Question

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

Answer 1

Hint: The saturated hydrocarbons contain the single bond between the carbon atoms. The chlorination is the addition of $\text{C}{{\text{l}}_{\text{2}}}$.The chlorination does not take place at atmospheric conditions.it requires a special source of energy to break down the $\text{C}{{\text{l}}_{\text{2}}}$ molecules into the chlorine radical $\overset{\text{}}{\mathop{\text{Cl}}}\,$, which facilitates the formation of chloroalkanes.

Complete step by step answer:
Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. They are the simplest class of hydrocarbons. They are called saturated because each carbon atom is bonded to as bonded as many hydrogen atoms as possible.
Chlorine reacts rapidly with saturated hydrocarbons in the presence of sunlight. The reaction is given below:
$\text{C}{{\text{H}}_{\text{4}}}\text{ + C}{{\text{l}}_{\text{2}}}\xrightarrow{\text{in the presence of sunlight}}\text{ C}{{\text{H}}_{\text{3}}}\text{Cl + HCl}$

The sunlight is a measure source of ultraviolet radiations. This uv radiation causes the homolytic fission of the chlorine molecule into the chlorine radical. This radical then attacks saturated hydrocarbons and generates haloalkanes.
\[\text{C}{{\text{l}}_{\text{2}}}\xrightarrow[\text{or UV radiation}]{\text{Sunlight}}\overset{\text{}}{\mathop{\text{2Cl}}}\,\]

So, as we can see that sunlight is necessary for the substitution reactions of saturated hydrocarbons. All the substitution reactions like chlorination or bromination of alkanes occur within the presence of sunlight. The ultraviolet light breaks down the halogen or halogen molecule into free radicals and reaction gets initiated.
The chain mechanism of chlorination of alkanes starts with the splitting or homolysis of a chlorine molecule to form two chlorine atoms. This process is initiated by the ultraviolet radiation of the sunlight. Then a chlorine atom having an unpaired electron acts as a free radical.
So, the correct answer is “Option A”.

Note: Alkanes react with halogen (such as chlorine and bromine) when the mixture is exposed to ultraviolet light (symbolized as hv and UV) or when heated to high temperature (about 200 or 400 degrees Celsius). The function of ultraviolet light is to provide energy for the homolytic cleavage of halogen (Cl-Cl or Br-Br).