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Substitution, addition, elimination and rearrangement reactions: NEET MCQs with answers

50 questions on this topic of Some Basic Principles of Organic Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Consider the conversion of propene into 1,2-dibromopropane using bromine dissolved in CCl₄. Which of the following statements is correct regarding this reaction?

  1. It is an electrophilic substitution reaction that retains the double bond
  2. It is a nucleophilic addition reaction involving a carbanion intermediate
  3. It is a free radical addition reaction initiated by homolytic cleavage of C-H bonds
  4. It is an electrophilic addition reaction involving a cyclic bromonium ion intermediate
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Answer: (D) It is an electrophilic addition reaction involving a cyclic bromonium ion intermediate

The halogenation of alkenes proceeds through an electrophilic addition mechanism where Br⁺ acts as an electrophile, forming a cyclic bromonium ion intermediate followed by attack of Br⁻.

medium

2. Consider the following reaction sequence: Chlorobenzene is heated with aqueous NaOH at 623 K and 300 atm pressure, followed by acidification to produce phenol. This conversion is classified mechanistically as:

  1. Electrophilic aromatic substitution
  2. Electrophilic addition
  3. Free-radical addition
  4. Nucleophilic aromatic substitution
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Answer: (D) Nucleophilic aromatic substitution

Dow's process involves the replacement of a chloro group on the benzene ring by a hydroxyl group under drastic conditions (623 K, 300 atm) via a nucleophilic aromatic substitution pathway (involving an elimination-addition benzyne intermediate).

hard

3. Given below are two statements regarding elimination reactions: Statement I: The E2 mechanism involves simultaneous cleavage of the C-H and C-X bonds in a single concerted transition state. Statement II: The E1 mechanism proceeds via a two-step pathway where the rate of reaction depends on the concentration of both alkyl halide and the attacking base. In light of the above statements, choose the correct 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: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because E2 is a bimolecular concerted process. Statement II is incorrect because E1 is unimolecular; its rate-determining step is the ionization of the substrate to form a carbocation, which depends only on the concentration of the alkyl halide, not the base.

medium

4. Match List-I with List-II to identify the type of reaction: List-I: (A) CH₃CH₂Cl + KCN(alc.) → CH₃CH₂CN + KCl (B) CH₃CH=CH₂ + Br₂ (at 500 °C) → CH₂=CH-CH₂Br + HBr (C) (CH₃)₂C=CH₂ + H₂O/H⁺ → (CH₃)₃C-OH (D) CH₃CH₂CH₂CH₃ (with AlCl₃/HCl, Δ) → (CH₃)₃CH List-II: (I) Free radical substitution (II) Nucleophilic substitution (III) Rearrangement (IV) Electrophilic addition Choose the correct answer from the options given below:

  1. (A)-(II), (B)-(I), (C)-(IV), (D)-(III)
  2. (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
  3. (A)-(II), (B)-(IV), (C)-(I), (D)-(III)
  4. (A)-(IV), (B)-(I), (C)-(II), (D)-(III)
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Answer: (A) (A)-(II), (B)-(I), (C)-(IV), (D)-(III)

(A) is an SN2 nucleophilic substitution by cyanide. (B) is allylic bromination proceeding by a free radical substitution pathway. (C) is acid-catalyzed hydration, an electrophilic addition. (D) is isomerization of n-alkane to iso-alkane via a rearrangement mechanism.

easy

5. Heating ammonium cyanate strongly gives urea according to the equation: NH₄CNO → NH₂CONH₂. This landmark historical synthesis represents which class of organic reactions?

  1. Addition reaction
  2. Elimination reaction
  3. Substitution reaction
  4. Rearrangement reaction
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Answer: (D) Rearrangement reaction

Wöhler's synthesis of urea from ammonium cyanate involves the reorganization of atoms within the same formula (an inorganic salt converting into an isomeric organic compound) without gaining or losing any atoms, which is an intramolecular rearrangement.

hard

6. When benzyl chloride is treated with aqueous KCN, benzyl cyanide is obtained as the principal product. However, if silver cyanide (AgCN) is used instead, benzyl isocyanide is the major product. This variation illustrates:

