Reaction of alkanes with halogens

When will an alkane react with a halogen?

In the presence of UV light

What kind of reaction is this?

Substitution reaction

What are the three steps of the mechanism?

1. Initiation

2. Propagation

3. Termination

Initiation

• Br-Br bond breaks in the presence of UV radiation

• Homolytic fission of covalent bond

• 2 bromine radicals are formed

(any halogen may substitute bromine)

What is homolytic fission?

The covalent bond breaks with each atom gaining one electron

What is a radical?

An atom with an unpaired electron

Propagation

Always two steps

1. Alkane + halogen radical —> alkyl radical + hydrogen halide

2. Alkyl radical + halogen molecule —> haloalkane + halogen radical

Number of radicals remains constant - use halogen radical then regenerated one (chain reaction)

Termination

Radicals combine by colliding together randomly

always three equations

1. halogen radical + halogen radical —> halogen

2. alkyl radical + halogen radical —> haloalkane

3. alkyl radical + alkyl radical —> alkane

Products of radical substitution of alkanes reaction

1. hydrogen halide

2. haloalkane

3. longer chain alkane

Why is a mixture of products obtained from this reaction?

• Any hydrogen atom on the carbon chain can be substituted

• Multiple hydrogen atoms can be substituted

• Various termination steps can occur