Chemistry: 10.2 Alkanes

10.2.1 Explain the low reactivity of alkanes in term of bond enthalpies and bond polarities.
Alkanes tend to be unreactive because of the relatively strong C-C and C-H bonds and because they have low polarity. They only readily undergo combustion reactions with oxygen, and substitution reactions with halogens when in ultraviolet light.

10.2.2 Describe, using equations, the complete and incomplete combustion of alkanes.
Hydrocarbon: contains only carbon and hydrogen atoms.
Complete: produces CO2 and water; some heat to initiate the reaction, plenty of oxygen
Incomplete: produces CO and water; some heat to initiate the reaction, insufficient O2 (oxygen is the limiting reagent)
General equation for the combustion of any hydrocarbon:

10.2.3 Describe, using equations, the reactions of methane and ethane with chlorine and bromine.
Alkanes can react with chlorine or other halogens in the presence of ultrviolet light. This forms hydrogen chloride and a substituted alkane. For instance, methane can react with chlorine to form chloromethane and ethane can reaction with bromine to form bromoethane.











10.2.4 Explain the reactions of methane and ethane with chlorine and bromine in terms of a free radical mechanism.
Heterolytic fission: both of the shared electrons go to one of the atoms, resulting in a positive and negative ion.

Homolytic fission: each of the two atoms that form the bond keeps one of the shared electrons, resulting in two free radicals.

Initiation: The bond between two halogen atoms is weaker than that of C-H and C-C in methane, and can break homolytically when UV light is present.

Propagation: As the free radicals each contain an unpaired electron they are highly reactive. When the chlorine free radicals come into contact with a methane molecule they each combine with a hydrogen to produce hydrogen chloride and a methyl radical. This methyl free radical is also highly reactive and so reacts with a chlorine molecule to form the product and regenerate another chlorine radical. This enables a chain reaction to occur as the process can repeat itself.

Termination: occurs when two radicals react together