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Reactions of Haloalkanes - Elimination Reactions

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Estimated time: 4 minutes
CBSE: Class 12

Key Points: Reactions of Haloalkanes — Elimination Reactions

β-Elimination Reaction:

  • When alkyl halides are heated with alcoholic KOH or KNH₂, they undergo β-elimination of HX to form an alkene (new π bond).
  • The carbon directly attached to X = α-carbon; the carbon adjacent to it = β-carbon.
  • Order of reactivity in elimination: R–Cl < R–Br < R–I

Saytzeff's Rule (Zaitsev's Rule):

  • In unsymmetrical alkyl halides, hydrogen is preferentially eliminated from the β-carbon with fewer hydrogen atoms → forms the more highly substituted alkene (major product).
  • e.g., 2-bromopentane → pent-2-ene (81%) [major] + pent-1-ene (19%) [minor]

Types of Elimination:

  1. α-elimination: Atom or group lost from the same carbon (gives carbene intermediates).
  2. β-elimination: H from β-carbon, X from α-carbon → alkene.
  • E₁ reaction: Two steps (similar mechanism to SN1)
  • E₂ reaction: One step (concerted, anti-periplanar geometry required — similar to SN2 but gives alkene)

Dehydrohalogenation:

  • Loss of HX from alkyl halide with alc. KOH → alkene.

\[\ce{\underset{Alky halide}{C_{n}H_{2n + 1}X} ->[Alcholic KOH] \underset{Alkene}{C_{n}H_{2n}} + KX + H2O}\]

  • With NaOH, Con. NH₃, t-BuONa, KNH₂, NaNH₂: elimination also occurs.

Shaalaa.com | Haloalkanes and Haloarenes part 22 (Elimination reaction, E1)

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Haloalkanes and Haloarenes part 22 (Elimination reaction, E1)
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Haloalkanes and Haloarenes part 23 (Elimination reaction)
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Related QuestionsVIEW ALL [22]

Observe the following compounds and answer the questions given below.


      (I)

\[\ce{\underset{\text{(II)}}{CH3 - CH2 - Br}}\]

  1. Identify the type of halides.
  2. Explain the nature of the C – Br bond in both of these halides.
  3. Which of these compounds will undergo aqueous alkaline hydrolysis readily? Write the reaction in support of your answer.

Identify the product ‘D’ in the following sequence of reactions:

\[\ce{H3C - CH2 - CH2 - Cl \underset{KOH}{\overset{Alc}{->}} 'B' \overset{HBr}{->} 'C' \underset{Elther}{\overset{Na}{->}}'D'}\]

Draw a neat, labelled energy profile diagram for SN1 reaction mechanism.

Identify the major product formed when 2-cyclohexylchloroethane undergoes a dehydrohalogenation reaction. Name the reagent which is used to carry out the reaction.

State and explain Markownikoff's rule with suitable example

Predict all the alkenes that would be formed by dehydrohalogenation of the following halide with sodium ethoxide in ethanol and identify the major alkene:

2-Chloro-2-methylbutane

How the following conversion can be carried out?

1-Bromopropane to 2-bromopropane

Write the structure of the major organic product in the following reaction:

\[\ce{(CH3)3CBr + KOH ->[ethanol][heat]}\]

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