IB Chemistry › R3.4 › R3.4.11

Electrophilic Addition Mechanisms

Two-step mechanism with curly arrows via carbocation intermediate.

Reactivity 3.4 HL Extension ⏱️ ~5 min revision
HL Extension

Electrophilic Addition Two-Step Mechanism

At Higher Level you must draw complete curly arrow mechanisms for the two-step electrophilic addition of alkenes with halogens (\(\text{Br}_2\)) and hydrogen halides (\(\text{HBr}\)), proceeding via a carbocation intermediate.

Mechanism: Ethene + HBr

R3.4.11 Electrophilic Addition Mechanisms - IB HL | ChemEasy Step 1 (slow): π bond attacks δ+ hydrogen H₂C CH₂ H-Br δ+ δ- → CH₃ CH₂⁺ Carbocation + Br⁻ Step 2 (fast): Br⁻ nucleophile attacks carbocation CH₃CH₂⁺ Br⁻ → CH₃CH₂Br Bromoethane The π bond electrons are donated to H⁺, forming a carbocation. Then Br⁻ donates a lone pair to the C⁺.

Step-by-Step Explanation

Step 1 (slow):

  1. The electron-rich π bond of the alkene acts as a nucleophile
  2. A curly arrow goes from the π bond to the δ+ H in HBr (new C-H bond forms)
  3. Simultaneously, the H-Br bond breaks heterolytically (curly arrow from bond to Br)
  4. Products: a carbocation intermediate + Br⁻

Step 2 (fast):

  1. Br⁻ acts as a nucleophile and donates a lone pair to the positive carbon
  2. A new C-Br bond forms → final product

Mechanism: Ethene + Br₂

Mechanism

Induced Dipole and Addition Steps

  1. Step 1: As \(\text{Br}_2\) approaches the \(\text{C}=\text{C}\;\pi\) bond, electron repulsion induces a dipole (\(\text{Br}^{\delta+}-\text{Br}^{\delta-}\)). The \(\pi\) electrons attack \(\text{Br}^{\delta+}\), breaking the \(\text{Br}-\text{Br}\) bond heterolytically → forms a carbocation and \(\text{Br}^-\).
  2. Step 2: The bromide ion (\(\text{Br}^-\)) attacks the carbocation → forms 1,2-dibromoethane.

Curly Arrow Checklist

Key Insight

Why π Bonds are Vulnerable to Electrophiles

The \(\pi\) bond reacts rather than the \(\sigma\) bond because \(\pi\) electrons reside above and below the internuclear axis, making them more accessible and having lower bond strength (\(\approx 270\text{ kJ mol}^{-1}\)).

Examiner Trap

Electrophilic Addition Mechanism Mistakes

  • Carbocation intermediate: Always explicitly draw the intermediate carbocation with its positive charge on carbon.
  • Arrow origin: First arrow must start from the \(\text{C}=\text{C}\) double bond, NOT from \(\text{HBr}\).
  • Induced dipole: Always mark partial charges (\(\delta+, \delta-\)) on \(\text{Br}_2\).
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