Question

One of the compounds that was initially formed in the primitive atmosphere was formaldehyde \(\left(\mathrm{H}_{2} \mathrm{C}=0\right)\). It is one of the precursor molecules that came to make up living organisms. It can be formed photolytically as shown in the following equation: $$ \mathrm{CH}_{4}+\mathrm{H}_{2} \mathrm{O}(\mathrm{hv}) \rightarrow \mathrm{H}_{2} \mathrm{C}=\mathrm{O}+4 \mathrm{H} $$ Propose a mechanism for this reaction.

Short answer

The proposed mechanism for the photolytic reaction of methane and water to form formaldehyde and hydrogen gas involves a series of steps: \(1)\) photoexcitation, \(2)\) homolytic bond cleavage, \(3)\) radical-radical combination, and \(4)\) elimination of water. Methyl and hydroxyl radicals are generated in homolytic cleavage, which then combine to form methanol. Finally, methanol undergoes dehydration to produce formaldehyde and water.

Step-by-step solution

Photoexcitation

In the presence of light (hv), energy is absorbed by the reactant molecules, resulting in photoexcitation. This leads to the weakening of bonds in methane (CH4) and water (H2O) molecules and the generation of radical species.

Homolytic bond cleavage

Photoexcitation eventually leads to the homolytic cleavage of one C-H bond of methane and one O-H bond of water, producing the methyl radical (CH3•) and hydroxyl radical (OH•). The overall reaction is as follows: \[ \mathrm{CH}_{4} \rightarrow \mathrm{CH}_{3} \cdot + \mathrm{H} \] \[ \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{OH} \cdot + \mathrm{H} \]

Radical-radical combination

The radicals formed in Step 2, methyl radical (CH3•) and hydroxyl radical (OH•), then combine to form a new species, methanol (CH3OH): \[ \mathrm{CH}_{3} \cdot + \mathrm{OH} \cdot \rightarrow \mathrm{CH}_{3} \mathrm{OH} \]

Elimination of water

In the presence of light, methanol undergoes dehydration, leading to the elimination of water (H2O), and results in the formation of formaldehyde (H2C=O): \[ \mathrm{CH}_{3} \mathrm{OH}(\mathrm{hv}) \rightarrow \mathrm{H}_{2} \mathrm{C}=0 + \mathrm{H}_{2} \mathrm{O} \]

Overall reaction

Combining all the steps, we have the following overall reaction: \[ \mathrm{CH}_{4}+\mathrm{H}_{2} \mathrm{O}(\mathrm{hv}) \rightarrow \mathrm{H}_{2} \mathrm{C}=\mathrm{O}+4 \mathrm{H} \] Thus, we propose that the photolytic reaction proceeds through a series of steps, involving photoexcitation, bond cleavage, radical-radical combination, and elimination of water.

Key concepts

Photoexcitation

When light interacts with molecules, it can lead to an incredibly important process known as photoexcitation. This is what happens when molecules absorb photons – tiny packets of light energy. Think of it like a light switch; when the light is off, the molecules are in a ground state. However, when the light is turned on, or in other words, when they absorb energy from light, they jump into an excited state.
This excitement of electrons causes shifts within the molecules. It is the initial step that sets the stage for various reactions.
In the context of the reaction between methane and water, the photoexcitation step causes bonds within these reactant molecules to weaken, creating an opportunity for further transformations. This is the beginning of a chain of chemical events that result in new substance formation.

Homolytic Bond Cleavage

Homolytic bond cleavage is a process during which a bond within a molecule is evenly split, leading to the generation of radicals. Radicals are atoms, ions, or molecules that have unpaired electrons, making them highly reactive.
During photoexcitation in our reaction, energy causes homolytic cleavage of one carbon-hydrogen (C-H) bond in methane and one oxygen-hydrogen (O-H) bond in water.
This means that each bond breaks in such a way that each of the two atoms involved in the bond takes away one electron from the bond. As a result, we obtain the methyl radical (CH3•) and the hydroxyl radical (OH•), along with additional hydrogen radicals. These radicals are key players in the continuing reaction process.

Radical-Radical Combination

Radical-radical combination is the term used to describe the process where two radicals meet and form a stable molecule. Radicals are highly reactive due to their unpaired electrons, and they are naturally inclined to pair up whenever possible to achieve stability.
In the sequence of transforming methane and water into formaldehyde, the methyl radical (CH3•) and hydroxyl radical (OH•) generated from homolytic bond cleavage are key participants in a radical-radical combination.
These radicals react with each other to form methanol (CH3OH). This reaction is crucial because it brings two unstable radicals together, creating a more stable molecular structure that can further participate in subsequent chemical processes.

Photolytic Reaction

A photolytic reaction, or photolysis, typically involves the breaking down of a chemical compound by photons, which are particles of light. In photochemistry, such reactions require the input of light energy.
In the transformation of methane and water to formaldehyde, the entire process is driven by a photolytic reaction. The presence of sunlight or another suitable light source is essential for the progression of the reaction.
The light provides the necessary energy not only to break bonds (as seen in the homolytic cleavage) but also to facilitate the dehydration of intermediate compounds like methanol to form the end product, formaldehyde. The overall reaction showcases how light energy can be harnessed to drive significant chemical transformations, simulating conditions thought to be present in the primitive atmosphere.