Reagent, 1-naphthylamine and sulphanilic acid in acetic acid is used for the detection of
(a) N2O
(b

Q: Why do we use acetic acid in this reaction instead of just mixing the two main reagents?

Acetic acid provides the necessary acidic medium. Without an acidic environment, the nitrite ion ($NO_2^-$) cannot be converted into nitrous acid ($HNO_2$). Nitrous acid is the actual active species required to initiate the diazotization of sulphanilic acid.

Q: Can we use aliphatic amines instead of sulphanilic acid for this test?

No, aliphatic primary amines form highly unstable diazonium salts that immediately decompose into alcohols and nitrogen gas. Aromatic amines are required because they form relatively stable diazonium salts due to resonance stabilization from the benzene ring.

Q: Why does the diazonium ion specifically attack 1-naphthylamine?

The diazonium ion is an electrophile (electron-loving). 1-naphthylamine has an amino group that strongly activates the aromatic ring via the +M (mesomeric) effect, making the ring highly electron-rich and very susceptible to electrophilic attack.

Q: What is the chemical significance of the red color that appears?

The red color comes from the formation of the azo dye. The $-N=N-$ double bond links two aromatic rings, creating a massive, extended conjugated system of alternating single and double bonds. This extended conjugation lowers the energy gap for electronic transitions, allowing the molecule to absorb visible light and appear red.

Q: Is this test also valid for detecting nitrate ($NO_3^-$) ions?

No, the Griess test is highly specific to nitrite ($NO_2^-$) ions. Nitrate ions are fully oxidized and do not form nitrous acid under these mildly acidic conditions, and therefore cannot initiate the required diazotization reaction.

Question

Reagent, 1-naphthylamine and sulphanilic acid in acetic acid is used for the detection of

Options:

(A) $N_2O$

(B) $NO_3^-$

(C) $NO$

(D) $NO_2^-$

Sigma Percentile · Accepted Answer

### Introduction to the Reagents

Welcome to a fascinating journey into the qualitative analysis of organic compounds! Today, we are exploring a classic and highly visual chemical test. The question asks us to identify which specific ion is detected using a mixture of **sulphanilic acid** and **1-naphthylamine** in an acetic acid medium.

This specific combination of reagents is famously known in analytical chemistry as the **Griess reagent**. It is a highly sensitive reagent primarily used for the detection of nitrite ions ($NO_2^-$). Let's dive into the beautiful two-step chemistry behind this colorful reaction.

### Step 1: The Diazotization Process

The first phase of this test relies on the generation of nitrous acid. When nitrite ions ($NO_2^-$) are present in an acidic medium (provided here by acetic acid), they are protonated to form nitrous acid ($HNO_2$).

$$NO_2^- + H^+ \rightarrow HNO_2$$

Once the nitrous acid is formed in situ, it immediately reacts with the first component of our reagent mixture: sulphanilic acid. Sulphanilic acid contains a primary aromatic amine group ($-NH_2$). In the presence of nitrous acid and an acidic medium, this amine group undergoes a classic **diazotization reaction**.

The primary amine is converted into a highly reactive diazonium group ($-N_2^+$). The resulting intermediate is known as a diazotised acid. This diazonium salt is relatively stable at low temperatures due to the resonance provided by the aromatic benzene ring, but it is incredibly hungry for electrons.

### Step 2: The Azo Coupling Reaction

Now comes the second, visually spectacular part of the test. We introduce the second reagent: **1-naphthylamine**. The diazonium salt we just formed acts as a powerful electrophile. It seeks out an electron-rich center to attack.

1-naphthylamine features a fused bicyclic aromatic system with an amino group that strongly activates the ring, making it highly electron-rich. The diazonium ion attacks the naphthylamine ring, typically at the para position relative to the activating amino group.

This type of electrophilic aromatic substitution is known as an **azo coupling reaction**. The two aromatic rings become linked together by a nitrogen-nitrogen double bond ($-N=N-$), forming an extended conjugated system.

### Conclusion and Environmental Significance

The resulting molecule is an **azo dye**. Because of the massive, extended conjugation across the two aromatic rings and the azo linkage, this molecule absorbs specific wavelengths of visible light, reflecting a brilliant, intense **red color**.

The sudden appearance of this red dye is the definitive positive confirmation for the presence of nitrite ions. Therefore, the correct answer to our question is option (d), $NO_2^-$. This Griess test is not just a textbook reaction; it is widely used in environmental chemistry and water quality testing to detect dangerous nitrite pollution in drinking water. Keep exploring these colorful reactions, as they beautifully bridge theoretical organic chemistry with real-world applications!

Animated Solution for Chemistry - Organic Chemistry: Reagent, 1-naphthylamine and sulphanilic acid in acetic acid is used for the detection of

Select Answer:

Visualized Solution

$Visual Anchor: The Reagents$

$Logic Bridge: The Griess Test$

$Raw Setup: Diazotization$

$Atomic Compute: Diazonium Intermediate$

$Atomic Compute: Azo Coupling$

$Final Answer: The Red Azo Dye$

$The Way Forward$

The Sigma Insight: Amines

Introduction to the Reagents

Step 1

Step 2

Conclusion and Environmental Significance

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