(CH3)(C6H5)Br}$

Answer 2

${{S}_{N}}1$ reaction involves the formation of carbonation intermediate. More the stability of carbonation, more is the reactivity of alkyl/aryl halides towards ${{S}_{N}}1$ reaction. The intermediate carbonations formed by given halides are as:
${{C}_{6}}{{H}_{5}}CH({{C}_{6}}{{H}_{5}})Br\xrightarrow[{}]{{}}{{({{C}_{6}}{{H}_{5}})}_{2}}\overset{+}{\mathop{C}}\,H+B{{r}^{-}}$ ${{C}_{6}}{{H}_{5}}CH(C{{H}_{3}})Br\xrightarrow[{}]{{}}{{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,H(C{{H}_{3}})+B{{r}^{-}}$(c)${{C}_{6}}{{H}_{5}}C(C{{H}_{3}})({{C}_{6}}{{H}_{5}})Br\xrightarrow[{}]{{}}$ ${{({{C}_{6}}{{H}_{5}})}_{2}}\overset{+}{\mathop{C}}\,(C{{H}_{3}})+B{{r}^{-}}$ ${{C}_{6}}{{H}_{5}}C{{H}_{2}}Br\xrightarrow[{}]{{}}{{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,{{H}_{2}}+B{{r}^{-}}$
The order of stability of these carbocations is as${{({{C}_{6}}{{H}_{5}})}_{2}}\overset{+}{\mathop{C}}\,(C{{H}_{3}})>{{({{C}_{6}}{{H}_{5}})}_{2}}\overset{+}{\mathop{C}}\,H$ $>{{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,H(C{{H}_{3}})>{{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,{{H}_{2}}$
Thus, ${{C}_{6}}{{H}_{5}}C(C{{H}_{3}})({{C}_{6}}{{H}_{5}})Br$ is most reactive towards ${{S}_{N}}1$ reaction.