Chapter 12 Alkanes

Question 1

What is an alkane and what type of bonding is present?

Answer 1

Alkane are saturated hydrocarbons contains single CxC and C-H bonds

This is because sigma-bonds (overlap of orbitals directly between the bonding atoms) are present allowing free rotation around the sigma-bonds

Question 2

What shape does an alkane have? what are the bond angles of these molecules? why is this?

Answer 2

Each carbon is surrounded by 4 electron pairs in 4 regions of electron density (in the form of a sigma-bond). Repulsion between these electron pairs results in a 3D tetrahedral arrangement around each carbon atom. Each bond angle is approximately 109.5

Question 3

How does the carbon chain length influence the boiling point of alkanes?

Answer 3

London forces act between molecules that are in close surface contact.
As the chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules.
The amount of London forces between molecules will be greater and so more energy is required to overcome the forces.
Therefore the increased carbon captain length increases the boiling point of alkanes

Question 4

How does branching influence the boiling point of alkanes?

Answer 4

There are fewer surface points of contact between alkanes with branches because the branches prevent other molecules from as getting close together compared to straight chain molecules.
This means that there are fewer London forces.
This means that less energy is required to break the intermolecular forces
Therefore increased branching of alkanes decreases the boiling point.

Question 5

Why are alkanes relatively unreactive?

Answer 5

1. C-C and C-H sigma-bonds are strong
2. C-C are not polar so don’t interact with polar solvents
3. The electronegativity of carbon and hydrogen are so similar that the C-H bond can be considered as non-polar

Question 6

Write the complete combustion equation of methane?

Answer 6

CH4(g) + 2O2(g) -> CO2(g) + H2O(l)

Question 7

Write the incomplete combustion of methane. Why does incomplete combustion occur and how are the products different to complete combustion?

Answer 7

CH4(g) + 1.5 O2(g) -> CO(g) + 2 H2O(l)
Incomplete combustion takes place when there is insufficient oxygen in the system. This results in the formation of carbon monoxide (and soot which is just Carbon as a solid)
This is dangerous because carbon monoxide is a colourless, odourless and highly toxic gas that binds (irreversibly) to haemoglobin preventing oxygen from being transported around the body

Question 8

What are the three steps of radical substitution?

Answer 8

Initiation
Propagation
Termination

Question 9

What is the initiation step of the ‘bromination of methane’?

Answer 9

In initiation the bromine undergoes homolytic bond fission to form two bromine radicals

Br-Br -> 2Br.

Question 10

What happens during propagation of ‘bromination of methane’?

Answer 10

Propagation happens in two steps:
1. The radicals attack the alkane
CH4 + Br. -> .CH3 + HBr
2. The alkyl (methyl in this case) radical reacts with another bromine molecule
.CH3 + Br2 -> CH3Br + Br.

In theory the propagation steps continue until all the reactants have been used up. In practice propagation is terminated whenever two radicals collide.

Question 11

What is the possible termination steps of the ‘bromination of methane’?

Answer 11

1. Two bromine radicals collide
Br. + .Br -> Br2
2. Two methyl radicals collide
.CH3 + .CH3 -> C2H6
3. A bromine and methyl radical collide
Br. + .CH3 -> CH3Br

Question 12

What are the 3 main limitations of radical substitution in organic synthesis?

Answer 12

1. A mixture of organic products is formed
2. Further substitution can take place to form dibromomethane
3. The radical could join the carbon chain at a different point (doesn’t apply to methane and ethane but is very important in longer chained molecules)