The addition of HF to 2-butene produces what?

The addition of HF to 2-butene (CH₃-CH=CH-CH₃) gives 2-fluorobutane (CH₃-CH₂-CHF-CH₃) as the only product.

This reaction is very slow. Remember that HF is a weak acid and, furthermore, is strongly hydrogen bonded to itself. The availability of the initiating H+ ions is very low.