alkanes

Question 1

what is meant by the term saturated

Answer 1

containing single C-C bonds only

Question 2

describe a sigma bond in words and how do they act upon attempted rotation

Answer 2

a sigma bond is formed by the direct overlap of two orbitals. sigma bonds can rotate freely

Question 3

expain the tetrahedral shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion

Answer 3

Each C atom in an alkane is bonded to 4 other atoms. There are 4 bonded pairs of electrons around carbon, these pairs of electrons repel each other and get as far apart as possible. The bond angle is 109.5˚.

Question 4

Describe and explain the variations in boiling points of alkanes with increasing carbon-chain length (e.g. methane to ethane to propane) and separately increasing branching (e.g. hexane, 2-methylpentane, 2,2-dimethyl butane) , in terms of induced dipole–dipole interactions (London forces)?

Answer 4

As chain length increases, the boiling point increases. This is because there are more electrons and therefore the London forces between molecules are stronger. These stronger forces require more energy to overcome increasing the boiling point.

Increasing branching decreases the boiling point. This is because there are fewer point of contact between branched molecules compared to straight chains. Fewer points of contact leads to weaker London forces which require less energy to overcome decreasing the boiling point.

Question 5

2 reasons that alkanes have low reactivity

Answer 5

•saturated molecules
•non polar

Question 6

Write equations for the complete combustion and incomplete combustion of propane.

Answer 6

Complete combustion:
CH3CH2CH3 + 5O2 -> 3CO2 + 4H2O

Incomplete combustion: CH3CH2CH3 + 3.5O2 ->3CO + 4H2O

Question 7

Can you describe the limitations of radical substitution in synthesis by the formation of a mixture of organic products, in terms of further substitution and reactions at different positions in a carbon chain?

Answer 7

Radical substitution is not a good way to synthesise a haloalkane as the yield of the product is low. This is because there is a large number of additional products formed from termination reactions, multiple substitution can occur and substitution of the halogen can occur anywhere along the alkane chain forming lots of different isomers.