Preparation of haloalkanes Part 1

Preparation of haloalkanes:

Haloalkanes can be produced from virtually all organic precursors. From the perspective of industry, the most important ones are alkanes and alkenes.

From Alkanes

Alkanes react with halogens by free radical halogenation. In this reaction a hydrogen atom is removed from the alkane, then replaced by a halogen atom by reaction with a diatomic halogen molecule. The reactive intermediate in this reaction is a free radical and the reaction is called a radical chain reaction.

The chain mechanism is as follows, using the chlorination of methane as a typical example:

1. Initiation: Splitting or homolysis of a chlorine molecule to form two chlorine atoms, initiated by ultraviolet radiation or sunlight. A chlorine atom has an unpaired electron and acts as a free radical.

2. Chain propagation (two steps): a hydrogen atom is pulled off from methane leaving a 1? methyl radical. The methyl radical then pulls a Cl• from Cl₂.

This results in the desired product plus another chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction. If there is sufficient chlorine, other products such as CH₂Cl₂ may be formed.

3. Chain termination: Recombination of two free radicals:

The last possibility in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.

The net reaction is:

or

From alkenes and alkynes:

In hydrohalogenation, an alkene reacts with a dry hydrogen halide (HX) like hydrogen chloride (HCl) or hydrogen bromide (HBr) to form a mono-haloalkane. The double bond of the alkene is replaced by two new bonds, one with the halogen and one with the hydrogen atom of the hydrohalic acid.

Markovnikov's rule states that in this reaction, the halogen is more likely to become attached to the more substituted carbon. This is an electrophilic addition reaction. Water must be absent otherwise there will be a side product of a halohydrin. The reaction is necessarily to be carried out in a dry inert solvent such as CCl₄ or directly in the gaseous phase. The reaction of alkynes are similar, with the product being a geminal dihalide once again, Markovnikov's rule is followed.