MCQ
Which one is most reactive towards ${S_{{N^1}}}$ reaction ?
- A$C_6H_5CH(C_6H_5)Br$
- B$C_6H_5CH(CH_3)Br$
- ✓$C_6H_5C(CH_3)(C_6H_5)Br$
- D$C_6H_5CH_2Br$
✓
Answer
Correct option: C.
$C_6H_5C(CH_3)(C_6H_5)Br$
c
Key ldea ${S_{{N^1}}}$ 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 halldes are as:
Key ldea ${S_{{N^1}}}$ 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 halldes are as:
$C_{6} H_{5} C H\left(C_{6} H_{5}\right) B r \rightarrow$$\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C} H+B r^{-} C_{6} H_{5} C H\left(C H_{3}\right) B r \rightarrow$$ C_{6} H_{5} \stackrel{+}{C} H\left(C H_{3}\right)+B r^{-}$
$C_{6} H_{5} C\left(C H_{3}\right)\left(C_{6} H_{5}\right) B r \rightarrow$$\left(C_{6} H_{5}\right)_{2}\stackrel{+}{C}\left(C H_{3}\right)+B r^{-} C_{6} H_{5} C H_{2} B r \rightarrow$$ C_{6} H_{5}\stackrel{+}{C} H_{2}+B r^{-}$
The order of stability of these carbocations is as
$\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C}\left(C H_{3}\right)>\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C} H$ $>C_{6} H_{5} \stackrel{+}{C} H\left(C H_{3}\right)>C_{6} H_{5} \stackrel{+}{C} H_{2}$
Thus, $C_{6} H_{5} C\left(C H_{3}\right)\left(C_{6} H_{5}\right) B r$ is most reactive towards ${S_{{N^1}}}$ reaction.
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