Question

Chlorine reacts with saturated hydrocarbon at room temperature in the
(A) Presence of water
(B) Presence of hydrochloric acid
(C) Absence of light
(D) Presence of sunlight

Answer 1

Hint - To solve this problem, firstly we should learn about the reactions of chlorine (halogen) and then we will learn what is free radical reaction, homolytic fission and by applying this knowledge we can easily approach our answer.

Complete step by step solution:
Free- Radical Reaction: It consist of 3 major steps
1) Chain initiation
2) Chain propagation
3) Chain termination
- Chain Initiation: In Chain Initiation, formation of a radical species. Generally, this is a homolytic cleavage, and takes place very rarely due to the high energy barriers involved.
$Cl - Cl\xrightarrow{{h\nu }}2Cl \bullet $
- Chain propagation: In chain propagation it describes the 'chain' part of chain reactions. As soon as the reactive free radical is generated, it can react with stable molecules to form new free radicals. These free radicals go on to generate more free radicals, and so on.
$Cl \bullet + C{H_4} \to HCl + \bullet C{H_3}$
$C{H_3} \bullet + Cl - Cl \to C{H_3}Cl + Cl \bullet $
-Termination: In this step, two free radical species react with each other to form a stable, non-radical adduct.
$Cl \bullet + \bullet C{H_3} \to C{H_3}Cl$
$ \bullet C{H_3} - C{H_3} \bullet \to C{H_3} - C{H_3}$
Radical reactions are very often initiated by light and medium. Uv light is very important as it initiates the initiation process.
- Homolytic Fission: It involves the breakdown of a molecule; one electron is retained by each of the original fragments of the molecule.
$A - B \to A \bullet + B \bullet $
- Heterolytic Fission: When a covalent bond present between two chemical atoms or molecules is broken in an unequal manner, resulting in the bond pair of electrons being retained by one of the chemical species.

Hence, the correct option is D presence of sunlight.

Note - In free-radical reaction in which the reagent is the chlorine atom. The sunlight (specifically, Uv light) is necessary to dissociate the chlorine molecule, it really is termed homolytic fission (homolysis). The Uv rays have energy which is exactly right to break the bonds in Cl molecules and hence Cl atoms are produced.