A carbene is a highly reactive intermediate that contains six valence electrons on the carbon atom. It is formed in a basic medium and acts as an electrophile. It is a neutral divalent substance in which two unpaired electrons are present in the separated nonbonding orbitals.
The unstable carbenes are short-lived species, however, stable carbenes are also prepared in the labs.
Carbenes are used in organic synthesis on the basis of their stability. For instance, stable carbenes are often used as catalysts in chemical reactions.
Singlet and triplet carbene
The carbenes are classified into singlet and triplet carbenes. A carbene is a neutral species with two electrons that are not shared with other atoms.
By principle, when the carbene has the opposite spin of these two electrons it is called singlet carbenes. On the other hand, when these nonbonding electrons have parallel spin, then these are termed triplet carbenes.
Note that, that singlet carbene has a pair of electrons in the single orbital whereas, the triplet has two unpaired electrons in separate orbitals.
Singlet and triplet carbene’s bond angles are 102° and 125-140º respectively. The triplet carbenes are more stable (8 kcal/mol) as compared to the singlet carbenes. This is because of the stabilization attribute to Hund’s rule of maximum multiplicity.
However, the singlet carbenes can also be stabilized by the substituents that can donate electrons.
Preparation of carbenes
Carbenes can be formed by the following methods.
From alkyl halide and base
The reaction of aqueous potassium hydroxide (KOH) and chloroform can form carbenes. In the first step, the base abstract the proton from the chloroform which results in the formation of an anion.
In the second step, the anion loses the chloride ion to give the divalent carbene that contains two unshared pairs of electrons.
By Simmons Smith reagent
The Simmons smith reagent carbenoid is formed by the reaction of diiodomethane with the zinc-copper couple.
This reagent is used in the synthesis of cyclopropane.
Generation of carbenes from diazo compounds
The diazo compounds form the carbenes by the removal of nitrogen. This reaction is catalyzed by heat, light, and copper.
Some reactions of the carbenes are given below:
Reactions of carbenes
As carbenes are very reactive intermediate, so It reacts in a number of different ways. They can undergo addition as well as substitution reactions depending upon the reaction conditions.
Adding the carbenes intermediates to the double and triple bonds is used for the preparation of cyclopropane and cyclopropene. In some cases, this reaction is 100% stereospecific while rationalized products are formed by diazo compounds.
In addition, they can also add to other nucleophiles such as thiolate, phenoxide ion, and hydroxide.
Stereospecific addition of the Carbenes
The addition of dichlorocarbene to the cis-2-butene is a 100% stereospecific reaction. This is because it gives only cis-1,2-dimethyl-3,3-dichlorocyclopropane. Similarly, the addition of the trans-2-butene gives only the trans isomer.
Non-stereospecific addition of carbenes
When non-stereospecific addition of the carbene occurs. The rationalized products are shown below:
The mechanism of the Reimer Tiemann reaction is started by the deprotonation of the chloroform. This can be done in the presence of a base such as a hydroxide. This carbanion quickly goes to elimination and gives dichloro carbene.
In the next step, the phenol reacts with dichlorocarbene in the presence of a hydroxide ion. This results in the formation of ortho and para hydroxy benzaldehyde.
Is carbene an electrophile?
Carbenes are strong electrophiles. They readily attack the nucleophilic center to undergo addition and substitution reactions.
Is carbene sp2 or sp3?
In carbene, the central carbon is sp3 hybridized.
Is carbene a Lewis acid or base?
In general, carbenes are lewis bases, however, various examples of their adduct as Lewis acid are also reported.
What is the difference between carbene and carbocation?
Carbene has a divalent carbon atom and bears no charge, whereas carbocation contains a trivalent carbon atom with a positive charge.
Is CCl3 a carbene?
CCl3 is a common example of carbene. In this compound, carbon has 6 valence electrons.
What is the stability of carbenes?
They are very unstable compounds due to the presence of unpaired electrons.
- Writing Reaction Mechanisms in Organic Chemistry By Audrey Miller
- Acid-Base Chemistry of Carbenes (wiley.com)