12. Alkanes
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What is the functional group of alkanes?
CnH2n+2
Is alkane saturated or unsaturated?
Saturated
Why is alkane saturated?
Because it only contains single bonds/sigma bonds.
Which alkanes don’t have any isomers?
• methane • ethane • propane
Are alkanes polar or non-polar? Why?
Non-polar, because: 1. the electronegativity of hydrogen and carbon are very similar 2. the intermolecular forces between molecules are weak London forces
How do the lengths of chain affect the melting point and boiling point of alkanes? Why?
As the chain is longer, the boiling point increases, because: 1. greater surface area of contact 2. stronger London forces
How do branches affect the melting point and boiling point of alkanes? Why?
Alkanes with branches have lower melting points, because: 1. less surface area of contact 2. weaker London forces
Are alkanes soluble or insoluble?
Insoluble
Are alkanes reactive? Why?
No, because they have strong carbon-carbon bonds.
What is the equation of the complete combustion of alkanes?
Alkane + oxygen > carbon dioxide + water
What is the equation of the incomplete combustion of alkanes?
Alkane + oxygen > carbon monoxide + water
What is the equation of the free radical substitution reaction of alkanes?
Alkane + halogen + UV light > haloalkane + hydrogen halide
What is required to start the free radical substitution reaction of alkanes?
UV light
What are the three stages in a free radical substitution reaction?
• initiation • propagation • termination
What happens in initiation in a free radical substitution reaction?
The covalent bond in a halogen molecule is broken by homolytic fission. This results in each halogen atom takes one electron from the pair.
What is homolytic fission?
The even breaking of a covalent bond which each bonding atom receives one electron from the bonded pair, forming two radicals.
What happens in propagation in a free radical substitution reaction?
• a halogen free radical takes a hydrogen atom from the alkane to form hydrogen halogen and a methyl free radical. • the methyl free radical is also reactive and reacts with a halogen molecule to form halogenoalkane and a new halogen free radical
What happens in termination in a free radical substitution reaction?
To remove free radicals: 1. two halogen free radicals react together to give a halogen molecule 2. two methyl free radicals react together to give an alkane 3. one halogen free radical reacts with one methyl free radical to give a halogenoalkane
Why is free radical substitution reaction not a suitable way for making halogenoalkane?
Because a mixture of isomers will be formed which are difficult to separate.