Question

Which of the following species is carbene? a) \(: \mathrm{CH}_{3}^{-}\) (b) \(: \mathrm{CCl}_{2}\) c) \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\) (d) [R]C([R])=O

Short answer

The carbene is \\(: \mathrm{CCl}_{2}\\).

Step-by-step solution

Understanding Carbenes

Carbenes are neutral species that consist of a carbon atom with two unshared electrons and two substituents. They generally have the formula \(: CR_2\), where R can be hydrogen or any other atom or group, making the carbon atom divalent.

Analyzing Option a

The species \(: \mathrm{CH}_{3}^{-}\) is an example of a carbanion, not a carbene. It has a negatively charged carbon with three hydrogen atoms and one lone pair of electrons, which doesn't fit the carbene definition.

Analyzing Option b

The species \(: \mathrm{CCl}_{2}\) fits the general formula for a carbene, as it has a carbon atom attached to two chlorine atoms and possesses two unshared electrons as a lone pair.

Analyzing Option c

The structure \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\) represents a cumulene compound, where the double bonds ensure all atoms have stable configurations, leaving no room for the characteristic lone pair required for a carbene.

Analyzing Option d

The representation of \(\mathrm{R}\mathrm{C}(\mathrm{R})=\mathrm{O}\) is typically indicative of a ketone or aldehyde, neither of which are carbenes, as they have a fully bonded carbon without the lone pair of electrons that distinguishes a carbene.

Conclusion: Identifying the Carbene

Among the provided options, \(: \mathrm{CCl}_{2}\) is the only species that fits the description of a carbene, having a carbon atom with two substituents and two unshared electrons.

Key concepts

Neutral Species

Carbenes are fascinating molecules, classified as neutral species. This means they carry no overall charge, unlike ions that are either positively or negatively charged. In the context of chemistry, being a neutral species implies that the molecule has an equal number of protons and electrons. This balance allows carbenes to actively engage in a variety of chemical reactions. The neutrality provides a unique point of reactivity, often making carbenes quite reactive and versatile intermediates in organic synthesis.

• **No Net Charge**: Equal number of protons and electrons.
• **Reactive Nature**: The balance allows for participation in diverse reactions.

This neutrality is critical in distinguishing carbenes from other charged species, such as carbanions or carbocations. It is the foundation for their role in many synthetic transformations, making them indispensable in organic chemistry.

Divalent Carbon

The defining feature of a carbene is its divalent carbon atom. This means the carbon is bonded to exactly two other atoms or groups and not more. In most organic molecules, carbon tends to form four bonds (making it tetravalent). However, in the case of carbenes, the carbon is divalent, forming exactly two sigma bonds.
This unique setup makes carbene carbon atoms very intriguing. They don't saturate all their valency which is ordinarily four, providing room for reactivity.

• **Two Bonds Only**: Only two single bonds are present.
• **Reactivity Potential**: The unsaturated valency allows for various reactions.

This configuration with two bonds and space for additional interactions often drives the carbene to become a participant in addition reactions, particularly with multiple bonds or radicals.

Lone Pair of Electrons

Carbenes are characterized by a lone pair of electrons residing on the divalent carbon. This lone pair is crucial because it influences both the geometry and reactivity of the carbene. Typically, this presence leads to a trigonal planar or bent shape, which is not the usual tetrahedral shape seen in saturated carbon centers with four single bonds.
The lone pair can engage in resonance or stabilize certain reactive intermediates, thereby influencing the path and products of the reactions they participate in.

• **Geometry Influence**: The lone pair can shape the molecule into trigonal planar or bent configurations.
• **Reaction Participation**: The electrons can be deployed in bonding or stabilize ions.

Understanding the role of this lone pair is key in predicting and rationalizing the behavior of carbenes in numerous chemical reactions, particularly in organic synthesis and mechanisms.