Question

Which of the following species is carbene? (a) \(\overline \mathrm{CH}_{3}\) (b) : \(\mathrm{CCl}_{2}\) (c) \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\)

Short answer

Option (b) \(: \mathrm{CCl}_2\) is the carbene.

Step-by-step solution

Understanding Carbenes

Carbenes are neutral species that consist of a carbon atom with two unshared valence electrons and are typically divalent. They have the general formula \(R_2C: \), where \(R\) can be hydrogen or hydrocarbon substituents.

Analyzing Option (a) \(\overline{\mathrm{CH}_{3}}\)

This species is a methyl anion, which has a negatively charged carbon atom with three hydrogen atoms. It doesn't qualify as a carbene because it does not have a divalent carbon atom with two free electrons.

Analyzing Option (b) \(:\,\mathrm{CCl}_{2}\)

This species is \(:\,\mathrm{CCl}_{2}\), which consists of a central carbon atom bonded to two chlorine atoms, and it has two unshared valence electrons. This structure fits the definition of a carbene (a divalent carbon atom with two free electrons). Therefore, \(:\,\mathrm{CCl}_{2}\) is a carbene.

Analyzing Option (c) \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\)

This compound is known as ketene. It has a linear structure where the carbon atom is part of the carbonyl group bonded with an adjacent methylene group. This does not fit the carbene criteria since the central carbon is involved in double bonds.

Determine the Correct Answer

Based on the analysis, option (b) \(: \mathrm{CCl}_2\) is the only species that fits the basic definition of a carbene.

Key concepts

Divalent Carbon

A divalent carbon atom signifies that the carbon possesses two bonds, meaning it has formed connections with exactly two other atoms. This is a characteristic feature of carbenes.

• In most organic compounds, carbon typically forms four bonds, maximizing its valence shell.
• However, in divalent carbon species, the carbon atom breaks this norm and is left with only two bonds.
• This limitation in bonding results in unique chemical properties.

Carbenes, for instance, exhibit this rare divalent structure. This makes them highly reactive and interesting to study in organic chemistry.
Not all carbon-containing molecules are blessed with this feature, making carbenes stand out for their distinct properties. For example, the ":CCl_2" species is divalent, as it binds with two chlorine atoms while maintaining two free electrons.

Unshared Valence Electrons

Unshared valence electrons, also known as lone pairs, are electrons remaining on an atom that are not involved in bonding.

• These electrons are important because they influence molecule shape and reactivity.
• Lone pairs can engage in interactions, such as forming bonds with other atoms.
• They can also contribute to the magnetic and electronic characteristics of the molecule.

In carbenes, the presence of these unshared valence electrons accounts for their radical-like behavior and high reactivity.
This is evident in ":CCl_2," where the carbon atom features two unshared electrons. These lonely electrons allow carbenes to participate in additional chemical reactions, often seeking partners to form bonds and stabilize the molecule.

Neutral Species

Neutral species in chemistry refer to molecules or atoms that carry no net electrical charge.
Carbenes fall into this category because they maintain this equilibrium between protons and electrons even though they have unique electronic structures.

• Most stable organic molecules are neutral, balancing out positively charged protons in the nucleus with an equal number of negatively charged electrons.
• Carbenes, despite having a peculiar electronic arrangement, manage to maintain this neutrality.

This neutrality is key in distinguishing them from ions, which carry a positive or negative charge.
In the exercise provided, ":CCl_2" is identified as a neutral carbene, which confirms that despite possessing two lone pairs of electrons, the molecule remains electrically balanced. This neutral characteristic enables carbenes to act as intermediaries in various chemical reactions, often stabilizing temporarily before transforming into a more stable form.