Question

Which of the following species on photolysis does give a carbene? (a) CC(C)=O (b) \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\) (c) \(\mathrm{CCl}_{4}\) (d) \(\mathrm{CHCl}_{3}\)

Short answer

Compounds (b) and (d) can form carbenes upon photolysis.

Step-by-step solution

Understanding the Definition of a Carbene

A carbene is a neutral molecule containing a carbon atom with two unshared valence electrons, typically having the formula R—C: :, where C represents the carbon atom with two non-bonded valence electrons.

Identifying Photolysis Potential

Photolysis involves the breaking of chemical bonds in a molecule by the absorption of light. It is important to understand how each compound might decompose under the influence of photolysis to form carbenes.

Analyzing Compound (a)

The compound with structure \( \text{CC(C)=O} \) does not form a carbene through photolysis as it represents a ketone and its structure does not lead to a carbene upon photolytic cleavage.

Analyzing Compound (b)

Compound \( \mathrm{CH}_{2}= ext{C}= ext{O} \) can potentially release CO upon photolysis, resulting in the formation of methylene \( \text{CH}_2 \), which is a carbene.

Analyzing Compound (c)

Compound \( \mathrm{CCl}_{4} \), under photolysis, can break down but forms radicals instead of a carbene.

Analyzing Compound (d)

Compound \( \mathrm{CHCl}_{3} \), known as chloroform, can be a source of the carbene \( \text{CCl}_2 \) upon photolysis due to the elimination of \( ext{HCl} \).

Confirming the Carbene Forming Compound

From the analysis, compounds (b) methylene and (d) chloroform are confirmed as capable of generating a carbene, whereas (a) and (c) do not form carbenes upon photolysis.

Key concepts

Photolysis

Photolysis is a fascinating chemical process where light energy, typically ultraviolet or visible light, is used to break down molecules. During this process, the energy from the light source cleaves certain chemical bonds within a compound. This leads to the formation of new substances, often including radicals and other reactive intermediates. Here’s how photolysis relates to carbene formation:

• Molecules absorbing light can undergo electronic excitation, leading to instability.
• This instability is often resolved by the breaking of bonds, sometimes resulting in open-shell species like radicals or neutral species like carbenes.
• Not all molecules break down through photolysis in ways that form carbenes; it depends on the specific structure of the compound and the type of light absorbed.

Photolysis plays a key role in atmospheric chemistry and is crucial in various synthetic applications where unique reactive intermediates like carbenes are desired.

Ketone Structure

Ketones are a class of organic compounds characterized by the presence of a carbonyl group (C=O) bonded to two carbon atoms. This structural feature profoundly influences its chemical behavior, including its response to light or photolysis:

• The carbonyl group in ketones is generally quite stable due to its strong double bond.
• This stability often prevents ketones from forming carbenes under photolytic conditions.
• Specifically, the compound \( \text{CC(C)=O} \) mentioned in the exercise is a ketone. In this case, photolysis tends not to generate carbenes because the bond scission typically does not lead to carbene formation.

This solid foundation of stable bonding in ketones contrasts with other compounds that readily form carbenes through photolysis due to weaker bonds or different structural arrangements.

Radicals Formation

Radicals are molecules or atoms that have unpaired electrons, making them highly reactive due to their tendency to pair up these electrons. They can form through multiple pathways, notably photolysis:

• During photolysis, molecules can absorb light energy, leading to the homolytic cleavage of bonds and formation of radicals.
• For example, \( \text{CCl}_4 \) which can undergo photolysis to form chlorine radicals \( \text{Cl} \) rather than carbenes.
• Radicals are often transient and react quickly with available species, playing key roles in various chemical reactions and degradation processes.

This ability to form radicals instead of carbenes sets certain molecules apart, emphasizing the nuances in chemical bond breaking and the importance of molecular structure in determining photolytic outcomes.

Methylene

Methylene \( \text{(CH}_2)} \) is one of the simplest carbenes, characterized by a divalent carbon atom with two unpaired electrons. Here's why methylene is special:

• It’s an unstable and highly reactive species due to the presence of these unpaired electrons.
• In the context of the exercise, \( \mathrm{CH}_{2}=\)\mathrm{C}=\mathrm{O} ) releases CO during photolysis to form methylene.
• This process showcases how specific molecular arrangements are primed to produce reactive intermediates like carbenes under the right conditions.

Methylene is extensively studied in organic chemistry for its role in various synthetic transformations and as an intermediate in chemical reactions due to its high reactivity.

Chloroform Reaction

Chloroform \( \mathrm{CHCl}_3 \) is a well-known organic solvent that plays a pivotal role in the formation of carbenes via photolysis:

• Upon exposure to light, chloroform can eliminate hydrochloric acid \( \text{HCl} \), leaving behind the dichlorocarbene species \( \text{CCl}_2 \).
• This type of carbene is similar to methylene but with halogens, making it highly reactive.
• The formation of dichlorocarbene highlights the variability of the photolysis reaction based on different substituents.

Chloroform's ability to serve as a carbene precursor underlines its significance in synthetic chemistry, where specific reaction pathways are harnessed for desired chemical transformations.