12.2 - Chemical reactions of the alkanes

Question 1

Why are alkanes unreactive?

Answer 1

• C-C and C-H bonds are strong.
• C-C bonds are non-polar.
• The electronegativity of C and H is so similar that the C-H bond can be considered to be non-polar.

Question 2

Why are alkanes used as fuels?

Answer 2

Combustion of alkanes produces heat.

Also, no toxic products released.

Question 3

Combustion of methane equation

Answer 3

CH₄ + 2O_{2 →} CO₂ + H₂O

Question 4

Incomplete combustion of alkanes

Answer 4

Happens in a limited supply of oxygen.

H atoms always oxidised to water, but combustion of C may be incomplete.

Question 5

What forms during incomplete combustion of the C in alkane?

Answer 5

Carbon monoxide (CO) or soot (C)

Question 6

How to write equations for incomplete combustion?

Answer 6

Balance C/CO as normal (can’t change due to C in CH_n)

Go back to O₂ and balance it.

Question 7

What type of reaction is the bromination of alkanes?

Answer 7

Free radical substirution

Question 8

1. Initiation

Answer 8

Covalent bond in Br₂ broken by homolytic fission.

Forms 2 Br free radicals.

Question 9

What is a radical?

Answer 9

A radical is a very reactive species with an unpaired electron.

Question 10

2. Propagation

Answer 10

1. A Br radical reacts with a C-H bond in the methane, forming methyl free radical and HBr.
2. Each methyl free radical reacts with another Br₂ molecule, forming CH₃Br and a new bromine radical.
3. This new Br radical reacts with another CH₄ molecule, causing a chain reaction.

Question 11

3. Termination

Answer 11

In the termination stage, two free radicals collide, forming a molecule with all electrons paired.

When two radicals collide, both radicals are removed from reaction mixture, stopping reaction.

Question 12

Limitations of radical substitution in organic synthesis

Answer 12

• Further substitution - in propagation, CH₃Br formed (not a radical). Another radical could collide with CH₃Br, substituting further H atom. Could continue until CBr₄.
• Substitution at different points on carbon chain - if carbon chain long e.g. pentane, multiple structural isomers can be formed depending on where substitution takes place.