(a) "Haloalkanes react with KCN to form alkyl cyanides as the main product, whereas with AgCN, they form isocyanides as the main product." Explain the statement.
Cyanide attacks through carbon when it is a free ion (KCN) and through nitrogen when carbon is bonded to silver (AgCN). Chlorobenzene on sulfonation gives mainly the para sulfonic acid.
Step 1: Understanding the Concept:
The cyanide ion \( \mathrm{CN^-} \) is an ambident nucleophile. It has two nucleophilic sites, the carbon atom and the nitrogen atom, and either one can attack the haloalkane. Part (b) is an electrophilic substitution (sulfonation) of chlorobenzene.
Step 2: Part (a) - Why KCN gives alkyl cyanide:
KCN is mostly ionic. It gives free \( \mathrm{CN^-} \) ions in solution.
The carbon end of \( \mathrm{CN^-} \) is the better nucleophilic centre, because a new C-C bond is stronger than a C-N bond.
So carbon attacks the alkyl halide in an \( S_N2 \) step and the main product is the alkyl cyanide.
\[ \mathrm{R-X + KCN \rightarrow R-CN + KX} \]
Step 3: Part (a) - Why AgCN gives isocyanide:
AgCN is largely covalent. The silver atom is bonded to the carbon atom of the cyanide group.
So the carbon atom is not free to attack. Only the lone pair on the nitrogen atom is available.
Nitrogen attacks the alkyl carbon, and the main product is the alkyl isocyanide.
\[ \mathrm{R-X + AgCN \rightarrow R-NC + AgX} \]
Step 4: Part (b) - Complete the equation:
Chlorobenzene is heated with conc. \( \mathrm{H_2SO_4} \), so sulfonation takes place. The \( \mathrm{-SO_3H} \) group replaces one ring hydrogen.
The chlorine atom is deactivating but it directs the new group to the ortho and para positions. The para product is the major one because of less steric hindrance.
The product is 4-chlorobenzenesulfonic acid (p-chlorobenzenesulfonic acid). It is a benzene ring with Cl and \( \mathrm{SO_3H} \) at opposite (1,4) positions.
\[ \mathrm{C_6H_5Cl + H_2SO_4 \xrightarrow{\Delta} 4\text{-}Cl{-}C_6H_4{-}SO_3H + H_2O} \]
Final Answer:
(a) KCN is ionic, so carbon of free \( \mathrm{CN^-} \) attacks and gives R-CN. AgCN is covalent, so only nitrogen has a free lone pair and gives R-NC. (b) The product is 4-chlorobenzenesulfonic acid.
\[ \boxed{p\text{-}\mathrm{ClC_6H_4SO_3H}} \]
