Dichloride Formation

Alkene Reactions: Dichloride Formation using Cl2 and Alkenes

Alkene Chlorination with Cl₂: Bromonium-Analog Anti Addition to Vicinal Dichlorides

Chlorine (Cl₂) reacts with alkenes to give vicinal dichlorides by a stereospecific anti addition. The transformation proceeds through a three-membered chloronium-ion intermediate, preventing free carbocation rearrangements. Because Cl₂ is more reactive than Br₂, the reaction is typically run in non-nucleophilic, aprotic solvents and under subdued light to avoid radical pathways.

Introduction

Chlorination of alkenes furnishes vicinal dichlorides. The regiochemistry is trivial (each vinylic carbon receives Cl), but the stereochemistry and intermediate matter: the alkene donates into Cl₂ to form a three-membered chloronium ion, and chloride attacks from the opposite face to open that bridge. Because no free carbocation forms, rearrangements are not observed and the outcome is a clean anti addition.


Quick Summary

  • Reagents: Cl₂
  • Outcome: Anti addition — each vinylic carbon receives Cl, yielding a vicinal dichloride.
  • Mechanism: Chloronium-ion pathway. Alkene → chloronium; Cl⁻ performs backside (anti) attack to open the ring.
  • Rearrangements: None. The bridged chloronium prevents hydride/alkyl shifts.
  • Stereochemistry: cis alkene → racemic enantiomeric pair; trans alkene (with symmetry) → meso product (both via anti addition).
  • Common pitfalls: Using nucleophilic solvents (gives halohydrins), proposing carbocations or syn addition, ignoring the need to suppress radical pathways.

Mechanism (Chloronium Anti Addition)

Step 1: chloronium ion formation from an alkene and Cl2.
Step 1 — Alkene polarizes Cl₂ and forms a three-membered chloronium ion.

Step 1 — Chloronium formation. The alkene π bond donates into one chlorine while the Cl–Cl σ bond heterolyzes. The result is a bridged three-membered chloronium ion plus Cl⁻; the positive charge is delocalized over both carbons and the bridging chlorine.


Step 2: chloride attacks the more substituted carbon of the chloronium from the backside.
Step 2 — Cl⁻ performs backside attack at the more substituted carbon, opening the chloronium anti.

Step 2 — Backside opening. Chloride approaches from the face opposite the chloronium bridge and attacks the carbon bearing greater positive character (typically the more substituted carbon). The SN2-like backside attack opens the ring with inversion at the attacked carbon, locking in anti stereochemistry.


Step 3: vicinal dichloride product showing anti relationship across the former double bond.
Step 3 — Anti vicinal dichloride product.

Step 3 — Product formation. Collapse of the chloronium delivers the neutral vicinal dichloride. The chlorine atoms remain on opposite faces of the former double bond (anti relationship).


Mechanistic Checklist (Exam Focus)

  • Draw the chloronium ion explicitly; do not depict a free carbocation.
  • Use two-electron curved arrows (no fishhooks) for these polar steps.
  • Show backside attack by Cl⁻ opening the chloronium with inversion at the attacked carbon.
  • Omit 1,2-shifts or rearrangements — the bridged intermediate prevents them.
  • Mention that nucleophilic solvents (H₂O, ROH) open the chloronium to halohydrins instead of dichlorides.

Worked Example — Exocyclic Alkene

  • Substrate: Isopropenylcyclohexane (exocyclic C=C).
  • Reagents: Cl₂
  • Pathway: The exocyclic double bond forms a chloronium; Cl⁻ opens from the opposite face at the more substituted ring carbon.
  • Outcome: Vicinal dichloride with the two chlorine atoms anti across the former double bond.
Substrate: isopropenylcyclohexane
Substrate — isopropenylcyclohexane
+
Reagent: liquid chlorine
Reagent — Cl₂
→
Product: anti vicinal dichloride
Product — anti vicinal dichloride

When Multiple Alkenes Are Present

Chlorination targets the alkene that forms the more stabilized chloronium and is most accessible to backside attack. Allylic or benzylic substitution biases formation of the bridged intermediate. Compare candidate chloronium ions and consider steric approach of Cl⁻ when justifying product predictions.


Practical Tips & Pitfalls

  • Stereocontrol: Verify chair/Newman projections to show anti addition; Cl atoms end up trans across the former double bond.
  • Contrast reactions: Nucleophilic solvents give chlorohydrins. NBS/H₂O or HOCl protocols deliver anti Cl/OH instead of dichlorides.

Exam-Style Summary

  • Regiochemistry: Both vinylic carbons receive Cl; focus on stereochemical control.
  • Mechanism: Chloronium formation followed by backside attack by Cl⁻ (anti opening).
  • Intermediates: Bridged chloronium ion; no discrete carbocation.
  • Stereochemistry: Anti; cis → racemic pair, trans (sym.) → meso product.
  • Competing pathway: Nucleophilic solvents (H₂O, ROH) yield halohydrins rather than dichlorides.

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FAQ / Exam Notes

Why is the addition anti? The chloronium ion shields one face of the alkene; Cl⁻ must attack from the opposite face, enforcing anti addition.

Why is Cl₂ more aggressive than Br₂? Chlorine is more electrophilic, so reactions run faster and are more prone to radical pathways; keep solutions cool and in the dark to avoid side reactions.

What if water or alcohol is present? Water or ROH opens the chloronium to give chlorohydrins (anti Cl/OH or Cl/OR) instead of dichlorides.


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