Statement 1 : Aniline on reaction with NaNO _2 /HCl at 0 ^\circ C followed by coupling with \beta -naphthol gives a dark blue coloured precipitate. Statement 2 : The colour of the compound formed in the reaction of aniline with NaNO _2 /HCl at 0 ^\circ C followed by coupling with \beta -naphthol is due to the extended conjugation.

Statement 1 is False, Statement 2 is True.

Statement 1 is True, Statement 2 is True; Statement 2 is a CORRECT explanation for Statement 1.

Statement 1 is True, Statement 2 is True; Statement 2 is a NOT CORRECT explanation for Statement 1.

Statement 1 is True, Statement 2 is False.

Compounds Containing Nitrogen PYQ — JEE Advanced 2008 · Shift 2 · Q11 with Solution

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Correct answer: D

Step 1: Diazotization of Aniline. Aniline, a primary aromatic amine, reacts with a mixture of sodium nitrite (NaNO $_2$ ) and hydrochloric acid (HCl) at a low temperature (0-5 $^\circ$ C). This reaction is called diazotization. It converts the amino group (-NH $_2$ ) into a diazonium group (-N $_2^+$ ). $C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$ The product formed is benzenediazonium chloride.
Step 2: Azo Coupling Reaction. The benzenediazonium salt then undergoes a coupling reaction with an electron-rich aromatic compound like $\beta$ -naphthol (2-naphthol). This is an electrophilic aromatic substitution reaction where the diazonium ion acts as the electrophile. The coupling occurs at the C-1 position of $\beta$ -naphthol, which is ortho to the activating -OH group.

The product is 1-phenylazo-2-naphthol, which is a type of azo dye.
Color of the Product. The compound 1-phenylazo-2-naphthol is an intensely coloured substance. However, its characteristic color is a bright orange-red precipitate, not dark blue. This reaction is a classic test for primary aromatic amines, known as the azo dye test.
Conclusion for Statement 1. Since the reaction yields an orange-red precipitate, not a dark blue one, Statement 1 is False.

Origin of Color in Organic Molecules. The color of organic compounds is due to the absorption of light in the visible region of the electromagnetic spectrum. This absorption promotes an electron from a lower energy molecular orbital (like the HOMO - Highest Occupied Molecular Orbital) to a higher energy molecular orbital (like the LUMO - Lowest Unoccupied Molecular Orbital).
Role of Conjugation. For absorption to occur in the visible region, the energy gap ( $\Delta$ E) between the HOMO and LUMO must be relatively small. In molecules with extensive systems of conjugated $\pi$ -bonds (alternating single and double bonds), the $\pi$ -electrons are delocalized over a large area. This delocalization stabilizes the molecular orbitals, decreasing the HOMO-LUMO gap.
Conjugation in the Azo Dye. The product, 1-phenylazo-2-naphthol, possesses a large, extended conjugated system. This system includes the benzene ring, the azo group (-N=N-), and the naphthalene ring system. The delocalization of $\pi$ -electrons across this entire structure significantly lowers the HOMO-LUMO energy gap, causing the molecule to absorb light in the blue-green part of the visible spectrum and appear as the complementary color, which is orange-red.
Conclusion for Statement 2. The statement that the color is due to extended conjugation is a correct and fundamental principle in the chemistry of dyes. Therefore, Statement 2 is True.

Statement 1 is False because the color of the azo dye formed is orange-red, not dark blue.
Statement 2 is True as the color of azo dyes is indeed due to the presence of an extended conjugated system.

Based on this analysis, the correct option is the one that identifies Statement 1 as False and Statement 2 as True.

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Correct answer: D

Step 1: Diazotization of Aniline. Aniline, a primary aromatic amine, reacts with a mixture of sodium nitrite (NaNO $_2$ ) and hydrochloric acid (HCl) at a low temperature (0-5 $^\circ$ C). This reaction is called diazotization. It converts the amino group (-NH $_2$ ) into a diazonium group (-N $_2^+$ ). $C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$ The product formed is benzenediazonium chloride.
Step 2: Azo Coupling Reaction. The benzenediazonium salt then undergoes a coupling reaction with an electron-rich aromatic compound like $\beta$ -naphthol (2-naphthol). This is an electrophilic aromatic substitution reaction where the diazonium ion acts as the electrophile. The coupling occurs at the C-1 position of $\beta$ -naphthol, which is ortho to the activating -OH group.

The product is 1-phenylazo-2-naphthol, which is a type of azo dye.
Color of the Product. The compound 1-phenylazo-2-naphthol is an intensely coloured substance. However, its characteristic color is a bright orange-red precipitate, not dark blue. This reaction is a classic test for primary aromatic amines, known as the azo dye test.
Conclusion for Statement 1. Since the reaction yields an orange-red precipitate, not a dark blue one, Statement 1 is False.

Origin of Color in Organic Molecules. The color of organic compounds is due to the absorption of light in the visible region of the electromagnetic spectrum. This absorption promotes an electron from a lower energy molecular orbital (like the HOMO - Highest Occupied Molecular Orbital) to a higher energy molecular orbital (like the LUMO - Lowest Unoccupied Molecular Orbital).
Role of Conjugation. For absorption to occur in the visible region, the energy gap ( $\Delta$ E) between the HOMO and LUMO must be relatively small. In molecules with extensive systems of conjugated $\pi$ -bonds (alternating single and double bonds), the $\pi$ -electrons are delocalized over a large area. This delocalization stabilizes the molecular orbitals, decreasing the HOMO-LUMO gap.
Conjugation in the Azo Dye. The product, 1-phenylazo-2-naphthol, possesses a large, extended conjugated system. This system includes the benzene ring, the azo group (-N=N-), and the naphthalene ring system. The delocalization of $\pi$ -electrons across this entire structure significantly lowers the HOMO-LUMO energy gap, causing the molecule to absorb light in the blue-green part of the visible spectrum and appear as the complementary color, which is orange-red.
Conclusion for Statement 2. The statement that the color is due to extended conjugation is a correct and fundamental principle in the chemistry of dyes. Therefore, Statement 2 is True.

Statement 1 is False because the color of the azo dye formed is orange-red, not dark blue.
Statement 2 is True as the color of azo dyes is indeed due to the presence of an extended conjugated system.

Based on this analysis, the correct option is the one that identifies Statement 1 as False and Statement 2 as True.