Question

Use the generalized reaction mechanism of photochemical smog formation to explain why aldehydes start at a very low concentration, build up in concentration until about 2:00 p.m. in the afternoon, and then decrease to their original concentrations?

Answer 1

The generalized mechanism is shown below to explain why aldehydes start at a very low concentration, build up in concentration until about 2:00 p.m. in the afternoon, and then decrease to their original concentrations.

The smog is an oxidising gas cloud filled with nitrogen dioxide, hydrocarbons and their photochemical product. It was shown that the automobile emission containing nitrogen dioxide and unburnt hydrocarbons in sunlight produce eye irritant such as formaldehyde. Initially, the concentration of aldehyde is low, but as the day progress the sunlight catalyse the reaction resulting in more formation of aldhehyde maximum at around 2 pm. Thereafter, the abstraction of aldehydic hydrogen leads to the formation of acetyl radical, which adds oxygen to form peroxyacetyl radical. This peroxyacetyl radical forms a stable adduct with N₂O forming peroxyacetal nitrate. Hence, the concentration of aldehyde eventually decreases.