Question

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

Answer 1

As we know light is a form of energy, so it provides the necessary activation energy for the reaction to take place. Some reactions do not take place in the dark but take place only in the presence of light or some other radiation. Such reactions are known as photochemical reactions. The most common example is Photosynthesis in which plants use solar energy to convert carbon dioxide and water into glucose and oxygen

Complete answer:
Chlorination reaction is the replacement of one or more hydrogen atoms in an organic compound by a chlorine. In general, if a replacement of hydrogen by any halogen occurs then it is called halogenation reaction.
The reaction of chlorination of saturated hydrocarbons as stated in question is also an example of halogenation reaction.
This reaction can only be carried out in presence of light. For example
$R - H + C{l_2}\xrightarrow{{hv}}R - Cl + HCl$
The energy for this reaction comes from sunlight.

Hence the correct choice is (B).

Additional Information: Halogenation reaction follows free radical mechanism. This is also a chain reaction which initially contains a very small number of free radicals but increases as the reaction proceeds. It is a fast reaction and completes very rapidly.

Note: Students may be confused and think that, like chlorine, other halogens also give suitable results with hydrocarbons, but it is not correct. The Reaction with iodine and fluorine do not give suitable results.