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One reason often cited for why alkenyl and phenyl halides do not undergo nucleophilic substitution by the $\ce {S_N1}$ mechanism is that the formation of the alkenyl and phenyl cations involve a prohibitively large activation energy since they are rather unstable. In fact, this reason has been discussed in various posts on this site; see for example, here and here. Recently, I chanced upon some calculated energies for these cations in the gas phase and it seems to suggest that they are not as unstable as various textbooks and educators claim. The following table is taken from p. 300 of Carey and Sundberg (2007):

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From the data above, we clearly see that the alkenyl carbocation is more stable than the primary carbocation although still significantly less stable than the secondary carbocation. Perhaps, we should be cautious when we discuss the stabilities of these carbocations because some sources seem to suggest that these alkenyl carbocations are so unstable that they are even less stable than primary carbocations. One thing that is consistent with most sources that the phenyl carbocation is the least stable of the lot.

There seems to be a cut-off point of stability, after which the $\ce {S_N1}$ reaction proceeds with much difficulty. This "cut-off point" seems to be between the stability of the alkenyl carbocation and the secondary carbocation. Is my interpretation correct?

Reference

  1. Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry Part A. Structure and Mechanisms (5th ed.). Springer, 2007.
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