Chlorination of alkanes

Question 1

What is a halogenoalkane?

Answer 1

An alkane with a halogen attached.

Question 2

What does the halogen replace in the alkane?

Answer 2

Hydrogen(s)

Question 3

In what process are halogenoalkanes formed in (chlorination of alkanes)

Answer 3

free-radical substitution

Question 4

What formula MUST free-radical equations be shown in?

Answer 4

Structural formula

Question 5

What is a free-radical substitution?

Answer 5

Free radical substitution: a type of substitution where a radical
replaces a different atom or group of atoms.

Question 6

What is a free-radical?

Answer 6

a species with an unpaired electron in
their outer shell - this unpaired electron makes them very reactive

Question 7

Demonstrate the first step of the chlorination of methane

Answer 7

1 - Initiation
Free-radical of halogen is formed

In the first step the halogen will break down to become the free-radical, so:
Cl2 –> 2Cl*
The dot in front of Cl represents a single unpaired electron. When we shine UV light onto the halogen molecule, the bond between them break equally and the electrons are shared between the two atoms.

Question 8

Demonstrate the second step of the chlorination of methane

Answer 8

2 - Propagation
A free-radical reacts and a new free-radical forms

Cl* + CH4 –> HCl + *CH3
The starting molecule (methane in this case) reacts with the chlorine free-radical. The chlorine free-radical is very reactive so it removes a hydrogen atom from the methane and forms HCl. This leaves us with a methyl free-radical. The dot representing the single unpaired electron must be drawn on the left next to carbon.

.CH3 + Cl2 –> CH3Cl + Cl.
The methyl free-radical reacts with the chlorine molecule to form the halogenoalkane (chloromethane) and a chlorine free-radical.

Looking at both equations, the *Cl free-radical appears at the start and end which means that it can be used over and over again until it it used up. The methyl free-radical also appears at the start and end. By removing these two radicals from both equations and adding up the reactants and products we are left with the equation:
CH4 + Cl2 –> HCl + CH3Cl

Question 9

Demonstrate the second step of the chlorination of methane

Answer 9

3 - Termination
Two free radicals react together to make a stable molecule, there are a few ways this can happen:

1. Cl* + Cl* –> Cl2
   Two chlorine free-radicals could react to form a chlorine molecule(can be further broken down by UV light so this reaction would not terminate)
2. *Cl + *CH3 –> CH3Cl
   A chlorine free-radical could react with a methyl free-radical to form chloromethane (more of desired product)
3. *CH3 + *CH3 –> CH3CH3
   A methyl free-radical could react with another methyl free-radical and form ethane(C2H6) (completely different product)