Illustrated Glossary of Organic Chemistry

Carbene: A molecule in which a carbon atom has one lone pair, an open octet, and no formal charge. Usually a highly unstable, reactive intermediate. A carbenoid has structure and properties closely resembling, but are not exactly the same as a carbene.

Molecular Structure of Methylene (:CH2), the Simplest Carbene

Lewis structure
Ball and spoke
Space-filling model
Molecular model kit

Deprotonation of chloroform with hydroxide ion (a strong base) gives a carbanion. This carbanion ejects chloride ion to give dichlorocarbene, :CCl2 (shown in red). This carbene reacts with an alkene to form a cyclopropane.