Structure and Reactivity of Alkyl Halides

Structure and Reactivity of Alkyl Halides MDCAT MCQs with answers

10 past paper MCQs on Structure and Reactivity of Alkyl Halides, 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.Identify the correct ascending order of reactivity of alkyl halides: (MDCAT 2024)

  1. Cl, Br, I, F
  2. F, Cl, Br, I
  3. Br, I, F, Cl
  4. I, F, Cl, Br
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Correct answer: (b) F, Cl, Br, I

The weaker the carbon-halogen bond the better the leaving group, and that bond weakens from fluorine to iodine.

2.The order of reactivity of following R-X for SN reaction is: (SIBA MDCAT 2025)

  1. RF>RCl>RBr>RI
  2. RBr>RF>RCl>RI
  3. RCl>RF>RBr>RI
  4. RI>RBr>RCl>RF
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Correct answer: (d) RI>RBr>RCl>RF

The weakest carbon halogen bond breaks most readily, and that is the bond to iodine.

3.Reactivity of order of halogen acid is: (SZABMU MDCAT 2023)

  1. HF ˃ HCl ˃ HBr ˃ HI
  2. HBr ˃ HCl ˃ HI
  3. HCl ˃ HBr ˃ HI ˃ HF
  4. HI ˃ HBr ˃ HCl ˃ HF
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Correct answer: (d) HI ˃ HBr ˃ HCl ˃ HF

The weakest bond breaks first, and the bond to hydrogen is weakest in hydrogen iodide.

4.Grignard reagent is formed when alkyl halide reacts in the presence of halogen with: (SZABMU MDCAT 2023)

  1. Calcium
  2. Potassium
  3. Sodium
  4. Magnesium
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Correct answer: (d) Magnesium

Magnesium in dry ether inserts itself into the carbon halogen bond to give the Grignard reagent.

5.In SN1 reactions the correct order of reactivity of alkyl halide is? (UHS MDCAT 2023)

  1. Primary>secondary>tertiary
  2. Tertiary>secondary>primary
  3. Secondary>primary>tertiary
  4. Primary>tertiary >secondary
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Correct answer: (b) Tertiary>secondary>primary

The first step makes a carbocation, and that is most stable for the tertiary halide.

6.Which one of the following is a strong nucleophile? (PMC MDCAT 2022)

  1. Cl-
  2. Br-
  3. OH-
  4. HSO4-
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Correct answer: (c) OH-

The hydroxide ion carries a full negative charge on a small atom, so it is the strongest nucleophile here.

7.Which of the following is a poor ‘leaving’ group in nucleophilic substitution reactions? (PMC Practice 2021)

  1. Cl-
  2. Br-
  3. I-
  4. OH-
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Correct answer: (d) OH-

The hydroxide ion is a strong base and holds on to its electrons, so it leaves reluctantly.

8.Which of the following alkyl halides is most reactive towards the Mg atom? (PMC Practice 2021)

  1. Alkyl bromide
  2. Alkyl fluoride
  3. Alkyl chloride
  4. Alkyl iodide
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Correct answer: (d) Alkyl iodide

The carbon iodine bond is the weakest, so the iodide reacts most readily with magnesium.

9.R-X on reaction with alcohols form: (PMC MDCAT 2020)

  1. R-OH
  2. ROR
  3. R-X-OH
  4. RH
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Correct answer: (b) ROR

The alkoxide displaces the halide and an ether is left, which is Williamson’s synthesis.

10.Which of the following acts as a nucleophile in the reaction of alkyl halide with alcoholic / aqueous ammonia? (UHS MDCAT 2018)

  1. H+
  2. NH3
  3. NO2+
  4. Br-
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Correct answer: (b) NH3

The lone pair of the nitrogen of ammonia attacks the carbon and displaces the halide.

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