WAEC Chemistry 2012 Paper

Concept:An alkene reacts with concentrated tetraoxosulphate (VI) acid by electrophilic addition, forming an alkyl hydrogen sulphate which is later hydrolysed to an alkanol.Explanation:When an alkene is passed into concentrated $\mathrm{H_2SO_4}$, the acid adds across the double bond.This addition produces an alkyl hydrogen tetraoxosulphate (VI), e.g. $\mathrm{C_2H_5HSO_4}$ from ethene.On boiling this product with water, hydrolysis occurs to give the corresponding alkanol and regenerate the acid.Representative equations:$\mathrm{C_2H_4 + H_2SO_4 \rightarrow C_2H_5HSO_4}$$\mathrm{C_2H_5HSO_4 + H_2O \rightarrow C_2H_5OH + H_2SO_4}$Bromine in tetrachloroethane produces a dibromoalkane, not an alkanol.Aqueous $\mathrm{KMnO_4}$ can give a diol, while sodium hydroxide solution does not form an alkanol from an alkene under these conditions.Answer:A. concentrated tetraoxosulphate (VI) acid