Organic Chemistry · Alkenes: Reactions and Synthesis
Addition of Carbenes to Alkenes: Cyclopropane Synthesis
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A carbene Neutral carbon with two bonds and 6 valence electrons (R₂C:) Full entry → is a neutral, divalent carbon species with only six valence electrons — two bonds plus two nonbonding electrons. Because carbon "wants" eight electrons, carbenes are among the most reactive intermediates in organic chemistry, and their signature reaction with alkenes forms a three-membered cyclopropane Three-membered ring, C₃H₆ Full entry → ring. This [2+1] cycloaddition is the standard route to cyclopropanes. Two practical variants dominate: free carbenes (from diazomethane CH₂N₂, a reagent that releases :CH₂ on losing N₂ Full entry →, or dichlorocarbene :CCl₂, generated from CHCl₃ and strong base Full entry → from chloroform and base) and the Simmons–Smith reagent, an iodomethylzinc carbenoid A metal-bearing reagent (e.g., ICH₂ZnI) that reacts like a carbene Full entry → that delivers CH₂ more gently and selectively. The key ideas: carbenes are electrophilic; singlet carbenes add stereospecifically while triplet carbenes do not; and the strained cyclopropane product has distinctive chemistry and appears in drugs and natural products.
Why this matters
Cyclopropane rings appear throughout biologically active chemistry: pyrethrin/pyrethroid insecticides (chrysanthemic acid derivatives) owe their potency to a cyclopropane unit; cyclopropane fatty acids help bacteria survive acid and heat stress; and medicinal chemists install cyclopropyl groups in drug candidates because the strained ring resists metabolic oxidation, extending drug lifetime. In synthesis, cyclopropanation is a classic C–C bond-forming reaction, and carbene chemistry connects forward to Chapter 30 and to the biological methylene transfer of Topic 11. For exams, the distinguishing facts are singlet vs triplet stereochemistry and Simmons–Smith's mildness and selectivity over free carbenes.
The college version
Core Concepts
What a carbene is
A carbene has the general formula R₂C: — a carbon with two substituents and two nonbonding electrons, giving 6 valence electrons total. Two electronic states exist:
- singlet carbene Carbene with paired nonbonding electrons, bent geometry Full entry →: the two nonbonding electrons are paired in one orbital (a lone pair); the carbon is bent (sp²-like) and electron-poor — an electrophile.
- triplet carbene Carbene with two unpaired electrons (diradical-like) Full entry →: the two nonbonding electrons occupy two different orbitals with parallel spins; the carbon is closer to linear and behaves like a diradical.
Methylene (:CH₂) from photolysis of diazomethane is often triplet; carbenes bearing π-donating substituents such as :CCl₂ are singlet. The spin state decides the mechanism and therefore the stereochemistry.
Generating carbenes
- Diazomethane (CH₂N₂) loses N₂ on photolysis or thermolysis to give :CH₂. It is explosive and highly toxic: handled only by trained chemists, in small quantities, with proper shielding and ventilation, per institutional safety rules.
- Dichlorocarbene (:CCl₂) forms when chloroform is treated with strong base (e.g., KOtBu): deprotonation gives :CCl₃⁻, which undergoes α-elimination of chloride. This is the classic route to gem-dichlorocyclopropanes.
Carbene addition: mechanism in words
The alkene π electrons attack the empty (or half-filled) orbital of the carbene carbon to form the first C–C bond. For a singlet carbene this is a concerted, stereospecific [2+1] addition: both bonds form in one step, so cis alkene → cis cyclopropane and trans alkene → trans cyclopropane. For a triplet carbene the addition is stepwise: one bond forms, the diradical intermediate can rotate, and only after spin pairing does the second bond form. This is slower and nonstereospecific: cis-2-butene gives a mixture of cis- and trans-1,2-dimethylcyclopropanes. Mnemonic: singlet = same side preserved; triplet = scrambled.
Simmons–Smith: a milder carbenoid route
The Simmons–Smith reaction CH₂I₂ + Zn(Cu) adds CH₂ across an alkene Full entry → treats an alkene with diiodomethane (CH₂I₂) and a zinc–copper couple. The active species is the carbenoid iodomethylzinc iodide, ICH₂ZnI, which transfers CH₂ in a concerted, syn, stereospecific step without ever forming a free carbene. Because it is so mild, Simmons–Smith tolerates alkenes bearing esters, alcohols, or ethers that free carbenes would attack, and CH₂ adds to the less hindered face.
Cyclopropane: structure and strain
In cyclopropane, three carbons form an equilateral triangle: C–C–C angles of 60° instead of the ideal 109.5°. The C–C bonds are "bent" (banana bonds) with poor overlap, and the ring carries roughly 27.5 kcal/mol (≈115 kJ/mol) of strain. This strain makes cyclopropanes more reactive than ordinary alkanes — they can add hydrogen, halogens, and acids, chemistry reminiscent of alkenes — and explains why the cyclopropyl group resists metabolic oxidation.
