During substitution, when rate of reaction depends only on one concentration term (i.e. commonly a substrate) e.g. alkyl halides. The reaction is of first order and called SN1 reactions. Here, rate ∝ substrate or, rate =k[RX] i.e. alkyl halide Step I : RX
Slow
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R++X− Step II : R+
OH −
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Fast
ROH Such type of reaction depends upon the stability of carb-cation formed in first step. It is favoured by heavy (bulky) groups.
To show faster SN1 reaction and option (d) is the correct answer.