**Abstract**

In organic chemistry, good knowledge of the chemistry of reactive intermediates is very important in understanding organic chemistry, which has a systematic like mathematics. Having this information, it is possible to predict the reactions and the products to be formed on the reactive intermediates. Otherwise, it becomes impossible to learn organic chemistry, which has a very wide scope. This chapter of the book will be a guide that summarizes the essential information about carbene, one of the important reactive intermediates. It is planned to include the following sub-headings in this chapter: Carbene definition and properties, Nomenclature of carbenes, Structure and reactivity of carbenes, Carbene synthesis, Carbene reactions.

**Keywords:** carbenes, triplet and singlet carbenes, diazo compounds, carbene synthesis, carbene reactions

## **1. Introduction**

Carbenes are neutral, divalent (two atoms attached to the central carbon atom) and highly reactive carbon intermediates generally written as "R2C:". Carbenes, which have six electrons in their outer orbital, have a nonbonding pair of electrons and their formal charge is zero. They are short-lived and exhibit extreme reactivity because of electron deficiency.

The simplest carbene is the CH2 compound and is called methylene, the term being first introduced during 19th century [1]. As early as 1835, Dumas [2] reported his attempts to prepare the parent carbene (CH2) by dehydration of methanol. Actually at that time, the quadrivalency of carbon atom was not exactly established and for this reason many ancient chemist believed that methylene would be a stable compound, and various experiments were carried out to produce it. At the end of the 19th century, Curtius [3] and Staudinger [4] showed that carbenes, produced from diazo compounds or ketenes, were highly reactive species. In addition to this, carbenes became popular as transient species [5] in the 1940–1950s, when Doering [6] discovered the cyclopropanation reaction. Then the word carbene was first used by Woodward, Doering and Winstein at a meeting of the American Chemical Society in 1951.

Carbenes are generally classified as triplet and singlet. As we will explain in detail later in singlet carbenes the two electrons are paired and the molecule is diamagnetic, while for triplet carbenes the two electrons are unpaired and the molecule is paramagnetic. Carbenes can be either linear or bent.
