Step 1: Identify the reaction type: A benzenediazonium salt reacts with phenol in a weakly alkaline medium. This is a coupling reaction. It is also called azo coupling. The diazonium ion is a weak electrophile and phenol is an activated ring.
Step 2: See what the base does: The OH- ion turns phenol into the phenoxide ion. The phenoxide ion is much more reactive towards electrophiles than phenol itself. The diazonium ion attacks the para position of the phenoxide ring and the nitrogen-nitrogen link is formed.
Step 3: Write the balanced equation: \[ \mathrm{C_6H_5N_2^+Cl^- + C_6H_5OH + OH^- \rightarrow p\text{-}HO{-}C_6H_4{-}N{=}N{-}C_6H_5 + Cl^- + H_2O} \] The product has an azo group (-N=N-) joining two benzene rings. It has OH on one ring, para to the azo group.
Step 4: Name the product: The product is p-hydroxyazobenzene, also written 4-(phenylazo)phenol. It is an orange coloured azo dye.
Final Answer: Benzenediazonium chloride couples with phenol at the para position and gives p-hydroxyazobenzene (an orange azo dye). \[ \boxed{\mathrm{p\text{-}HO{-}C_6H_4{-}N{=}N{-}C_6H_5}} \]
