Coupling of benzenediazonium chloride with N,N-dimethylaniline is carried out at pH 4-5 and not at pH 10 because at pH 10

1. AN,N-dimethylaniline is converted into its anilinium ion
2. Bthe dimethylamino group is hydrolysed by alkali
3. Cnitrogen gas is evolved from the amine
4. Dthe diazonium ion is largely converted into a non-electrophilic diazohydroxide / diazotate species