Reaction Mechanisms - Substitution of Alkanes - Arrow Formalism

Question 1

What is the substitution reaction of alkanes?

Answer 1

Halogenation of alkanes

A chlorination reaction

Question 2

What are the three steps to a free-radical chain reaction?

Answer 2

Chain initiation step

Chain propagation step

Chain termination step

Question 3

Describe the chain initiation step

2

Answer 3

There’s a Cl2 molecule

The Cl-Cl bond breaks in the presence of light or heat

This forms two chlorine free radicals

Each atom retains one of the two electrons shared in the bond (homolytic fission)

Question 4

Describe the first chain propagation step

2

Answer 4

A chlorine atom then attacks a methane molecule (any alkane)

This forms a methyl free radical

Question 5

Describe the second chain propagation step.

3

Answer 5

The methyl free radical attacks a chlorine molecule

This forms chloromethane and a Cl atom

The Cl atom can now attack a methane like in step 2 to set up a chain reaction

Question 6

Describe the termination step.

Answer 6

There is three ways that the chain reaction can come to an end, all involve the free radicals or atoms combining with each other to form molecules

Question 7

What three molecules can bring about the termination of a chain reaction?

Answer 7

Chlorine (Chlorine)

Chloromethane (CH3Cl)

Ethane (C2H6)