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I was reading of different ways to perform Friedel Craft alkylations.

One of the methods I came across involved an alkene in the presence of H+ or a mineral acid like $\ce{HF}$. Here $\ce{H+}$ or $\ce{HF}$ caused an electromeric shift of the pi electrons and the $\ce{H+}$ bonded there resulting in a carbocation.

However I felt like the bond strength of $\ce{HF}$ is so high that it couldn't be possible to cleave it for the donation of $\ce{H+}$ (If that's what happens. Correct me if I'm wrong).

Why does it still generate a carbocation?

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    $\begingroup$ Well, it sure can do that if it's concentrated enough. Somewhat depending on alkene and temperature, though, I guess. $\endgroup$
    – Mithoron
    Jun 15, 2020 at 19:34
  • $\begingroup$ You felt wrong. HF is a typical acid, behaves like an acid, and (most likely) tastes like an acid. $\endgroup$ Jun 15, 2020 at 19:36
  • $\begingroup$ @IvanNeretin But HF is a really weak acid, owing to its extremely strong bond with hydrogen $\endgroup$ Jun 15, 2020 at 19:37
  • $\begingroup$ @Mithoron But what could cause the HF bond cleavage? Surely the pi electrons wouldnt suffice? $\endgroup$ Jun 15, 2020 at 19:38
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    $\begingroup$ chemistry.stackexchange.com/questions/34818/… $\endgroup$
    – Mithoron
    Jun 15, 2020 at 19:48

1 Answer 1

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However I felt like the bond strength of $\ce{HF}$ is so high that it couldn't be possible to cleave it for the donation of $\ce{H+}$

HF has the highest single bond energy and hence you were right in expecting that it may not dissociate.However in concentrated solutions, due to the formation of stable hydrogen bonded complex $\ce{[HF_2]-}$, the reaction is pushed in the forward direction making it a resonably strong acid (in fact strongest among corresponding concentrated solutions of all the hydrogen halides).So, you could generate a carbocation with that.However, as generally dilute solutions are used which are economically feasible, more stronger mineral acids in dilute solutions like $\ce{HCl, HBr}$ and $\ce{HI}$ are used.

EDIT:

As suggested by Oscar Lanzi in the comments, acids like $\ce{HCl, HBr}$ and $\ce{HI}$ are safer to handle when compared to $\ce{HF}$ as the reactions of $\ce{HF}$ are associated with highly negative enthalpy changes [highly exothermic]

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    $\begingroup$ Not just an economic advantage. HF is more dangerous to work with than HCl or HI, especially if you need a strong acid and thus would have to concentrate the HF. $\endgroup$ Jul 3, 2020 at 20:01
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    $\begingroup$ @OscarLanzi Thanks for adding value.I will update my answer to include this as well. $\endgroup$ Jul 3, 2020 at 20:18
  • $\begingroup$ Also more dangerous than HBr. Left that out! $\endgroup$ Jul 3, 2020 at 20:19

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