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Amines: nomenclature, classification, structure and basicity: NEET MCQs with answers

56 questions on this topic of Organic Compounds Containing Nitrogen (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following compounds is a tertiary (3°) aromatic amine?

  1. N-Methylaniline
  2. Triphenylmethanamine
  3. N,N-Dimethylaniline
  4. 2,4,6-Tribromoaniline
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Answer: (C) N,N-Dimethylaniline

In N,N-dimethylaniline, the nitrogen atom is bonded directly to one benzene ring and two methyl groups, having three hydrocarbon attachments and no hydrogen on nitrogen, making it a 3° aromatic amine. Triphenylmethanamine is a primary amine because the -NH₂ group is bonded to a single tertiary carbon atom.

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2. How many structural isomeric primary (1°) aliphatic amines are possible for the molecular formula C₄H₁₁N?

  1. 4
  2. 3
  3. 5
  4. 2
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Answer: (A) 4

For C₄H₁₁N, the primary amines must have the -NH₂ group attached to a butyl skeleton: butan-1-amine (CH₃CH₂CH₂CH₂NH₂), butan-2-amine (CH₃CH₂CH(NH₂)CH₃), 2-methylpropan-1-amine ((CH₃)₂CHCH₂NH₂), and 2-methylpropan-2-amine ((CH₃)₃CNH₂). Thus, there are exactly 4 structural primary amine isomers.

hard

3. Why is cyclohexanamine considerably more basic than aniline in aqueous medium?

  1. The lone pair on nitrogen in cyclohexanamine occupies an unhybridized p orbital
  2. In cyclohexanamine, the nitrogen lone pair is localized on sp³ carbon skeleton, whereas in aniline it is delocalized over the benzene ring
  3. Cyclohexyl group exerts a strong -I effect while phenyl exerts a +I effect
  4. The conjugate acid of aniline is stabilized by five resonating structures
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Answer: (B) In cyclohexanamine, the nitrogen lone pair is localized on sp³ carbon skeleton, whereas in aniline it is delocalized over the benzene ring

In cyclohexanamine, the lone pair on nitrogen is localized because the cyclohexane ring is non-aromatic and lacks conjugation, allowing ready protonation. In contrast, in aniline the nitrogen lone pair is delocalized into the aromatic π-system, decreasing electron density on nitrogen and making it a much weaker base.

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4. What is the correct IUPAC name for the compound CH₃-NH-CH₂-CH(CH₃)₂?

  1. N-Methyl-2-methylpropan-1-amine
  2. 1-(Methylamino)-2-methylpropane
  3. N,2-Dimethylpropan-1-amine
  4. N-Isobutylmethanamine
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Answer: (C) N,2-Dimethylpropan-1-amine

The parent chain is a propane chain containing 3 carbon atoms with the amine group at C1, giving propan-1-amine. It has a methyl substituent at C2 and another methyl group attached to the nitrogen atom, so its IUPAC name is N,2-dimethylpropan-1-amine.

easy

5. How many acyclic structural isomers of formula C₃H₉N contain a primary (1°) amine group?

  1. 1
  2. 3
  3. 2
  4. 4
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Answer: (C) 2

For the molecular formula C₃H₉N, the primary amines must have the -NH₂ group attached to a C₃H₇ (propyl) skeleton. Two propyl groups exist: n-propyl (giving CH₃CH₂CH₂NH₂, propan-1-amine) and isopropyl (giving CH₃CH(NH₂)CH₃, propan-2-amine). Thus, there are exactly 2 primary amine isomers.

hard

6. The correct order of basic strength of methyl-substituted amines in aqueous solution is:

  1. NH3 > CH3NH2 > (CH3)2NH > (CH3)3N
  2. CH3NH2 > (CH3)2NH > (CH3)3N > NH3
  3. (CH3)2NH > CH3NH2 > (CH3)3N > NH3
  4. (CH3)3N > (CH3)2NH > CH3NH2 > NH3
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Answer: (C) (CH3)2NH > CH3NH2 > (CH3)3N > NH3

In water, basicity depends on the +I effect, solvation by hydrogen bonding and steric hindrance together. For methyl amines the order is (CH3)2NH > CH3NH2 > (CH3)3N > NH3.

