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Carbene

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Last updated date: 29th Feb 2024
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What is Carbene ?

A carbene is a molecule with two unshared valence electrons and the valence of two neutral carbon atoms. The conventional carbene formula is R-(C:)-R' or R=C:. The term "carbene" can also be referred to as the chemical H2C and it is commonly known as methylene. It is the parent hydride from which all the other carbene compounds are formed loosely.


Overview of Carbene

Depending on the carbene structure, carbenes are classified as singlets or triplets. Despite the fact that persistent carbenes exist, most carbenes have a short life period. Cl2C (dichlorocarbene) is a well-known carbene that can be made in-situ using chloroform and a strong base.


This is the simplest carbene definition. Let us discuss what is carbene in detail, carbene reactions, the difference between singlet and triplet carbene, and more concepts related to it, here.


Singlet and Triplet Carbenes

Singlet and triplet carbenes are the two types of carbenes. Carbenes with singlets are spin-paired. The molecule has an sp2 hybrid carbene structure, according to valence bond theory. Carbenes with two unpaired electrons are known as triplets. They can be sp or sp2 hybridized, and they can be linear or curved. Except for those with nitrogen, oxygen, or sulphur atoms, and halides directly linked to the divalent carbon, most carbenes have a nonlinear triplet ground state.


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Depending on how many electronic spins they have, carbenes are classified as singlet or triplet. Triplet carbenes are paramagnetic and, if they last long enough, can be detected via electron spin resonance spectroscopy. Singlet carbenes have no total spin, but triplet carbenes have one (in units of \[\hbar\] ). Triplet methylene has a bond angle of 125-140°, while singlet methylene has a bond angle of 102° (as determined by EPR). In the gaseous state, triplet carbenes are more stable, whereas singlet carbenes are more common in aqueous media.

Triplet carbenes have 8 kcal/mol (33 kJ/mol) lower energy than singlet carbenes for simple hydrocarbons (see also Hund's law of maximum multiplicity), hence triplet is the more stable state (the ground state), and singlet is the excited state species. The singlet state can be stabilized by substituents that can donate electron pairs by delocalizing the pair into an empty p-orbital. The energy of the singlet state can be lowered to the point where it becomes the ground state. There are currently no viable solutions for triplet stabilization. It was discovered that the carbene 9-fluorenylidene is a fast equilibrating mixture of singlet and triplet states with an energy difference of roughly 1.1 kcal/mol (4.6 kJ/mol).

However, whether diaryl carbenes, such as fluorene carbene, are real carbenes is debatable because the electrons can delocalize to the point where they become biradicals. The silico experiment implies that electropositive heteroatoms, such as those found in silyl and silyloxy carbenes, can thermodynamically stabilize triplet carbenes, particularly the trichlorosilyl carbenes.


Carbene Reactions or Reactivity

Carbene formula is R-(C:)-R'. Let us look at its reactivity here.


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Carbene Addition to Alkenes

The reactivity of singlet and triplet carbenes differs. Singlet carbenes can act as either electrophiles or nucleophiles in cheletropic reactions. Electrophilicity should be present in singlet carbenes with an unfilled p-orbital. Diradicals, triplet carbenes, and stepwise radical additions can all be considered. Singlet carbenes can react in a single concerted step, whereas triplet carbenes must move through an intermediate containing two unpaired electrons.

Singlet methylene reactions are stereospecific, but triplet methylene reactions are stereoselective, due to these two forms of reactivity. This difference can be utilized to figure out what a carbene is made of. For example, the reaction of diazomethane methylene with cis-2-butene or trans-2-butene forms a single diastereomer of the 1,2-dimethyl cyclopropane product: cis from cis and trans from trans, proving that the methylene is a singlet.

If the methylene were a triplet, one would anticipate the result to be substantially identical in each case, regardless of the starting alkene geometry.

The substituent groups determine the reactivity of a specific carbene. Metals can influence their reactivity. Insertions into C-H bonds, skeletal rearrangements, and additions to double bonds are just a few of the processes that carbenes can do. Nucleophilic, electrophilic, and amphiphilic carbenes are the three types of carbenes.

If a substituent can give a pair of electrons, for example, a carbene is unlikely to be electrophilic. Methylene does not distinguish between primary, secondary, or tertiary C-H bonds, therefore alkyl carbenes enter considerably more selectively.


Cyclopropanation

Let us look at the cyclopropanation and carbene formation here.

Cyclopropanes are formed when carbenes combine with double bonds. For singlet carbenes, a coordinated mechanism is available. The stereochemistry of triplet carbenes is lost in the final product molecule. Exothermic reactions are common in addition reactions.

In most cases, carbene formation is the slowest step. Simmons-Smith reagent is a well-known reagent for alkene-to-cyclopropane reactions. The active reagent is thought to be iodomethyl zinc iodide, which is a system of copper, zinc, and iodine. As hydroxy groups complicate the reagent, addition usually takes place in the opposite direction as the hydroxyl group.


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C—H Insertion

Carbene reactions can also take the form of insertions. The carbene simply interposes itself into a relationship that already exists. The following is a usual order of preference: 

  1. X–H bonds where X is not carbon

  2. X–H bonds when X is carbon;

  3. X–H bonds where X is 2. the carbon-hydrogen bond

  4. The C–C bond Insertions might happen in a single step or not.


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Intramolecular insertion processes open up new avenues for synthetic development. In general, rigid carbene structures encourage such insertions. There are no intermolecular insertions when an intramolecular insertion is possible. Five-membered ring formation is favoured over six-membered ring formation in flexible carbene structure. Asymmetric induction can be achieved in both intermolecular and intramolecular insertions by using chiral ligands on metal centers.


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The formation of cyclopentene moieties is attractive to alkylidene carbenes. A ketone can be exposed to trimethylsilyl diazomethane to produce an alkylidene carbene.


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Stabilization strategies for triplet carbenes remain challenging. The carbene 9-fluorenylidene has been discovered to be a fast equilibrating mixture of singlet and triplet states with an energy difference of about 1.1 kcal/mol (4.6 kJ/mol).


However, because electrons can delocalize to the point that they become biradicals, whether diaryl carbenes, such as fluorene carbene, are true carbenes is debatable. In silico experiments, electropositive heteroatoms, such as those found in silyl and silyloxy carbenes, can thermodynamically stabilize triplet carbenes, particularly trichlorosilyl carbenes.

A component of bitumen that is soluble in carbon disulfide but not in carbon tetrachloride. Any group of chemical compounds with an uncharged divalent carbon atom that is produced as intermediates in chemical processes and is frequently very reactive.

FAQs on Carbene

1. Give the difference between singlet and triplet carbene.

Singlet carbenes have two unpaired electrons, whereas triplet carbenes do not. In organic chemistry, a carbene is a functional group. Depending on the electrical carbene structures, carbenes can be classified as singlets or triplets.

2. Give the carbene stability of both singlet and triplet carbenes, which is more stable, and explain why?

Singlet carbene has a spin multiplicity of one, while triplet carbene has a spin multiplicity of three. The species with the greater spin multiplicity value will be more stable than the other, according to Hund's rule of spin multiplicity. As a result, triplet carbene outperforms singlet carbene stability.