Alkanes are saturated hydrocarbons.
Formation of the sigma bond in alkanes:
One sp3 orbital from each carbon overlaps to forma single C-C bond called a sigma bond – rotation can occur around a sigma bond.
Properties of alkanes
Boiling point: the boiling point increases with each successive member of the homologous series as the bigger molecules causes tronger induced dipole-dipole interactions between molecules that require more energy to overcome than those found in smaller molecules. Long alkane chains have a larger surface area of contact for London Dispersion forces to form. Whereas branched chains have fewer surface contact points making them have weaker boiling points as the London Dispersion forces are weaker,
Reactivity: alkanes have low reactivity due to high bond enthalpies that require a lot of energy to overcome, making them difficult to break and react. There is also very low polarity of the sigma bonds present.
- equal distribution of electrons when a bond breaks, each atom gets one electron
- When a bond breaks in homolytic fission free radicals are formed which are high energy particles that do not carry a charge possessing an unpaired electron
- unequal distribution of electrons in which one atom gets both electrons in a pair
- A curly arrow represent the movement of th eelectron pair – which will always start from the lone pair of electrons or the centre of a bond.
- Heterolitic fission creates ions
In the presence of UV light halogen molecules or hydrogenated halogens can undergo homolytic fission to produce radicals which react with alkanes. The mechanism is called free radical substitution.There are three steps: initiation, propagation and termination.
Initiation: Essential condition is UV light, which supplies energy to break the halogen bond.
Propagation: The free radicals are very reactive and remove a hydrogen from the methane, leaving a free radical. The free radical alkyl will react with other members of the halogenous reactant to produce themain product. As the halogen radical is regenerated it can react with several more alkane molecules in a chain reaction.
Termination: this is the collision of two freeradicals which stops the process as there are no further free radicalsgenerated
Excess amounts of the halogen reactant will promote further substitution – polychlorination, polybromination.
Alkanes and their properties
Alkanes can be used as fuels, meaning that they release energy when burnt. In complete combustion the only products are carbon dioxide and water but in incomplete combustion carbon monoxide and soot are produced – incomplete combustion produces less energy per mole.
Soot can cause global dimming – reflection of the sun's light.
Carbon monoxide: it is an odourless, colourlessgas that is highly toxic that can cause death when it builds up in enclosed spaces. It forms a strong bond with haemoglobin in red blood cells which is much stronger than the bind formed with oxygen and prevents oxygen from attaching to haemoglobin resulting in less oxygen in the body and eventual suffocation.
Cracking: converting large hydrocarbons to smaller molecules by breaking the C-C bonds.
The demands for shorter carbon chain molecules are in much higher demand that long chain fractions(e.g. Petrol). To make more use of longer chains they are broken into more useful smaller ones. This processinvolves high temperatures.
Catalytic cracking – turns straight alkanes into branches cyclic alkanes and aromatic hydrocarbons.
Conditions: low pressure; high temperature (450degrees centigrade), zeolite catalyst.
This process is used for making motor fuels.Branched and cyclic hydrocarbons burn more cleanly.
Drafted by Eunice (Chemistry)