The reaction proceeds in two main stages:

1. Cyanohydrin Formation: The reaction of Butanone ($CH_3-CH_2-CO-CH_3$) with HCN is a nucleophilic addition reaction. The cyanide ion ($CN^-$) attacks the carbonyl carbon, forming a cyanohydrin intermediate (G): 2-hydroxy-2-methylbutanenitrile. $G: CH_3-CH_2-C(OH)(CH_3)-CN$
2. Hydrolysis and Dehydration: Heating with concentrated (95%) $H_2SO_4$ performs two functions:

• Hydrolysis: The nitrile group ($-CN$) is hydrolysed to a carboxylic acid group ($-COOH$).
• Dehydration: The acid acts as a dehydrating agent, removing a water molecule (the $-OH$ group and a hydrogen from an adjacent carbon). According to Zaitsev's rule, the major product is the more substituted alkene. Thus, hydrogen is removed from the $CH_2$ group (C3) rather than the $CH_3$ group, forming a double bond between C2 and C3.

The final product H is 2-Methylbut-2-enoic acid ($CH_3-CH=C(CH_3)-COOH$).