See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Azo coupling is an electrophilic aromatic substitution reaction where a diazonium ion (Ar-N≡N+) acts as an electrophile and couples with an activated aromatic compound (here N,N-dimethylaniline, an amine-activated ring). Step 1: Identify the coupling partner. N,N-dimethylaniline is an electron-rich aromatic amine. It can couple with diazonium salts via electrophilic aromatic substitution at the para position. Step 2: Consider pH dependence. At low pH (strongly acidic conditions), the amine group of N,N-dimethylaniline becomes protonated to form the anilinium ion (Ar-NH+(CH3)2). This protonated form is electron-withdrawing (deactivating), making the ring much less reactive toward electrophilic attack by the weakly electrophilic diazonium ion. The reaction is therefore inhibited at low pH. Step 3: At high pH (strongly basic conditions), the diazonium ion itself is converted to the diazohydroxide or diazotate (Ar-N=N-OH or Ar-N=N-O-), which are far less electrophilic than the diazonium cation and do not couple efficiently with amines. Step 4: At intermediate (mildly acidic to neutral, roughly pH 5–7) pH, two conditions are simultaneously satisfied: (a) the diazonium ion remains intact and sufficiently electrophilic, and (b) the amine coupling partner is predominantly in its free-base (unprotonated) form, keeping the ring activated toward electrophilic substitution. This dual condition at intermediate pH enables the C—N azo coupling reaction to proceed optimally. Why other options fail: - (a) Low pH: amine is protonated, ring deactivated, coupling does not occur. - (c) High pH: diazonium converts to non-electrophilic diazotate, coupling does not occur. - (d) Any pH: the reaction is pH-dependent and does not work at all pH values. Therefore, the correct answer is B.