Advanced Topic 766 of 1102 Organic Chemistry
Carbenes
Neutral divalent carbon species with six valence electrons.
In-depth explanation of Carbenes
A carbene is a neutral species in which a carbon atom has two bonds, two non-bonding electrons and only six valence electrons in total. Carbenes can exist in a singlet state, where both electrons are paired in one orbital, or a triplet state, where they are unpaired in different orbitals, and the ground state depends on the substituents. They are highly reactive and add to alkenes to give cyclopropanes in a concerted or stepwise process. Carbenes are generated from precursors such as diazomethane, or from chloroform with a strong base.
Going deeper: Carbenes
Key ideas: Neutral divalent carbon with six valence electrons, written :CR2. Singlet carbenes (paired electrons, sp2, empty p orbital) add stereospecifically to alkenes to give cyclopropanes; triplet carbenes (two unpaired electrons) add non-stereospecifically. Generated from CHCl3 with strong base (dichlorocarbene), diazomethane with light, or the Simmons-Smith reagent (a carbenoid).
Common mistakes and connections: Students call carbenes charged or stable; they are highly reactive and short-lived. Another mistake is forgetting that dichlorocarbene is the intermediate in the Reimer-Tiemann reaction and the carbylamine test. Links to cycloaddition and phenol chemistry.
Visualization of Carbenes
Formaldehyde has a planar sp2 carbon with a C=O pi bond; a carbene :CH2 is similar with two bonds, a lone pair and an empty p orbital. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Dichlorocarbene, :CCl2, generated from chloroform and base, adds to cyclohexene to form a bicyclic cyclopropane.
Frequently asked questions about Carbenes
What is Carbenes?
Neutral divalent carbon species with six valence electrons.
Is Carbenes a beginner, intermediate or advanced topic?
Carbenes is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Carbenes?
Study these earlier topics in Organic Chemistry first: Free Radicals, Carbocations, Carbanions.
What comes after Carbenes?
The next topics in Organic Chemistry are Homolytic and Heterolytic Fission and Electrophiles and Nucleophiles.
Can I practise Carbenes with quizzes?
Yes. The interactive lesson includes a self-check question on Carbenes, and the Organic Chemistry quiz draws on every topic in this area.
Where Carbenes fits in Organic Chemistry
Carbenes is topic 38 of 148 in Organic Chemistry.