Physical and Chemical Properties
The alkanes are a homologous series of hydrocarbons with the general formula CnH2n.
Lighter members of the series are gases.Heavier members of the series are solids.
Alkanes contain only C-C and C-H bonds. Alkanes are very unreactive because of very strong bonds.
Alkanes are saturated hydrocarbons as they only contain single bonds between carbon atoms.
Bonds in alkanes are essentially non-polar and are only held together by weak van der Waal forces.
The strength of the van der Waal forces increases as the surface area of the molecule increases. Which causes the boiling point of the alkanes to increase as the number of carbon atoms increases.
The surface area of the molecule is affected by the arrangement of the carbon atoms in the molecule. larger surface area = more van der Waal forces.
Sources of Alkanes
Alkanes are the major component of crude oil and natural gas.
Crude oil is a complex mixture of hydrocarbons and can be separated by fractional distillation to produce more useful mixtures.
The crude oil is heated to around 350oC. The vapour produced enters the distillation column and cools as it rises. Compounds with different boiling points condense and are collected at different heights in the column. The products collected at different heights are known as fractions.
A fraction is a liquid mixture containing compounds collected over a range of temperature.
Refinery gas Diesel oil
Gasoline (petrol) Lubricating oil
Structure and Bonding
Structures with the same chemical formula but different arrangements of atoms are known as structural isomers.
2,2-dimethyl propane CH3C(CH3)2CH3
The name of an alkane is based on the length of the longest carbon chain in the molecule.
Chain length Type of alkane Chain length Type of alkane
1 methane 6 hexane
2 ethane 7 heptane
3 propane 8 octane
4 butane 9 nonane
5 pentane 10 decane
Hexane - the molecule has a chain of six carbon atoms.
Naming Alkanes cont.
Functional groups containing C and H atoms are known as alkyl groups. These functional groups are indicated by brackets when writing condensed formula for a molecule, and are written after the carbon atom they are attached to.
Common alkyl groups:
Methyl, -CH3 Ethyl, -C2H5 Propyl -C3H7
CH3CH(CH3)CH(CH3)CH3 2,3-dimethyl butane
A structural formula indicates how the atoms are connected it does not describe the shape of the molecule.
Naming Alkanes cont.
Key rules for naming alkanes:
- the type of alkane is determined by the length of the longest carbon chain.
- the groups attached to the longest chain are listed in alphabetical order.
- the poistion of groups attached to the longest chain is indicated using the smallest possible numbers.
Combustion of Alkanes
Burning alkanes in a plentiful supply of air (oxygen) produces carbon dioxide and water.
Complete combustion of butane,
C4H10(g) + 13/2O2(g) -> 4CO2(g) + 5H2O(l)
Burning alkanes in a limited supply of air (oxygen) produces carbon monoxide and water. (also produces soot)
Incomplete combustion of butane,
C4H10(g) + 9/2O2(g) -> 4CO(g) + 5H2O(l)
Environmental impacts of combustion
The extraction, transport and use of fossil fuels damages the environment in several ways including:
- Damage to the landscape from mining and drilling.
- Air pollution from the transport and burning of fossil fuels
- Damage to plant and aquatic life from acid-rain created by burning fossil fuels.
Leads to global warming, the green house effect and acid rain from the large quantities of sulphur dioxide and nitrogen oxides.
Catalytic converters in the exhaust from cars are used to remove the carbon monoxide and unburnt hydrocarbons. A catalytic converter contains a "honeycomb" made of ceramic material. Small particles of metals such as palladium, rhodium and platinum.
The use of leaded petrol reduces the efficiency of the catalytic converter. Lead "poisons" the catalyst by bonding strongly to the surface of the metal particles and preventing exhaust gases bonding to the surface.
Environmental impacts of combustion cont.
The metal particles catalyse:
- the oxidation of carbon monoxide
- the reduction of nitrogen oxides
- the oxidation (combustion) of unburnt hydrocarbons from petrol/diesel.
The bonding between the metal particles and exhaust gases is an example of a process known as chemisorption and is an example of a heterogeneous catalyst. When the exhaust gases chemisorb on the metal:
- chemical bonds form between the reactants and the surface of the catalyst
- the bonds within the reactants get weaker
- the activation energy for the reaction decreases as the bonding in the reactants gets weaker.
A heterogenous catalyst is a catalyst that is in a different physical state than the reactants.
Cracking of Alkanes
Cracking is breaking larger alkanes from higher boiling fractions into smaller alkanes.
Cracking reactions can be carried out by heating alkanes to around 1000oC; a process known as thermal cracking.
Cracking can also be carried out at much lower temperatures (500oC) in the presence of a catalyst; a process known as catalytic cracking. Ceramic materials known as zeolites are used a catalysts in the cracking of alkanes.
Photochemical Halogenation of Alkanes
Alkanes are very unreactive and will only react with very reactive compounds.
Halogens such as chlorine and bromine will form radicals when they absorb UV light.
The equal distributution of bonding electrons amongst the atoms forming a bond when the bond is broken is known as homolytic fission.
Eg chloromethane is formed when methane and chlorine are exposed to UV light.
CH4(g) + Cl2 (g) -> CH3Cl(g) + HCl(g)
This is an example of a photochemical reaction: a reaction that only occurs when radiation is absorbed by the reactants: therefore radiation is not a catalyst.
The photochemical chlorination of hydrocarbons such as methane is an example of a chain reaction. In a chain reaction the products of one reaction become the reactants for the next reaction.
Photochemical Halogenation of Alkanes cont.
Mechanism for the photochemical chlorination of methane: (Bromine also reacts in the same way)
Initiation Cl2 (g) -> 2Cl *
UV light produces the chlorine radicals needed to react with methane by homolytic fission of Cl2 molecules.
Propagation CH4(g) + Cl* -> CH3Cl(g) + HCl(g)
Chlorine radicals abstract hydrogen from methane to form HCl.
CH3*(g) + Cl2(g) -> CH3Cl(g) + Cl*(g)
Methyl radicals abstract chlorine atom from Cl2 to form chloromethane. This process generates a chlorine radical that can be used to react with another molecule of methane.
Termination 2CH3*(g) -> C2H6(g) 2Cl*(g) -> Cl2(g) CH3*(g) + Cl*(g) -> CH3Cl(g)
Termination reactions remover the meythl and chlorine radicals needed for the propagation steps.