Question

Which of the following statements regarding free radical halogenation of alkane is not true?
A.Hydrogen attached by halogen radical in the propagation step is exothermic in both chlorination and bromination.
B.Hydrogen abstraction by halogen radical in the propagation is exothermic in chlorination but endothermic in bromination.
C.A single halogen radical may bring about halogenation to thousands of alkane molecules.
D.Continuous source of energy is required to continue the halogenation reaction.

Answer 1

During the free radical halogenation of alkane, the hydrogen is abstracted very fast that causes an exothermic reaction, whereas the fraction of hydrogen by bromine is low thereby it requires energy and hence the reaction is endothermic.

Complete step by step solution:
Halogenation of alkane is done by a free radical substitution mechanism. Free radical is a species with one odd electron and substitution reactions are those reactions in which one group is substituted by the other group.
The most common example of halogenation by free radicals is the halogenation of methane using chlorine gas in the presence of sunlight. The resultant products are methyl chloride and hydrochloric acid.
If the chlorine is used in the access then a chain of reaction occurs and at each step chlorine is consumed with the release of one molecule of hydrogen chloride and ultimately all the hydrogen of methane gets substituted by chlorine and tetrachloromethane is formed.
There are mainly 3 steps involved in the free radical mechanism that are chain initiation step, chain propagation step chain termination step.
The hydrogen attached with halogen radical is exothermic in case of chlorine but endothermic in case of from in the given statement A is incorrect
Statement B is correct; single hydrogen radical is enough because the radical is regenerated in the cause of reaction and hence can be used over and over again. The only thing required is the excess of chlorine.
A constant source of energy in the form of light or heat is required to carry out the halogenation reaction so as to break the molecule.
Hence, the correct option to the question is A.

Note:
However, usually, chlorine gas is used in the above reaction. Alkyl fluoride prepared by the above method is very exclusive and hence it is prepared using the Swarts reaction by reacting alkyl halide with silver chloride.