# Why does HF generate a carbocation with an alkene

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?

• Well, it sure can do that if it's concentrated enough. Somewhat depending on alkene and temperature, though, I guess. – Mithoron Jun 15 '20 at 19:34
• You felt wrong. HF is a typical acid, behaves like an acid, and (most likely) tastes like an acid. – Ivan Neretin Jun 15 '20 at 19:36
• @IvanNeretin But HF is a really weak acid, owing to its extremely strong bond with hydrogen – Vamsi Krishna Jun 15 '20 at 19:37
• @Mithoron But what could cause the HF bond cleavage? Surely the pi electrons wouldnt suffice? – Vamsi Krishna Jun 15 '20 at 19:38
• chemistry.stackexchange.com/questions/34818/… – Mithoron Jun 15 '20 at 19:48

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]

• 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. – Oscar Lanzi Jul 3 '20 at 20:01
• @OscarLanzi Thanks for adding value.I will update my answer to include this as well. – Chem-Learner Jul 3 '20 at 20:18
• Also more dangerous than HBr. Left that out! – Oscar Lanzi Jul 3 '20 at 20:19