Formation of Dichlorocyclopropanes of Alkenes with :CCl₂

Chloroform treated with strong base generates dichlorocarbene (:CCl₂), a singlet carbene that reacts with alkenes in a stereospecific [1+2] cycloaddition. The resulting 1,1-dichlorocyclopropane retains the original alkene geometry (cis→cis, trans→trans). Because the carbene reacts in a concerted, closed-shell fashion, no carbocation intermediates or rearrangements occur.

Introduction

Deprotonation of chloroform (CHCl₃) by strong base (NaOH, KOH) forms the trichloromethyl anion, which undergoes α-elimination of chloride to give singlet dichlorocarbene. In the organic phase, :CCl₂ reacts with an alkene in a concerted, syn addition. The carbene carbon becomes the apex of the cyclopropane and retains both chlorine substituents, affording a 1,1-dichlorocyclopropane with complete retention of alkene stereochemistry.

Quick Summary

• Reagents: CHCl₃ + strong base (NaOH/KOH), often with a phase-transfer catalyst (PTC); inert organic solvent (toluene, CH₂Cl₂).
• Intermediate: Singlet dichlorocarbene (:CCl₂).
• Outcome: 1,1-dichlorocyclopropane; substituent relationships from the alkene are retained (syn addition).
• Mechanism: Base deprotonation → α-elimination → concerted cyclopropanation.
• Stereochemistry: Cis alkene → cis-substituted cyclopropane; trans → trans.
• Competing pathways: Hydride/alkyl shifts and carbocations are absent; avoid radical conditions that could generate different products.

Mechanism (Dichlorocarbene Formation & Cyclopropanation)

The hydroxide (or other base) removes the acidic proton from CHCl₃, placing the electron pair on carbon to yield CCl₃⁻ along with water.

The carbanion collapses, pushing out chloride to generate singlet dichlorocarbene (:CCl₂).

The alkene π bond donates into the empty p-orbital of :CCl₂ while the carbene lone pair forms the second C–C bond, giving a cyclopropane with both chlorines on the apex carbon.

Neutral aqueous workup separates the organic product from salts (Cl⁻, residual base) and phase-transfer catalyst residues.

Mechanistic Checklist (Exam Focus)

• Show base deprotonation of CHCl₃ to form CCl₃⁻.
• Depict α-elimination (lone pair collapses, Cl⁻ departs) to generate :CCl₂.
• Draw the concerted two-arrow cyclopropanation (π → carbene carbon; carbene lone pair → opposite alkene carbon).
• Retain alkene stereochemistry—no carbocations, no rearrangements.

Worked Examples

Example A — cis-2-butene → cis-1,1-dichloro-2,3-dimethylcyclopropane

• Substrate: cis-2-butene (C/C=C\C).
• Reagents: CHCl₃, 50% NaOH, phase-transfer catalyst (e.g., TBAB).
• Outcome: Cis relationship of the methyl groups is retained in the cyclopropane.

Example B — trans-stilbene → trans-1,1-dichloro-1,2-diphenylcyclopropane

• Substrate: trans-stilbene (PhCH=CHPh).
• Reagents: CHCl₃, KOH (aq), PTC, CH₂Cl₂.
• Outcome: Trans relationship of the phenyl groups is preserved; no racemization.

When Multiple Alkenes Are Present

:CCl₂ adds fastest to the most electron-rich, least hindered double bond. Conjugated alkenes (allylic, benzylic) react rapidly. If multiple alkenes are available, the one that is best positioned sterically and electronically to be approached from one face will dominate.

Practical Tips & Pitfalls

• Phase-transfer catalysis: In biphasic systems (organic CHCl₃ + aqueous base), PTCs (TBAB, TEBA) shuttle HO⁻ into the organic layer.
• Temperature: 0–25 °C limits side reactions; avoid strong light or peroxides that could form triplet carbenes/radicals.
• Safety: Chloroform is toxic and can release phosgene under UV/heat—work in a hood, use inhibitor-containing bottles, and avoid strong heating. Concentrated base is caustic.
• Common errors: Drawing radical arrows, invoking carbocation intermediates, or losing the original cis/trans relationship.

Exam-Style Summary

• Base deprotonates CHCl₃ → CCl₃⁻.
• α-Elimination gives :CCl₂.
• Singlet :CCl₂ adds syn across the alkene to produce a 1,1-dichlorocyclopropane with retention of geometry.
• No carbocation or radical intermediates; stereospecific addition.

Interactive Toolbox

• Compare dichlorocarbene addition with the Simmons–Smith reaction (CH₂I₂, Zn/Cu).
• Practice carbene additions vs. other cyclopropanation methods in the Reaction Solver.
• Use the Mechanism Solver to visualize the closed-shell two-arrow cyclopropanation.

FAQ / Exam Notes

• Why is stereochemistry retained? The addition is concerted and suprafacial; both new C–C bonds form from the same face of the alkene.
• Can rearrangements occur? No; no carbocation is formed.
• What if the alkene is trans? You obtain the trans cyclopropane; no racemization.
• Alternative carbenes? Simmons–Smith (CH₂I₂/Zn(Cu)) forms non-halogenated cyclopropanes; diazomethane generates :CH₂ but carries significant safety risks.