Substitution versus Elimination

Substitution versus Elimination MDCAT MCQs with answers

18 past paper MCQs on Substitution versus Elimination, from the Alkyl Halides unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.In $\mathrm{S_N1}$ reactions the correct order of reactivity of alkyl halides is: (MDCAT 2023)

  1. primary $>$ secondary $>$ tertiary
  2. tertiary $>$ secondary $>$ primary
  3. secondary $>$ tertiary $>$ primary
  4. tertiary $>$ primary $>$ secondary
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Correct answer: (b) tertiary $>$ secondary $>$ primary

The step that decides the rate is the formation of the carbocation, and a tertiary carbocation is the most stable.

2.Identify the correct statement related to substitution and elimination of alkyl halides: (MDCAT 2024)

  1. strong bases cause substitution in preference to elimination
  2. the role of leaving groups in elimination is similar to that in substitution
  3. substitution is favoured more than elimination by decreasing solvent polarity
  4. a decrease in temperature will favour elimination more than substitution
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Correct answer: (b) the role of leaving groups in elimination is similar to that in substitution

Both reactions need the halide to leave, so a better leaving group speeds up either one; strong bases and higher temperatures favour elimination.

3.Alkyl halides involving C-X bond breakage and C-Nu bond formation simultaneously would follow which one of the following mechanisms? (MDCAT 2024)

  1. $\mathrm{S_N1}$
  2. $\mathrm{S_N2}$
  3. E1
  4. E2
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Correct answer: (b) $\mathrm{S_N2}$

In $\mathrm{S_N2}$ the nucleophile attacks as the halide leaves, in one step through a single transition state.

4.Racemic mixture formed in SN1 reaction is due to presence of empty: (NUMS MDCAT 2025)

  1. s-orbital
  2. p-orbital
  3. sp3-orbital
  4. sp2-orbital
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Correct answer: (b) p-orbital

The carbocation of the first step is flat, so the nucleophile can come in from either face.

5.Reaction of 2-Bromobutane with alcoholic KOH yields: (SZABMU MDCAT 2025)

  1. 2-Butanol
  2. 2-Butene
  3. 2-Butyne
  4. 1-Butanol
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Correct answer: (b) 2-Butene

Alcoholic potash favours elimination, so hydrogen bromide is lost and the alkene is left.

6.Dehydrohalogenation of alkylhalide is carried out in presence of (KMU MDCAT 2024)

  1. Alcoholic KOH
  2. Aqueous KOH
  3. Conc. H2SO4
  4. Zn dust
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Correct answer: (a) Alcoholic KOH

Alcoholic potash favours elimination; aqueous potash would substitute and give the alcohol.

7.When 2-bromopropane reacts with Sodium ethoxide, the major product is/are? (UHS MDCAT 2023)

  1. Propane
  2. Propene
  3. Ethyl isopropyl ether
  4. All are formed
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Correct answer: (b) Propene

Ethoxide is a strong base, so elimination wins over substitution and propene is left.

8.Which alkyl halide gives SN2 reactions? (PMC Practice 2021)

  1. Secondary
  2. Primary
  3. Tertiary
  4. All of these
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Correct answer: (b) Primary

A primary halide is least crowded, so the nucleophile can reach the carbon from behind.

9.During the course of an SN1 reaction, the intermediate species formed is: (PMC Practice 2021)

  1. A carbocation
  2. A free radical
  3. A carbanion
  4. An intermediate complex
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Correct answer: (a) A carbocation

The halide leaves first and a carbocation is left behind.

10.Which of the following compete with each other? (PMC Practice 2021)

  1. E1, E2
  2. E1, S2
  3. E2, S1
  4. E2, S2
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Correct answer: (d) E2, S2

Both need a nucleophile attacking the same molecule, so elimination and substitution compete.

11.What is the value of molecularity and order of SN1 reactions? (PMC Practice 2021)

  1. 2, 1
  2. 1, 1
  3. 0, 1
  4. 0, 2
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Correct answer: (b) 1, 1

The slow step involves the halide alone, so it is unimolecular and first order.

12.SN1 reaction is a? (PMC Practice 2021)

  1. Multi-step reaction
  2. Two-step reactions
  3. Concerted reaction
  4. Three-step reaction
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Correct answer: (b) Two-step reactions

The halide leaves in the first step and the nucleophile adds in the second.

13.Alkyl halides involving -C-X bond breakage and -C-Nu bond formation simultaneously would follow the mechanism: (PMC MDCAT 2020)

  1. SN1
  2. SN2
  3. E1
  4. E2
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Correct answer: (b) SN2

In an SN2 reaction the nucleophile comes in as the halide goes out, in one step.

14.Which product is obtained by the hydrolysis of 1-chlorobutane with the aqueous sodium hydroxide? (UHS MDCAT 2019)

  1. 1-butanal
  2. 1-butanol
  3. 1-butene
  4. Butanone
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Correct answer: (b) 1-butanol

Aqueous alkali substitutes hydroxide for the halogen and gives the alcohol.

15.Which compound is obtained by the elimination of bromopropane? (UHS MDCAT 2018)

  1. Propene
  2. Ether
  3. Aldehyde
  4. Ester
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Correct answer: (a) Propene

Losing hydrogen bromide from bromopropane leaves propene.

16.In elimination reactions, ______ is used: (UHS MDCAT 2017)

  1. Acidic K2Cr2O7
  2. CuCl
  3. Acidic NaOH
  4. Alcoholic KOH
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Correct answer: (d) Alcoholic KOH

Alcoholic potash favours elimination and leaves the alkene.

17.Among the alkyl halides, which always follows SN2 mechanism: (UHS MDCAT 2017)

  1. Primary alkyl halides
  2. Secondary alkyl halides
  3. Tertiary alkyl halides
  4. Both Secondary & Tertiary alkyl halides
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Correct answer: (a) Primary alkyl halides

A primary halide is least crowded, so the nucleophile can reach the carbon from behind.

18.Which of the following is an intermediate compound in SN1: (UHS MDCAT 2017)

  1. Ethoxide ion
  2. Alkyl halide
  3. Alkene
  4. Carbocation
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Correct answer: (d) Carbocation

The halide leaves first and a carbocation is left behind.

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