  1. A rearrangement reaction of the benzyl cation intermediate into a nitrene
  2. An addition-elimination mechanism involving aromatic ring activation
  3. Substitution by an ambident nucleophile depending on the covalent or ionic nature of the reagent
  4. Electrophilic substitution at the ortho and para positions of the benzene ring
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Answer: (C) Substitution by an ambident nucleophile depending on the covalent or ionic nature of the reagent

Cyanide ion (:C≡N:⁻) is an ambident nucleophile. Ionic KCN provides free cyanide ions attacking primarily through the more nucleophilic carbon atom, while largely covalent AgCN exposes the lone pair on nitrogen for nucleophilic attack, yielding an isocyanide.

easy

7. The reaction between acetone and hydroxylamine (NH₂OH) to form acetoxime is fundamentally classified as:

  1. Free radical substitution
  2. Electrophilic addition
  3. Addition followed by elimination
  4. Intramolecular rearrangement
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Answer: (C) Addition followed by elimination

The reaction of carbonyl compounds with ammonia derivatives involves nucleophilic addition of the amino group to the carbonyl carbon followed by the elimination of a water molecule. Hence, it is an addition-elimination reaction.

easy

8. The reaction CH₄ + Cl₂ → CH₃Cl + HCl (in light) is an example of:

  1. Addition
  2. Elimination
  3. Rearrangement
  4. Substitution
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Answer: (D) Substitution

A hydrogen atom of methane is replaced by a chlorine atom, so this is a substitution reaction.

NCERT Class 11

hard

9. In the Hofmann elimination of quaternary ammonium hydroxides, the less substituted alkene (Hofmann product) is formed as the major product because:

  1. The transition state has carbocation character stabilized by hyperconjugation
  2. The leaving group departs first forming a stable free radical intermediate
  3. The more substituted alkene is thermodynamically unstable under all thermal conditions
  4. Steric hindrance and the high electron-withdrawing effect of the bulky leaving group favour abstraction of the more acidic, less hindered β-proton
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Answer: (D) Steric hindrance and the high electron-withdrawing effect of the bulky leaving group favour abstraction of the more acidic, less hindered β-proton

The bulky -N⁺(CH₃)₃ group imposes steric hindrance, making the more exposed, less sterically hindered β-hydrogens on the less substituted carbon preferred for base abstraction. Furthermore, its strong -I effect increases the acidity of these terminal protons, leading to the Hofmann alkene.

easy

10. Which of the following organic transformations is an example of an electrophilic addition reaction?

  1. CH₄ + Cl₂ (in presence of hν) → CH₃Cl + HCl
  2. CH₃CHO + NaHSO₃ → CH₃CH(OH)SO₃Na
  3. CH₃CH₂Br + aq. KOH → CH₃CH₂OH + KBr
  4. CH₃-CH=CH₂ + HBr → CH₃-CH(Br)-CH₃
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Answer: (D) CH₃-CH=CH₂ + HBr → CH₃-CH(Br)-CH₃

Propene undergoes electrophilic addition with HBr where the electrophile (H⁺) attacks first to form a stable secondary carbocation, which is then attacked by Br⁻. Halogenation of methane is a free-radical substitution, addition of NaHSO₃ to acetaldehyde is a nucleophilic addition, and reaction of ethyl bromide with aqueous KOH is a nucleophilic substitution.

medium

11. A reaction in which the atoms or groups of a molecule rearrange to give a structural isomer is called:

  1. Addition
  2. Elimination
  3. Substitution
  4. Rearrangement
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Answer: (D) Rearrangement

In rearrangement reactions, migration of atoms or groups within the molecule produces a new structural isomer.

NCERT Class 11

hard

12. Heating n-hexane with anhydrous AlCl₃ and dry HCl gas at 300 °C produces a mixture of 2-methylpentane and 3-methylpentane. This reaction is fundamentally categorized as:

  1. Chain rearrangement involving carbocation intermediates
  2. Free radical addition followed by elimination
  3. Electrophilic aromatic substitution
  4. Homolytic cleavage followed by disproportionation
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Answer: (A) Chain rearrangement involving carbocation intermediates

Isomerization of alkanes catalyzed by anhydrous AlCl₃/HCl involves hydride abstraction to generate carbocations, which undergo 1,2-hydride and 1,2-methide shifts (skeletal rearrangement) before hydride recapture.

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