Question

 

In acid-catalyzed addition of water to alkene, alcohol is the product. The conjugate base of the acid may compete with water for the newly formed carbocation and give a different product (other than alcohol). Among the following acids, which is the best acid catalyst? Which is the worst? Depend your answers.

               HOAc     HCl      H₂SO₄       CF₃COOH

Answer 1

Alkenes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates. Let's consider the alkene is propene.
Propene reacts with sulfuric acid to give propyl hydrogensulphate.

CH3CH=CH2 + H2SO4 CH3CH(OSO2OH)CH3

The product is diluted with water, water reacts with the propyl hydrogensulphate to produce 2-propanol which distils off.

CH3CH(OSO2OH)CH3 + H2O CH3CH(OH)CH3

In case of HCl

Propene reacts with HCl to give an intermediate 2^o propyl carbocation and a Cl- ion. Now, Cl- ion will compete with H2O. Again Cl- ion is better neucleophile that H2O.

CH3CH=CH2 + HCl [CH3CHCH3]⁺ + Cl^-

[CH3CHCH3]⁺ + Cl^-     CH3CH(Cl)CH3

[CH3CHCH3]⁺ + H2O   CH3CH(OH)CH3

H2SO4 and HCl are strong acids, whereas HOAc and CF3COOH are weak acids and acid catalyzed reaction of alkene requires strong acid.

Hence, the best catalyst is H2SO4 and worst catalyst is HCl.