What is general requirement for free radical substitution reaction of ethane and halogen?

Explanation

Initiation requires the halogen molecule to split homolytically into two radicals, and only ultraviolet light or a high temperature supplies enough energy to break that bond. In the dark and in the cold, an alkane and a halogen simply do not react, which is a standard demonstration of how unreactive alkanes are. Once started, the chain propagates on its own until two radicals combine.