Addition to alkenes | Introduction to Electrophilic addition to alkenes

Reactions of alkanes | Halogenation of alkanes | Reactivity and selectivity

REACTIONS OF ALKANES

Alkanes are saturated hydrocarbons. Having only carbon and hydrogen in their structures and only carbon-carbon single bonds, these molecules are of the least polarity. Therefore, their affinity towards any nucleophile or electrophile is very low. Owing to their low reactivity, they are also known as "paraffins".

However, they still undergo some chemical reactions. An account of these is given below.

COMBUSTION

Alkanes are burnt in the presence of oxygen to yield carbon dioxide and water.

Alkanes might also react through a free radical mechanism.

FREE RADICAL MECHANISM

Free radicals are species with an unpaired electron. They are unstable just like the cations and anions and highly reactive. Free radicals cause homolytic cleavage of the bonds and the reaction proceeds in three steps:

1. Initiation step

This is the step wherein the free radicals are generated

2. Propagation step

The free radicals generated in the initiation step react with anything available to them, giving rise to new free radicals which react further. 

3. Termination step

This is the step where two free radicals may react together to form a bond and get quenched. As soon as the radicals are out of the system, the chain of reactions stop.

Free radical reactions are also known as chain reactions. 

An example of such types of reactions involving the alkanes is "halogenation"

HALOGENATION OF ALKANES

Alkanes react with halogens under light or at high temperature, giving rise to alkyl halides. This is a substitution reaction in which a hydrogen of the alkane is replaced by a halogen atom.

Let's see this with the help of an example.

In case of higher alkanes, such as propane, butane and branched alkanes, more than one products could possibly form. However, one of them will be formed predominantly in higher percentage. The reason for this is the possibility of formation of a primary, secondary or tertiary radical. Keeping in view the stability order of these radical types, tertiary radicals will be formed more than secondary, which will be produced more than the primary radical, thus giving rise to a high percentage of product originating from a tertiary radical. This type of selectivity in the formation of products is known as "Regioselectivity".

This whole concept is explained in the video at the top.