Chapter 9: Amines Application Questions | chemistry Class 12 CBSE

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Chapter 9: Amines Application Questions | chemistry Class 12 CBSE | ByteNotes.in

Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 1

Applied Concept

A chemist wants to prepare a pure sample of propan-1-amine starting from ethyl chloride without getting a mixture of amines. Which method should be used, and why is it preferred over ammonolysis?

The chemist should use Gabriel phthalimide synthesis: ethyl chloride reacts with the potassium salt of phthalimide, followed by alkaline hydrolysis to give pure propan-1-amine—wait, ethyl chloride would give ethanamine. To get propan-1-amine (3 carbons) from a 2-carbon starting material, reduction of propionitrile (prepared from ethyl chloride via NaCN treatment) with LiAlH4 or H2/Ni is the correct approach (ascent of amine series).
Ammonolysis of ethyl chloride gives a mixture of ethylamine, diethylamine, triethylamine, and tetraethylammonium salt—separation is difficult and wasteful.
Nitrile reduction is preferred here because it gives exclusively the primary amine (propan-1-amine) without any secondary or tertiary products, by adding one carbon via CN and then reducing.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

In a pharmaceutical laboratory, the drug adrenaline (containing a secondary amino group) needs to be identified functionally. Which chemical test would confirm it is a secondary amine, and what would be observed?

Hinsberg's test is used: adrenaline (secondary amine) reacts with benzenesulphonyl chloride (Hinsberg's reagent) to form N,N-disubstituted benzenesulphonamide.
The product formed from a secondary amine has no N–H proton; it is therefore not acidic and remains insoluble when treated with NaOH solution.
Compared to primary amines (whose sulphonamide is soluble in NaOH) and tertiary amines (which do not react at all), insolubility in alkali after reaction confirms secondary amine.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

With reference to the preparation of diazonium salts, why must the diazotisation of aniline be conducted strictly at 273–278 K? What happens if the temperature rises?

At 273–278 K (0–5 °C), aniline reacts with NaNO2 and HCl to form the stable benzenediazonium chloride; the low temperature slows decomposition and allows the diazonium salt to accumulate in solution.
Arenediazonium salts are thermally unstable; if the temperature rises above 278 K toward 283 K, the diazonium salt undergoes hydrolysis, and the diazonium group is replaced by –OH, giving phenol (ArOH) instead of the desired salt.
Further temperature rise causes complete decomposition with loss of N2 gas and formation of unwanted by-products, making the diazonium salt unavailable for subsequent synthetic steps.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 4

Applied Concept

A student performs carbylamine test on three unknown compounds and observes foul smell in only one case. What conclusion can be drawn, and which class of amine does the positive compound belong to?

The carbylamine test is positive (foul-smelling isocyanide formed) only for primary amines; secondary and tertiary amines do not give this test.
The reaction involved: R–NH2 + CHCl3 + 3KOH (ethanolic) → R–NC (isocyanide) + 3KCl + 3H2O; only primary amines (both aliphatic and aromatic) show this response.
The compound that gives foul smell is a primary amine; the other two compounds showing no smell must be secondary or tertiary amines, confirming that the test is specific for the primary amine class.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 5

Applied Concept

A researcher needs to convert benzene into aniline for industrial synthesis. Outline the steps involved and identify the reagents at each stage.

Step 1—Nitration: benzene is treated with a mixture of conc. HNO3 and conc. H2SO4 (nitrating mixture) at 328–333 K to give nitrobenzene via electrophilic aromatic substitution.
Step 2—Reduction: nitrobenzene is reduced to aniline using H2/Pd in ethanol, or alternatively with Sn/HCl or Fe/HCl (preferred industrially); Fe/HCl is preferred because FeCl2 hydrolyses in situ to regenerate HCl, requiring only a catalytic amount of acid.
The overall transformation—benzene → nitrobenzene → aniline—is a two-step sequence combining electrophilic aromatic substitution with catalytic or chemical reduction.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

With reference to azo dye synthesis, explain how benzenediazonium chloride is used to prepare an orange dye. Write the relevant equation and state the conditions.

Benzenediazonium chloride (from aniline + NaNO2/HCl at 273–278 K) undergoes coupling with phenol in alkaline medium (OH–) at the para position to form p-hydroxyazobenzene, an orange dye.
Equation: C6H5N2+Cl– + C6H5OH → C6H5–N=N–C6H4–OH (p-hydroxyazobenzene) + HCl + H2O; the reaction is carried out in alkaline medium to form the phenoxide ion, which is a better coupling partner.
The product has an extended conjugated system spanning both benzene rings connected by the –N=N– group, giving rise to the intense orange colour; such azo compounds are widely used as textile dyes.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

A mixture contains aniline, diethylamine, and triethylamine. How would you use Hinsberg's reagent to separate and identify each component? Describe the experimental observation at each step.

