Q.Write a short note on coupling reactions.

Step 1. Define coupling reactions (section 13.7.2), contrasting with 13.7.1. Unlike the displacement reactions of section 13.7.1 (where the diazo group is LOST as N2), a coupling reaction RETAINS the diazo group intact: the whole arene diazonium salt reacts, as an electrophile, with a second, particularly reactive aromatic compound -- specifically a phenol or an aromatic amine -- in an electrophilic aromatic substitution.

Step 2. Describe the product class. The product of coupling is an azo compound: two separate aromatic rings joined together through a shared -N=N- (azo) linkage, with an extended, conjugated system of alternating double bonds spanning both rings. This extended conjugation is what makes azo compounds brightly coloured, and it is exactly why they are so widely used as synthetic dyes (a 'Do you know?' aside even notes that the familiar acid-base indicator methyl orange is itself an azo dye).

Step 3. Note the regiochemistry. Because this is an electrophilic aromatic substitution on the SECOND ring (the phenol or amine), the incoming diazonium electrophile substitutes predominantly at the position PARA to that ring's own activating -OH or -NH2 group. …

Explaining coupling reactions from section 13.7.2 -- diazo-group retention, electrophilic aromatic substitution mechanism, azo-compound p …

• Confusing coupling reactions (section 13.7.2, diazo group RETAINED) with the displacement reactions of section 13.7.1 (Sandmeyer, Gatterman etc., diazo group LOST as N2) -- these are the two genuinely distinct 'two types of reactions' the diazonium salt can undergo, and mixing them up is the most common error. …