What process involves the gradual substitution of hydrogen by halogen atoms in a molecule?

The process of gradual substitution of hydrogen by halogen atoms in a molecule is known as free radical substitution. This reaction typically occurs in alkanes when they are exposed to halogens under conditions such as heat or ultraviolet light, which generate free radicals.

In free radical substitution, the halogen molecules (such as Cl2 or Br2) undergo homolytic cleavage, breaking the halogen bonds to form two halogen radicals. These radicals are highly reactive and can abstract hydrogen atoms from alkanes, forming alkyl radicals. The alkyl radicals can then react with more halogen molecules, leading to further substitution of hydrogen atoms. This chain reaction can continue, resulting in the progressive substitution of hydrogen for halogens in multiple steps.

This process distinguishes itself from others like electrophilic addition, which typically involves the addition of atoms to a double bond rather than substitution. Chain reactions can also refer to processes like polymerization rather than specific halogen substitutions. Complete combustion is a reaction where hydrocarbons react with oxygen to produce carbon dioxide and water, not involving the substitution of hydrogen with halogens.

Thus, free radical substitution aptly describes the process whereby hydrogen atoms in a molecule are systematically replaced by halogens due to the action of free radicals formed during the reaction