The dye test is used to distinguish between primary, secondary, and tertiary amines, as well as phenols like β-naphthol. β-naphthol is a phenol (naphthalene with an -OH group), and phenols undergo coupling reactions with diazonium salts to form azo dyes, which are highly colored compounds.

For the coupling reaction to occur efficiently, the phenol must be in its phenoxide ion form, which is more nucleophilic. This requires an alkaline medium to deprotonate the phenolic -OH group:

$\mathrm{ArOH}+\mathrm{OH}-\to \mathrm{ArO}-+H_{2}O$

In acidic conditions, the diazonium salt itself may decompose, and the phenol remains protonated and less reactive. A neutral medium does not provide sufficient phenoxide ions. Organic solvents like dichloromethane are not suitable because the reaction is typically aqueous-based and requires the ionic phenoxide species.

Therefore, an alkaline solution of β-naphthol is necessary for the dye test to ensure effective formation of the azo dye.

Related Topics

Diazotization and Coupling Reactions: These are key reactions in organic chemistry where diazonium salts (formed from primary amines) react with phenols or aromatic amines to form azo compounds. The pH is critical: coupling with phenols occurs in alkaline medium, while coupling with amines occurs in weakly acidic medium.

Phenol Acidity: Phenols are weakly acidic and can be deprotonated by strong bases to form phenoxide ions, which are strong nucleophiles for electrophilic aromatic substitution reactions like the coupling reaction.

Key Formulae

Formation of diazonium salt:

$\mathrm{Ar}{\mathrm{NH}}_2+\mathrm{NaNO}{2}_{\mathrm{}}+2\mathrm{HCl}\to \mathrm{Ar}{N}_2^{+}{\mathrm{Cl}}^{-}+\mathrm{NaCl}+2H_{2}O$

Coupling reaction (with phenoxide):

$\mathrm{Ar}{N}_2^{+}{\mathrm{Cl}}^{-}+\mathrm{Ar}\prime O-\to \mathrm{Ar}-N=N-\mathrm{Ar}\prime \mathrm{OH}$