4.2 Alkanes

They are saturated hydrocarbons consisting only of carbon and hydrogen joined bye single bonds only.

Main components of natural gas and crude oil. They are mainly used as fuel as they react with O2 and have low reactivity to other compounds/ elements.

Only single covalent bonds, known as sigma bonds, as a result as the end-on overlap of atomic orbitals from each bonding atom.

Alkanes are tetrahedral shape with bond angled of 109.5. There are 4 pairs of electrons in sigma bonds to hydrogen or carbon atoms.

CnH2n+2

Crude oil contains hundreds of alkanes that can be separated by cracking in fractional distillation.

They act between molecules that are in close surface contact.

Boiling point increases as chains get longer because: -Large SA so more points of surface contact. -More London forces that are greater so more energy is required to break forces of attraction.

Branched isomers of alkanes have lower boiling points because: -Less points of SA contact -Smaller chains -Fewer London forces -Branches get in way from molecules being closer together.

-C-C sigma bonds are strong -C-C bonds are non-polar -Electronegativity of C and H are similar that C-H can be considered non-polar.

All combustion process give out heat.

When there is a plentiful supply of oxygen, alkanes burn to produce carbon dioxide and water.

When there is a limited supply of oxygen, hydrogen atoms are oxidised to form water, but also carbon monoxide, which is toxic.

Carbon monoxide is a toxic, colourless and odourless gas. It combines irreversibly with haemoglobin to form carboxyhaemaglobin preventing binding oxygen being transported around the body. Its danger is the lack of colour as a person may be being poisoned but don’t know it.

In the presence of sunlight, alkanes react with halogens due to UV radiation that provides energy for a reaction.

Substitution reaction as hydrogen atom has been substituted by halogen atom.

Can be represented by a series of steps (reaction mechanism), showing how electrons move.

When the reaction is started when a covalent bond is broken by homolytic fission. Each atom of molecule take on electron forming 2 radicals. Eg., Br-Br = Br. + Br.

The reaction propagates through 2 propagation steps, a chain reaction. 1) CH4 + Br. = .Ch3 +HBr 2).Ch3 + Br2 = CH3Br + Br.

When 2 radicals collide, forming a molecule with all electron pairs. There are a number of different possible termination steps with different radicals. When 2 radicals collide, both radicals are removed, stopping the reaction.

Although we can make haloalkanes, it limits the importance of synthesis of one organic compound.

When another radical can collide with the molecule, substituting a further H atom. This can continue until all H atoms have been substituted.