Diiodide Formation

Alkene Reactions: Diiodide Formation using I2 and Alkenes

Alkene Iodination with I₂: Iodonium-Ion Anti Addition to Vicinal Diiodides

Iodine (I₂) can add across alkene π bonds to form vicinal diiodides via an anti addition. The reaction proceeds through a three-membered iodonium ion, so rearrangements are not observed. Because the addition is endergonic relative to elimination of HI, iodination is often reversible unless an oxidant or scavenger removes I⁻/HI; nonetheless, the same halogenation logic and stereochemistry apply.

Introduction

Iodine adds to alkenes using the same halonium-ion logic seen for Cl₂ and Br₂, yielding vicinal diiodides with anti stereochemistry. The iodonium intermediate prevents carbocation rearrangements. However, the addition is less favorable thermodynamically; HI elimination readily reverses the transformation, so iodination is mainly used as a mechanistic probe unless driven forward by oxidizing conditions (e.g., I₂ in the presence of H₂O₂ or I₂/NaHCO₃).


Quick Summary

  • Reagents: I₂
  • Outcome: Anti addition — each vinylic carbon receives I, giving a vicinal diiodide.
  • Mechanism: Iodonium-ion pathway. Alkene → iodonium; I⁻ performs backside (anti) attack to open the ring.
  • Rearrangements: None. The bridged iodonium blocks hydride/alkyl shifts.
  • Stereochemistry: cis alkene → racemic enantiomeric pair; trans alkene (with symmetry) → meso product (anti addition).
  • Common pitfalls: Ignoring reversibility (HI elimination), using nucleophilic solvents that yield iodohydrins, or proposing syn addition/carbocation steps.

Mechanism (Iodonium Anti Addition)

Step 1: iodonium ion formation from an alkene and I2.
Step 1 — Alkene polarizes I₂ and forms a three-membered iodonium ion.

Step 1 — Iodonium formation. The alkene π bond donates into one iodine while the I–I σ bond heterolyzes. A bridged iodonium ion is produced along with I⁻; the positive charge is delocalized across both carbons and the bridging iodine.


Step 2: iodide attacks the more substituted carbon of the iodonium from the backside.
Step 2 — I⁻ performs backside attack at the more substituted carbon, opening the iodonium anti.

Step 2 — Backside opening. Iodide attacks from the face opposite the iodonium bridge, targeting the carbon with greater positive character (usually more substituted). This SN2-like backside attack opens the ring with inversion at the attacked carbon, enforcing anti stereochemistry.


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

Step 3 — Product formation. Collapse of the iodonium yields the vicinal diiodide with iodine atoms on opposite faces of the former double bond.


Mechanistic Checklist (Exam Focus)

  • Draw the iodonium ion explicitly; avoid depicting a free carbocation.
  • Use conventional two-electron curved arrows for the ionic steps.
  • Show backside attack by I⁻ opening the iodonium with inversion at the attacked carbon.
  • Do not include rearrangements; the bridge prevents hydride or alkyl shifts.
  • Note that nucleophilic solvents (H₂O, ROH) form iodohydrins instead of diiodides.

Worked Example — Exocyclic Alkene

  • Substrate: Isopropenylcyclohexane (exocyclic C=C).
  • Reagents: I₂
  • Pathway: The exocyclic double bond forms an iodonium; I⁻ opens from the opposite face at the more substituted ring carbon.
  • Outcome: Vicinal diiodide with the iodine atoms anti across the former double bond (product may require workup to isolate due to reversibility).
Substrate: isopropenylcyclohexane
Substrate — isopropenylcyclohexane
+
Reagent: solid iodine
Reagent — I₂
→
Product: anti vicinal diiodide
Product — anti vicinal diiodide

When Multiple Alkenes Are Present

Iodination favors the alkene that forms the more stabilized iodonium and permits backside attack by I⁻. Conjugated or electron-rich alkenes react faster, but equilibrium control may limit conversions. Compare potential iodonium ions and consider whether the more substituted site also minimizes steric hindrance for I⁻ attack.


Practical Tips & Pitfalls

  • Stereocontrol: Verify stereochemical drawings (Newman/chair) to reflect anti addition; the diiodide is typically obtained as racemic or meso mixtures depending on substrate symmetry.

Exam-Style Summary

  • Regiochemistry: Both vinylic carbons receive I; stereochemical control is the highlight.
  • Mechanism: Iodonium formation followed by backside attack by I⁻ (anti opening).
  • Intermediates: Bridged iodonium ion; no carbocation rearrangements.
  • Stereochemistry: Anti; cis → racemic pair, trans (sym.) → meso product.
  • Competing pathway: HI elimination reverses the reaction; nucleophilic solvents yield iodohydrins.

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FAQ / Exam Notes

Why is the reaction reversible? Addition of I₂ is less exergonic; HI elimination can regenerate the alkene. Driving the reaction requires oxidizing I⁻ or removing HI.

Does the addition remain anti? Yes. The iodonium ion enforces backside attack, so the two iodine atoms end up anti regardless of equilibrium considerations.

What if water or alcohol is present? They open the iodonium to give iodohydrins (anti I/OH or I/OR). This pathway is often more favorable than forming the diiodide.


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