Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. An ice cold solution of aniline in \(\mathrm{HCl}\) is treated with \(\mathrm{NaNO}_{2}\). The compound (A) thereby formed is tested for nitrogen using Lassaigne's test. The result would be (a) a Prussian blue colour confirming the presence nitrogen in aniline. (b) a green colour confirming the presence of nitrogen in aniline (c) an absence of any coloured solution to confirm the presence of nitrogen (d) evolution of ammonia which gives a colour with Nessler's reagent.

Short answer

Answer: (c) An absence of any coloured solution to confirm the presence of nitrogen.

Step-by-step solution

Formation of Compound (A)

The first step is to create compound (A) by reacting aniline with HCl and NaNO2. Aniline reacts with HCl to form anilinium ion, which then reacts with NaNO2 to form diazonium chloride. Reaction: Aniline + HCl \(\rightarrow\) Anilinium ion Anilinium ion + NaNO2 \(\rightarrow\) Diazonium chloride Compound (A) = Diazonium chloride

Lassaigne's Test

Next, we will perform Lassaigne's test on compound (A). Lassaigne's test is a test used to detect the presence of nitrogen, halogens, and sulfur in an organic compound. When the compound is heated with molten sodium metal, it extracts these elements in the form of their respective sodium salts. Then, these resulting salts are dissolved in water and tested for the presence of nitrogen, halogens, and sulfur. In this case, we are specifically looking for the presence of nitrogen in the compound (A) after the reaction.

Expected Result with Lassaigne's Test

Lassaigne's test for nitrogen typically involves the formation of ferrocyanide complex ions (Prussian blue) when the dissolved salts are treated with FeSO4 and concentrated HCl. This complex forms only if the original compound contains nitrogen. However, diazonium chloride is not stable and decomposes into nitrogen gas and HCl, which then reacts with the sodium metal and does not form any colored complex. Reaction: Diazonium chloride \(\rightarrow\) N2 (gas) + HCl Nitrogen gas will not form any colored complex during the Lassaigne's test, and thus, the result would be an absence of any colored solution. So, the correct answer is (c) an absence of any coloured solution to confirm the presence of nitrogen.

Key concepts

Aniline

Aniline is a simple aromatic amine with the chemical formula \(C_6H_5NH_2\). It's a benzene ring attached to an amino group. Aniline is an important industrial chemical, used in the manufacture of dyes, plastics, and explosives. It serves as a building block for many larger, more complex organic molecules.

Aniline is known for its distinctive smell and is slightly oily in nature. Its properties make it fairly reactive.

• Found naturally in coal tar.
• Used in the synthesis of various chemicals and pharmaceuticals.
• Acts as a primary amine, where the amino group is directly attached to a phenyl group.

When it comes to reactions, aniline is highly reactive towards electrophilic substitution due to the electron-donating effect of its amino group, making the benzene ring more reactive. This property plays a crucial role in various chemical transformations.

Diazonium Chloride

Diazonium chloride is a crucial intermediate in organic synthesis, formed by the reaction of a primary aromatic amine like aniline with nitrous acid (generated in situ from \(NaNO_2\) and \(HCl\)). This compound is highly reactive and can decompose easily, often requiring an ice-cold environment for stabilization.

Formation of diazonium chloride involves converting the amine into a diazonium ion, which has the form \(RN_2^+Cl^-\).

• Unstable at higher temperatures, often decomposing to release nitrogen gas.
• Plays a foundational role in introducing functional groups onto an aromatic ring via substitution reactions.
• Key reaagents for synthesizing azo dyes and aromatic halides.

It's important to handle diazonium chlorides with care due to their instability, particularly for their propensity to liberate nitrogen gas explosively upon decomposition.

Lassaigne's Test

Lassaigne's test is a classic analytical procedure used in organic chemistry to detect the presence of elements like nitrogen, sulfur, and halogens in an organic compound. Here's how it works:

A sample is fused with a small piece of sodium metal, converting elements within the organic compound into inorganic, soluble salts. For nitrogen detection, cyanide ions (from the formation of sodium cyanide) react with ferrous ions to form ferrocyanide, yielding a Prussian blue color when ferrocyanide reacts with ferric ions.

• The Prussian blue color indicates nitrogen presence in the original compound.
• This test can also detect sulfur and halogens through further specific reactions.

However, for compounds like diazonium chloride, nitrogen is not detected as a Prussian blue color. Instead, nitrogen escapes as a gas when heated with sodium metal, leaving no colored complex. This explains the absence of color in the test for diazonium chloride.

Nitrogen Detection

Detecting nitrogen within organic compounds hinges on converting it to an easily recognizable form. Historically, Lassaigne's test served this purpose. This test specifically forms sodium cyanide, which interacts with iron salts to form a complex identifiable by its bright blue hue.

Yet, as seen with diazonium chloride, certain compounds deviate from expected behaviors. Here, instead of forming a detectable nitrogen complex, nitrogen escapes as a gas:

• For nitrogen detection, cyanide ion acts as an indicator of nitrogen presence.
• In some compounds, nitrogen is lost during the procedure, leading to no color change.

This procedure highlights the limitations of nitrogen detection in certain contexts. For diazonium salts, where rapid decomposition occurs, this traditional test may not be conclusive.