Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is singlet carbene? (a) \(\left(\mathrm{CH}_{3}\right)_{3} \dot{\mathrm{C}}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5}{\vec{C}}-\mathrm{H}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \mathrm{HCH}_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

Option (b) \(\mathrm{C}_{2} \mathrm{H}_{5}{\vec{C}}-\mathrm{H}\) is a singlet carbene.

Step 1: Understanding Carbene

A carbene is a neutral molecule containing a divalent carbon atom with six electrons in its valence shell. Carbenes can exist in two electronic states: singlet or triplet.

Step 2: Identifying Singlet Carbene

A singlet carbene has its two non-bonded electrons paired in the same orbital, resulting in a diamagnetic species.

Step 1: Examination of Options

Let's examine each option to determine if any is a carbene. Only option (b), \(\mathrm{C}_{2} \mathrm{H}_{5}{\vec{C}}-\mathrm{H}\), represents a carbene structure.

Step 2: Determining the State

In option (b), the lone pair of electrons on the carbon atom are paired, indicating that it is a singlet carbene.

Step 3: Conclusion

Options (a), (c), and (d) do not represent carbenes. Only option (b) has the appropriate electronic configuration for a singlet carbene.

Key Concepts

Electronic States

Electronic states describe the distribution and behavior of electrons in an atom or molecule. In the case of carbenes, these states determine whether the carbene is classified as a singlet or triplet. A singlet carbene has both of its valence electrons paired in the same orbital, which results in a diamagnetic nature due to the absence of unpaired electrons.
This contrasts with a triplet carbene, where the electrons occupy separate orbitals, leading to a paramagnetic nature due to the presence of unpaired electrons.

• Singlet State: Electrons are paired, making it diamagnetic.
• Triplet State: Electrons are in different orbitals, making it paramagnetic.

The distinction between these electronic states is crucial to understand reactivity and properties of carbenes in chemical reactions.

Divalent Carbon

Divalent carbon refers to a carbon atom that forms only two covalent bonds instead of the typical four, thereby having two non-bonded electrons. This is a characteristic feature of carbenes.
In carbenes, the divalent carbon carries two lone electrons, resulting in a highly reactive species due to its incomplete octet.
This lack of full electron sharing or octet fulfillment makes the carbene eager to participate in chemical reactions to attain stability.
The most common types of carbenes feature a divalent carbon, leading to different reactivity patterns based on their electronic states, singlet or triplet. For instance:

• Singlet carbenes react more as electrophiles due to the paired electron nature.
• Triplet carbenes tend to behave more like free radicals due to unpaired electrons.

Understanding divalent carbon is essential for predicting the behavior of carbenes.

Diamagnetic Species

Diamagnetic species are those which do not have any unpaired electrons. This feature plays a significant role in determining the magnetic properties of molecules.
For instance, when electrons are paired in all orbitals, the magnetic effects cancel each other out, resulting in diamagnetism.
In the case of singlet carbene, the electrons are paired, making it a diamagnetic species.
This pairing not only impacts its magnetic properties but also affects its stability and the way it interacts with other molecules.

• Characteristics: No unpaired electrons, not attracted to a magnetic field.
• Example: Singlet carbenes are diamagnetic due to electron pairing.
• Contrast with Paramagnetic: Paramagnetic species have unpaired electrons and are attracted to a magnetic field.

Recognizing whether a species is diamagnetic or paramagnetic is integral to understanding its reactivity and interactions in chemical processes.