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Suppose I have 2 compounds, 3-chloropropene and 1-bromo-1-methylcyclohexane. Without knowing the values, how can we compare the rate of solvolysis?

My attempt: Resonance is quite dominant character thus alkyl chloride resonance I think should dominate over hyperconjugation but then we have an exception that 3 degree carbocation is more stable than benzyl carbocation hence there might be some disparity in orders.

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  • $\begingroup$ Resonance effect is more stronger than hyperconjugation irrespective of the number of $\alpha$H. In first case allylic carbocation is formed which is stabilised by resonance. So solvolysis rate is more for the first compound. $\endgroup$
    – Manu
    Jul 1, 2020 at 15:50
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    $\begingroup$ 3 degree carbocation is more stable than benzyl carbocation - only for trimethyl carbocation; other 3 degree carbocations are less stable than benzyl carbocation. See chemistry.stackexchange.com/questions/74943/… $\endgroup$ Jul 1, 2020 at 17:25
  • $\begingroup$ Now problem is that in answer of this question they have given the reverse order but I think concepts matter more so what is reality that should be right answer. Thanks $\endgroup$
    – Charlie
    Jul 2, 2020 at 8:16

2 Answers 2

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Rate of solvolysis depends on two factors:

  1. $\ce{C-X}$ bond energy:
    $\ce{C-Br}$ bond is weaker than $\ce{C-Cl}$ bond because, $\ce{C-Br}$ bond is longer than $\ce{C-Cl}$ bond.
  2. Stability of the carbocation:
    The stability of a carbocation can be measured (or compared with others) with the help of their hydride ion affinity(HIA) values. More is the HIA value lesser is that stability of the carbocation. Here are some HIA values for some carbocations: $$\begin{array}{|c|c|} \hline \textrm{Carbocations} & \textrm{HIA values} \\ & (\textrm{in Kcal/mol}) \\ \hline \ce{CH2=CH-\overset{+}{C}H2}& 256 \\ \ce{(CH3)2\overset{+}{C}H} & 246 \\ \hline \end{array}$$

The previous answer states that,

Resonance effect is more stronger than hyperconjugation irrespective of the number of α-hydrogens.

I feel this is wrong. Because hyperconjugation is a type of resonance. Effect of hyperconjugation (with one alpha hydrogen) might not be effective as resonance effect but, it is comparable with resonance effect with increase in the number of alpha hydrogens. This can be understood from the HIA values.

Now let's compare the stability of 1-methylcyclohexyl cation and isopropyl cation. Stabilization of cation by inductive effect is more in case of 1-methylcyclohexyl cation because, in 1-methylcyclohexyl cation it is surrounded by more number of carbons than that of in isopropyl cation. As there are more number of alpha hydrogens in 1-methylcyclohexyl cation than in isopropyl cation, the effect of hyperconjugation will be more in case of 1-methylcyclohexyl cation. By combining these two effects we can say that 1-methylcyclohexyl cation is more stable than isopropyl cation. So the HIA value for 1-methylcyclohexyl cation will be less than 246.

So by having a rough idea on the HIA values we can say that 1-methylcyclohexyl cation is more stable than allyl cation. So by combining these two factors we can say that the rate of solvolysis is more for 1-bromo-1-methylcyclohexane than 3-chloropropene.

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Resonance effect is more stronger than hyperconjugation irrespective of the number of αH. In first case allylic carbocation is formed which is stabilised by resonance. So solvolysis rate is more for the first compound

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