NCERT Class 12

medium

7. Which of the following compounds has the highest basic strength in aqueous medium?

  1. Aniline
  2. Benzylamine
  3. N-Methylaniline
  4. Diphenylamine
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Answer: (B) Benzylamine

In benzylamine (C₆H₅CH₂NH₂), the -NH₂ group is attached to an sp³ hybridized carbon atom, so the lone pair on nitrogen is not delocalised into the benzene ring. In contrast, aniline and its derivatives involve delocalisation of the nitrogen lone pair into the aromatic ring, making them significantly weaker bases.

medium

8. Given below are two statements: Statement I: The C-N-C bond angle in trimethylamine is 108°, which is slightly larger than the tetrahedral angle (109.5°). Statement II: The increase in bond angle in trimethylamine compared to ammonia is due to the steric repulsion between bulky methyl groups. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because 108° is slightly less than the tetrahedral angle of 109.5°, not larger. Statement II is correct because the bond angle in (CH₃)₃N (108°) is indeed larger than in NH₃ (107°) owing to steric repulsion between bulky methyl groups overcoming lone pair-bond pair repulsion.

hard

9. Arrange the following amines in increasing order of their pKb values: (I) Benzylamine (II) Aniline (III) N,N-Dimethylaniline (IV) p-Methoxyaniline

  1. I < III < IV < II
  2. II < III < IV < I
  3. I < IV < III < II
  4. IV < I < III < II
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Answer: (C) I < IV < III < II

A stronger base has a lower pKb value. Benzylamine (I) is an aliphatic amine (pKb ≈ 4.70), making it the strongest base (lowest pKb). Among the aromatic amines, p-methoxyaniline (IV, pKb ≈ 8.66) is strengthened by the +R effect of -OCH₃, N,N-dimethylaniline (III, pKb ≈ 8.94) has electron-releasing methyl groups, and aniline (II, pKb ≈ 9.38) is the weakest base (highest pKb). Hence, the pKb order is I < IV < III < II.

easy

10. Which of the following compounds is classified as a secondary (2°) amine?

  1. (CH₃)₂CH-NH₂
  2. CH₃-NH-CH(CH₃)₂
  3. (CH₃)₃C-NH₂
  4. C₆H₅-CH₂-NH₂
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Answer: (B) CH₃-NH-CH(CH₃)₂

Amines are classified as 1°, 2°, or 3° based on the number of carbon groups attached directly to the nitrogen atom. In CH₃-NH-CH(CH₃)₂, the nitrogen is attached to two alkyl groups, making it a secondary (2°) amine, while the others are primary amines.

easy

11. In the molecule of ammonia and simple aliphatic amines, the nitrogen atom exhibits sp³ hybridisation. The shape of trimethylamine molecule is:

  1. Trigonal planar
  2. Tetrahedral
  3. T-shaped
  4. Pyramidal
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Answer: (D) Pyramidal

According to NCERT, like ammonia, the nitrogen atom in amines is trivalent and carries an unshared pair of electrons. Due to the presence of three bond pairs and one lone pair, the geometry is tetrahedral while the shape is pyramidal.

easy

12. What is the correct IUPAC name of the compound H₂N-(CH₂)₆-NH₂?

  1. Hexane-1,6-diamine
  2. 1,6-Diaminohexane
  3. Hexamethylenediamine
  4. Hexyl-1,6-diamine
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Answer: (A) Hexane-1,6-diamine

According to IUPAC rules, when two identical functional groups are present on the alkane parent chain, both locants and the prefix 'di' are used with the suffix. Thus, H₂N-(CH₂)₆-NH₂ is systematically named hexane-1,6-diamine.

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