Treat the mixture with benzenesulphonyl chloride and then shake with NaOH solution: aniline (primary amine) gives a sulphonamide soluble in NaOH; diethylamine (secondary) gives a sulphonamide insoluble in NaOH; triethylamine (tertiary) does not react and remains as a separate layer.
The NaOH-soluble fraction (aniline sulphonamide) is acidified to reprecipitate the sulphonamide, then hydrolysed to regenerate aniline. The NaOH-insoluble residue from diethylamine is hydrolysed to recover diethylamine.
Triethylamine, being unreacted, is recovered directly from the reaction mixture; this systematic use of Hinsberg's test allows complete separation of all three amine classes.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 8

Applied Concept

Explain why direct nitration of aniline produces a mixture of ortho, para, and meta nitroanilines, and describe how to obtain predominantly p-nitroaniline.

In the strongly acidic nitrating mixture (HNO3/H2SO4), aniline gets protonated to form the anilinium ion (C6H5NH3+), which is a meta-directing deactivating group; hence a significant proportion of meta-nitroaniline (about 2%) and also o-nitro (51%) and p-nitro (47%) products form.
Additionally, direct nitration can lead to tarry oxidation products because of the highly activating –NH2 group, making it difficult to control.
To obtain predominantly p-nitroaniline: acetylate aniline with (CH3CO)2O/pyridine to give acetanilide; nitrate with HNO3/H2SO4 at 288 K to give mainly p-nitroacetanilide; hydrolyse the amide bond with OH– or H+ to yield p-nitroaniline.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 9

Applied Concept

In biological systems, quantitative estimation of amino acids in a protein sample is required. Which reaction of amines is exploited for this, and what is the underlying principle?

The reaction of primary aliphatic amines (amino groups in amino acids) with nitrous acid is exploited: R–NH2 + HNO2 → ROH + N2 + HCl; the nitrogen gas is liberated quantitatively.
The volume of N2 gas collected is measured under known temperature and pressure conditions; from the volume, the moles of N2 are calculated, which equals the moles of primary amino groups present.
This method, known as van Slyke's method, is used for quantitative estimation of amino acids and proteins based on the principle that one mole of –NH2 group liberates one mole of N2.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

With reference to the Sandmeyer reaction, explain how cyanobenzene (benzonitrile) can be prepared from aniline. Why is this route preferred over direct substitution?

Aniline is diazotised (NaNO2 + HCl, 273–278 K) to form benzenediazonium chloride; treatment with CuCN/KCN (Sandmeyer conditions) replaces the diazonium group by CN–, giving benzonitrile (cyanobenzene) and releasing N2.
Direct nucleophilic substitution of CN– onto chlorobenzene is not feasible under ordinary conditions because the aryl ring is electron-rich and strongly resists SN2 attack; high temperatures and pressures are needed (Dow process) making it impractical.
The Sandmeyer route from aniline proceeds under mild conditions via a radical or organometallic mechanism with Cu(I), giving good yields of benzonitrile without requiring extreme conditions.

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Chapter 9: Amines | Application Questions

A chemist wants to prepare a pure sample of propan-1-amine starting from ethyl chloride without getting a mixture of amines. Which method should be used, and why is it preferred over ammonolysis?

In a pharmaceutical laboratory, the drug adrenaline (containing a secondary amino group) needs to be identified functionally. Which chemical test would confirm it is a secondary amine, and what would be observed?

With reference to the preparation of diazonium salts, why must the diazotisation of aniline be conducted strictly at 273–278 K? What happens if the temperature rises?

A student performs carbylamine test on three unknown compounds and observes foul smell in only one case. What conclusion can be drawn, and which class of amine does the positive compound belong to?

A researcher needs to convert benzene into aniline for industrial synthesis. Outline the steps involved and identify the reagents at each stage.

With reference to azo dye synthesis, explain how benzenediazonium chloride is used to prepare an orange dye. Write the relevant equation and state the conditions.

A mixture contains aniline, diethylamine, and triethylamine. How would you use Hinsberg's reagent to separate and identify each component? Describe the experimental observation at each step.

Explain why direct nitration of aniline produces a mixture of ortho, para, and meta nitroanilines, and describe how to obtain predominantly p-nitroaniline.

In biological systems, quantitative estimation of amino acids in a protein sample is required. Which reaction of amines is exploited for this, and what is the underlying principle?

With reference to the Sandmeyer reaction, explain how cyanobenzene (benzonitrile) can be prepared from aniline. Why is this route preferred over direct substitution?

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