Identification Condition of Beta Naphthol Using Dye Test

To identify $\beta$-naphthol using the azo dye test, it is essential to prepare an alkaline solution of $\beta$-naphthol.

Detailed Chemical Explanation:

1. Role of Alkaline Medium:

   • Phenols, such as $\beta$-naphthol ($\text{C}_{10}\text{H}_7\text{OH}$), are weakly acidic and dissolve in basic solutions (such as dilute $\text{NaOH}$) to form the naphthoxide ion ($\text{C}_{10}\text{H}_7\text{O}^-$): $\text{C}_{10}\text{H}_7\text{OH} + \text{OH}^- \rightleftharpoons \text{C}_{10}\text{H}_7\text{O}^- + \text{H}_2\text{O}$
   • The phenoxide/naphthoxide anion ($-\text{O}^-$) is a significantly stronger electron-donating group than the neutral hydroxyl group ($-\text{OH}$). This increases the electron density of the aromatic naphthalene ring through resonance, making it sufficiently reactive toward weak electrophiles like the diazonium cation ($\text{Ar-N}_2^+$).

2. Coupling Reaction:

   • When an alkaline solution of $\beta$-naphthol is mixed with freshly prepared benzene diazonium chloride ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$) at $0-5^\circ\text{C}$, an electrophilic aromatic substitution reaction occurs at position $1$ ($\alpha$-position) of $\beta$-naphthol: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_{10}\text{H}_7\text{O}^- \xrightarrow{\text{Alkaline Medium}} \text{C}_6\text{H}_5-\text{N}=\text{N}-\text{C}_{10}\text{H}_6\text{OH} + \text{Cl}^-$
   • This forms 1-(phenylazo)-2-naphthol, an intense orange-red precipitate/dye, confirming the presence of $\beta$-naphthol.

Thus, an alkaline solution of $\beta$-naphthol is required for the azo dye test.

Correct Option: D