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Classification, isomerism, nomenclature, preparation and properties: NEET MCQs with answers

68 questions on this topic of Hydrocarbons (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. During the preparation of an alkane by heating anhydrous sodium propanoate with soda lime (NaOH + CaO in 3:1 ratio), which alkane is produced?

  1. Methane
  2. Butane
  3. Propane
  4. Ethane
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Answer: (D) Ethane

Decarboxylation of a sodium salt of a carboxylic acid with soda lime eliminates a molecule of CO₂ (as Na₂CO₃) and yields an alkane having one less carbon atom than the parent carboxylic acid. Sodium propanoate (CH₃CH₂COONa) thus yields ethane (CH₃CH₃).

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2. The number of moles of O₂ required for complete combustion of 1 mol of the alkyne C₃H₄ is:

  1. 5 mol
  2. 4 mol
  3. 6 mol
  4. 3 mol
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Answer: (B) 4 mol

C₃H₄ needs 3 O₂ for carbon and 4/4 O₂ for hydrogen: 3 + 1 = 4 mol.

hard

3. Consider the following reaction sequence: CaC₂ + 2H₂O → A A --(Red hot iron tube, 873 K)--> B B --(CH₃COCl / Anhydrous AlCl₃)--> C The product C is:

  1. Toluene
  2. Benzophenone
  3. Acetophenone
  4. Benzaldehyde
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Answer: (C) Acetophenone

Calcium carbide hydrolyses to ethyne (A). Ethyne undergoes cyclic trimerization over red hot iron tube at 873 K to form benzene (B). Benzene undergoes Friedel-Crafts acylation with acetyl chloride and anhydrous AlCl₃ to yield acetophenone (C).

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4. The number of moles of O₂ required for complete combustion of 1 mol of the alkene C₅H₁₀ is:

  1. 7.5 mol
  2. 8 mol
  3. 10 mol
  4. 5 mol
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Answer: (A) 7.5 mol

C₅H₁₀ needs 5 O₂ for carbon and 10/4 O₂ for hydrogen: 5 + 2.5 = 7.5 mol.

easy

5. During Kolbe's electrolytic synthesis, an aqueous solution of potassium acetate is electrolyzed. Which gas is liberated at the anode along with carbon dioxide?

  1. Ethene
  2. Methane
  3. Hydrogen
  4. Ethane
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Answer: (D) Ethane

At the anode, acetate ions lose electrons to form acetate radicals, which rapidly lose CO₂ to yield methyl free radicals. Two methyl radicals combine to produce ethane (C₂H₆) gas. Hydrogen gas is evolved at the cathode.

hard

6. What is the major organic product formed when 3,3-dimethylbut-1-ene reacts with dilute aqueous H₂SO₄?

  1. 3,3-Dimethylbutan-2-ol
  2. 3,3-Dimethylbutan-1-ol
  3. 2,3-Dimethylbutan-2-ol
  4. 2,3-Dimethylbutan-1-ol
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Answer: (C) 2,3-Dimethylbutan-2-ol

Electrophilic addition of H⁺ forms a secondary carbocation [(CH₃)₃C-CH⁺-CH₃], which undergoes a 1,2-methyl shift to form a more stable tertiary carbocation [(CH₃)₂C⁺-CH(CH₃)₂]. Attack of H₂O followed by deprotonation yields 2,3-dimethylbutan-2-ol.

medium

7. Given below are two statements: Statement I: The eclipsed conformation of ethane is more stable than the staggered conformation. Statement II: In the staggered conformation of ethane, torsional strain is minimized due to maximum separation between adjacent C-H bonds. In 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

The staggered conformation of ethane is more stable than the eclipsed conformation by about 12.5 kJ/mol because electron pairs in adjacent C-H bonds are as far apart as possible, minimizing torsional and steric repulsion. Thus, Statement I is incorrect while Statement II is correct.

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8. The number of moles of O₂ required for complete combustion of 1 mol of the alkene C₃H₆ is:

  1. 6 mol
  2. 3 mol
  3. 5 mol
  4. 4.5 mol
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Answer: (D) 4.5 mol

C₃H₆ needs 3 O₂ for carbon and 6/4 O₂ for hydrogen: 3 + 1.5 = 4.5 mol.

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9. What is the correct IUPAC name of the branched alkene: CH₂=C(CH₂CH₃)-CH(CH₃)₂?

  1. 3-Isopropylbut-1-ene
  2. 2-Ethyl-3-methylbut-1-ene
  3. 3,4-Dimethylpent-1-ene
  4. 2-Isopropylbut-1-ene
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Answer: (B) 2-Ethyl-3-methylbut-1-ene

The principal functional group is the double bond, so the longest carbon chain containing the double bond is selected. Numbering starts from the double bond: C1=C2(CH₂CH₃)-C3H(CH₃)₂. The principal chain has 4 carbons (but-1-ene) with an ethyl group at C2 and a methyl group at C3, giving 2-ethyl-3-methylbut-1-ene.

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10. The number of moles of O₂ required for complete combustion of 1 mol of the alkane C₆H₁₄ is:

  1. 9.5 mol
  2. 9 mol
  3. 6 mol
  4. 13 mol
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Answer: (A) 9.5 mol

C₆H₁₄ needs 6 O₂ for carbon and 14/4 O₂ for hydrogen: 6 + 3.5 = 9.5 mol.

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11. Consider the following statements regarding the stability and reactivity of benzene: Statement I: Benzene readily undergoes electrophilic substitution reactions rather than addition reactions. Statement II: The cyclic delocalization of six π electrons imparts exceptional aromatic stability to the benzene ring. In 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: (A) Both Statement I and Statement II are correct

Benzene has a delocalized π-electron system of 6 electrons satisfying Hückel's (4n+2) rule, giving it high resonance stabilization energy. Addition reactions would destroy this aromatic stability, so benzene predominantly undergoes electrophilic substitution, which preserves the aromatic sextet.

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12. In the Newman projection of ethane, as the dihedral angle changes from 60° to 0°, the conformational change is from:

  1. Eclipsed to skew conformation, accompanied by an increase in potential energy
  2. Eclipsed to staggered conformation, accompanied by a decrease in potential energy
  3. Staggered to skew conformation, accompanied by a decrease in torsional strain
  4. Staggered to eclipsed conformation, accompanied by an increase in torsional strain
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Answer: (D) Staggered to eclipsed conformation, accompanied by an increase in torsional strain

In ethane, a dihedral angle of 60° corresponds to the staggered conformation (energy minimum), and 0° corresponds to the eclipsed conformation (energy maximum). Moving from 60° to 0° transforms staggered to eclipsed with an increase in torsional strain of about 12.5 kJ/mol.

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