Question
How do polar solvents help in the first step in $S_N1$ mechanism?
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Answer
$S_N1$ occurs in two steps.
In step I, the polarised $C—Br$ bond undergoes slow cleavage to produce a carbocation and a halide ion.
Step II involves the attack of nucleophile upon the carbocation thus formed to complete the substitution reaction.
Since 'step $I'$ is slowest and reversible. It causes breaking of $C—X$ bond, for which the energy is obtained through solvation of halide ion with the proton of protic solvent.
In step I, the polarised $C—Br$ bond undergoes slow cleavage to produce a carbocation and a halide ion.
Step II involves the attack of nucleophile upon the carbocation thus formed to complete the substitution reaction.
Since 'step $I'$ is slowest and reversible. It causes breaking of $C—X$ bond, for which the energy is obtained through solvation of halide ion with the proton of protic solvent.
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