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| "A carbene is a carbocation." | A carbocation is positively charged with 6 valence electrons and no lone pair; a carbene is neutral with 6 valence electrons and a lone pair (singlet) or two unpaired electrons (triplet). |
| "All carbene additions are stereospecific." | Only singlet carbenes add concertedly; triplet carbenes give mixtures. |
| "Simmons–Smith makes a free carbene." | It makes a carbenoid (ICH₂ZnI); no free carbene is involved, which is why it is so mild. |
| "Cyclopropane reacts like a normal alkane." | The 60° angles create ~27.5 kcal/mol of strain, making cyclopropane far more reactive than typical alkanes. |
| "Cyclopropane is planar and aromatic like benzene." | The three carbons are coplanar only because three points define a plane; the ring is saturated, strained, and not aromatic. |
| "Simmons–Smith inserts CH₂ into a C–H bond." | It adds across the C=C; C–H insertion is a different (free-carbene) reaction. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A carbene is a carbon atom that shows up with only two arms instead of four. An alkene is two atoms holding hands. The two-armed carbon grabs one hand, and the other atom reaches back to grab the carbene's other arm — now all three are holding hands in a little triangle. That triangle is a cyclopropane, and it's special because three atoms squeezed into a tiny circle are very squished and want to pop open.
Worked example
Example 1: Simmons–Smith on cyclohexene
Question: Cyclohexene reacts with CH₂I₂/Zn(Cu). What is the product?
Step 1 — recognize the reaction: Simmons–Smith adds one CH₂ across the double bond.
Step 2 — the ring: cyclohexene (C1CCCC=C1) becomes bicyclo[4.1.0]heptane (C1CCC2CC2C1), commonly called norcarane — a cyclohexane ring fused to a cyclopropane ring.
Step 3 — stereochemistry: the CH₂ adds from the less hindered face (syn), so the product is a single stereoisomer.
Example 2: Singlet vs triplet stereochemistry
Question: cis-2-butene reacts with (a) :CCl₂ (singlet) and (b) triplet :CH₂. What happens to the alkene geometry?
(a) Singlet :CCl₂ adds concertedly: the two methyl groups stay on the same face → cis-1,1-dichloro-2,3-dimethylcyclopropane, a single product.
(b) Triplet :CH₂ adds in two steps; the diradical intermediate rotates before the second bond forms → a mixture of cis- and trans-1,2-dimethylcyclopropanes.
The same starting alkene, two spin states, two very different outcomes.
Example 3: Stoichiometry with dimensional analysis
Question: How many grams of CH₂I₂ (267.84 g/mol) are required to cyclopropanate 0.0500 mol of an alkene in the Simmons–Smith reaction (1:1 stoichiometry)?
Step 1 — mole relation: one CH₂I₂ delivers one CH₂ to one alkene:
nCH2I2 = 0.0500 mol alkene × 1 mol CH2I21 mol alkene = 0.0500 mol
Step 2 — convert to mass:
m = n × M = 0.0500 mol × 267.84 g1 mol = 13.4 g
The mol units cancel, leaving grams of diiodomethane.
Key takeaways
- Carbene: neutral, divalent carbon with 6 valence electrons — extremely reactive, electrophilic.
- Singlet carbene + alkene = concerted, stereospecific (cis → cis, trans → trans).
- Triplet carbene + alkene = stepwise, nonstereospecific (mixtures).
- Diazomethane → :CH₂ + N₂; CHCl₃ + strong base → :CCl₂ (dichlorocarbene).
- Simmons–Smith (CH₂I₂/Zn–Cu) forms ICH₂ZnI, a carbenoid that transfers CH₂ syn and stereospecifically — mild and chemoselective.
- Cyclopropane: 60° angles, ~27.5 kcal/mol strain, bent "banana" bonds, alkene-like reactivity.
- gem-Dichlorocyclopropanes from :CCl₂ can be hydrolyzed to cyclopropanones — a useful follow-up.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
How many valence electrons does a carbene carbon have, and why is that number unusual?
Show answer
Six — two bonding electrons in each of two bonds plus two nonbonding electrons. Carbon normally achieves an octet (8), so 6 makes the carbene electron-deficient and explains its extreme reactivity.
Why is cis-2-butene converted to a single cis cyclopropane by singlet :CCl₂ but to a mixture by triplet :CH₂?
Show answer
Singlet :CCl₂ forms both new bonds in one concerted step, so the alkene's geometry is retained; triplet :CH₂ forms one bond at a time, and rotation in the diradical intermediate scrambles the geometry.
What is the active species in the Simmons–Smith reaction, and why is it preferred over free :CH₂?
Show answer
Iodomethylzinc iodide, ICH₂ZnI — a carbenoid that transfers CH₂ in a concerted, syn, stereospecific step without forming a free carbene, so other functional groups survive.
Describe in words the product of dichlorocarbene addition to cyclohexene.
Show answer
7,7-dichlorobicyclo[4.1.0]heptane — a cyclohexane ring fused to a gem-dichlorocyclopropane (both chlorines on the same carbon of the new ring).
What structural feature makes cyclopropane so much more reactive than propane?
Show answer
The C–C–C angles are compressed to 60° (ideal ~109.5°), creating ~27.5 kcal/mol of ring strain; the bent "banana" bonds also overlap poorly, making the ring eager to react.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- carbene
- Neutral carbon with two bonds and 6 valence electrons (R₂C:)
- singlet carbene
- Carbene with paired nonbonding electrons, bent geometry
- triplet carbene
- Carbene with two unpaired electrons (diradical-like)
- diazomethane
- CH₂N₂, a reagent that releases :CH₂ on losing N₂
- dichlorocarbene
- :CCl₂, generated from CHCl₃ and strong base
- carbenoid
- A metal-bearing reagent (e.g., ICH₂ZnI) that reacts like a carbene
- Simmons–Smith reaction
- CH₂I₂ + Zn(Cu) adds CH₂ across an alkene
- cyclopropane
- Three-membered ring, C₃H₆
- ring strain
- Extra energy from bond-angle distortion in small rings
Sources & references
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