What is the mechanism of substitution reactions in alkanes?

The mechanism of substitution reactions in alkanes involves the replacement of a hydrogen atom by a halogen atom.

In a substitution reaction, one atom or group of atoms in a molecule is replaced by another atom or group of atoms. In the case of alkanes, these are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms and are filled to capacity with hydrogen atoms. The most common type of substitution reaction in alkanes is halogenation, where a hydrogen atom is replaced by a halogen atom (fluorine, chlorine, bromine, or iodine).

The mechanism of this reaction involves three stages: initiation, propagation, and termination. In the initiation stage, ultraviolet light provides the energy needed to break the bond of a halogen molecule, producing two highly reactive halogen atoms. This process is called homolytic fission.

In the propagation stage, the reactive halogen atom collides with an alkane molecule. It 'steals' a hydrogen atom from the alkane, forming a hydrogen halide and leaving behind a reactive alkyl radical. This alkyl radical can then react with another halogen molecule, replacing another hydrogen atom and producing a halogenoalkane and another halogen atom. This process can continue in a chain reaction.

Finally, in the termination stage, two radicals collide and combine to form a stable molecule, ending the chain reaction. This could be two alkyl radicals forming an alkane, two halogen atoms forming a halogen molecule, or an alkyl radical and a halogen atom forming a halogenoalkane.

It's important to note that these reactions are not very selective. In larger alkanes, the halogen can replace a hydrogen atom at any position, leading to a mixture of products. The reaction conditions, such as temperature and pressure, can influence which product is most abundant.