When benzene is treated with chlorine under the influence of ultra violet light (a) chlorobenzene is obtained (b) benzyl chloride is obtained (c) 1,4 -dichlorobenzene is obtained (d) benzene loses its aromaticity

Let's evaluate each option to see which one correctly represents the product formed in the reaction. (a) chlorobenzene is obtained Chlorobenzene is formed when a hydrogen atom in the benzene ring is substituted by a chlorine atom. This type of reaction is known as electrophilic aromatic substitution. It can happen under the influence of ultraviolet light. (b) benzyl chloride is obtained Benzyl chloride is formed when a chlorine atom is attached to the benzene ring's side chain specifically at the CH₂ group, adjacent to the benzene ring. This type of reaction is known as free radical substitution, which also occurs under the influence of ultraviolet light. (c) 1,4 -dichlorobenzene is obtained 1,4-dichlorobenzene is formed when two chlorine atoms are added to the benzene ring at positions 1 and 4 (opposite each other). This type of reaction can be an electrophilic aromatic substitution, but it generally requires a more vigorous condition than that provided by the ultraviolet light, such as a catalyst or elevated temperature. (d) benzene loses its aromaticity Aromaticity is a significant stabilizing property of benzene, and it is preserved in electrophilic aromatic substitution reactions. It is unlikely that benzene will lose its aromaticity under the given conditions.

From our evaluation in step 1, we can see that options (a) chlorobenzene and (b) benzyl chloride are the two most viable products of this reaction under ultraviolet light. The reaction conditions mentioned in the exercise are typical for free radical substitutions, such as forming benzyl chloride. However, electrophilic aromatic substitution can also happen under the influence of ultraviolet light, leading to the formation of chlorobenzene. Considering the reaction conditions and the more common outcome for these conditions, the correct answer is: (b) benzyl chloride is obtained