Question

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

Short answer

Option (d) \\(\text{CHCl}_3\\) gives a carbene upon photolysis.

Step-by-step solution

Understanding Photolysis

Photolysis refers to the decomposition of a chemical compound by photons. To identify which species gives a carbene upon photolysis, we must consider the mechanism of light-induced decomposition and the formation of carbenes.

Identifying Carbene Formation

Carbenes are neutral species with a divalent carbon atom that has only six electrons in its valence shell. They can be formed from compounds such as diazo compounds or compounds with geminal dihalides.

Analyzing Molecule CHCl3

In photolysis of \( ext{CHCl}_3\), a chlorine atom can be removed upon exposure to light, resulting in the formation of \(: ext{CCl}_2\), a dihalocarbene. This is a known pathway for carbene formation.

Conclusion - Photolysis Identification

Among the given options, \(\text{CHCl}_3\) is the compound that results in a carbene, specifically dichlorocarbene, upon photolysis. Hence, the correct answer is option (d).

Key concepts

Photolysis

Photolysis is a chemical process in which a chemical compound is decomposed by the absorption of photons, particularly under the influence of light energy. - The energy from the light waves is absorbed by the molecules, resulting in the breaking of chemical bonds. - This is a fundamental process in numerous natural and artificial chemical reactions, such as photosynthesis and the degradation of certain pollutants. During photolysis, the energy absorbed from photons is often sufficient to break specific chemical bonds in a compound, which can lead to the creation of highly reactive species, such as radicals or carbenes. These intermediates are significant because they can participate in forming many other compounds, indicating their vital role in chemical synthesis.

Chemical Decomposition

Chemical decomposition refers to the process where a single compound breaks down into two or more elements or new compounds. This process can occur spontaneously, but it can also be induced by external factors: - Heat (thermal decomposition) - Light (photolysis) - Catalyst presence In the context of photolysis leading to carbene formation, chemical decomposition happens when the energy from light causes a compound to disassociate into simpler molecules or atoms. This is crucial for understanding carbene chemistry because carbenes themselves can be highly unstable and are often formed and reacted with in brief moments during decomposition reactions.

Divalent Carbon Species

Divalent carbon species, particularly carbenes, contain a carbon atom with two unshared electrons in their valence shell and only two bonds connecting it to other atoms. - These carbon atoms possess a total of six valence electrons, making them highly reactive. - Carbenes are unique because they exist in a singlet state with paired electrons, or in a triplet state with unpaired electrons. Due to their electron-deficient nature, carbenes readily participate in various chemical reactions. Their formation often requires special conditions, such as high energy inputs from photolysis, to stabilize their transient existence. Recognizing the unusual properties of divalent carbon species is fundamental for understanding their reactivity and application in synthetic chemistry.

Dihalocarbene

Dihalocarbenes are a subtype of carbenes, where two halogen atoms are bonded to the divalent carbon atom. - They are typically formed through the removal of a halogen from a "geminal" halide, which is a compound with two halogen atoms attached to the same carbon. - In the photolysis of chloroform (CHCl₃), another chlorine atom is removed, resulting in the formation of dichlorocarbene, \\(:\mathrm{CCl}_2\), a classic example of dihalocarbene.Dihalocarbenes are known for their utility in chemical synthesis, specifically in cyclopropanation reactions, where they help in forming three-membered carbon rings. Understanding the formation and reactivity of dihalocarbenes is essential for chemists who wish to manipulate such reactive intermediates in synthetic routes.