AS Organic Chemistry - Alkanes


Alkanes - Nomenclature

(1) Names :

The names of alkanes are composed of two parts -

(i) a prefix which comes from the number of carbon atoms in the longest straight chain in the molecule.

1 carbon atom : meth___ 19 carbon atoms : nonadec___
2 carbon atoms : eth___ 20 carbon atoms : eicos___
3 carbon atoms : prop___ 21 carbon atoms : heneicos___
4 carbon atoms : but___ 22 carbon atoms : docos___
5 carbon atoms : pent___ 23 carbon atoms : tricos___
6 carbon atoms : hex___ 24 carbon atoms : tetracos___
7 carbon atoms : hept___ 25 carbon atoms : pentacos___
8 carbon atoms : oct___ 26 carbon atoms : hexacos___
9 carbon atoms : non___ 27 carbon atoms : heptacos___
10 carbon atoms : dec___ 28 carbon atoms : octacos___
11 carbon atoms : undec___ 29 carbon atoms : nonacos___
12 carbon atoms : dodec___ 30 carbon atoms : triacont___
13 carbon atoms : tridec___ 31 carbon atoms : hentriacont___
14 carbon atoms : tetradec___ 32 carbon atoms : dotriacont___
15 carbon atoms : pentadec___ 33 carbon atoms : tritriacont___
16 carbon atoms : hexadec___ 34 carbon atoms : tetratriacont___
17 carbon atoms : heptadec___ 35 carbon atoms : pentatriacont___
18 carbon atoms : octadec___ 36 carbon atoms : hexatriacont___

(ii) a suffix which shows which type of organic compound it is. With alkanes this suffix is ___ane.

Examples -

ethane - octane -
(2) Branched alkanes :

When the alkane is not just a simple straight chain of carbon atoms joined together the names become a little more complex.

The longest connected chain of carbon atoms must be found as before and the alkane name generated as usual.

Then the name for the pendent group is found, again by counting the number of carbon atoms present, and used as a prefix.

CH3- group : methyl___
CH3CH2- group : ethyl___
CH3CH2CH2- group : propyl___
CH3CH2CH2CH2- group : butyl___

The numbers used to indicate the positions of the pendent groups must be the lowest numbers possible, so always check them from both ends of the molecule.

Example -

3-ethylpentane -

Some examples to try out -


(3) Skeletal formulae :

There are a number of different ways of representing an organic molecule, and all of them will probably examined in AS and A2 question papers.

The simplest is a molecular formula, which is simply the number of each type of atom totaled up.

A structural formula separates the atoms into groups, such as CH3 and CH2 parts of a molecule.

A displayed structure shows all the bonds between the atoms, including all the bonds going to hydrogen atoms.

A skeletal structure is similar to the displayed structure, though all the hydrogens are removed, including any bonds to hydrogen atoms, and the labels for carbon atoms are removed as well. Labels for atoms other than carbon are kept (see the nomenclature sections of other homologous series for examples of their skeletal formulae).

e.g. for propane,

molecular formula : C3H8
structural formula : CH3CH2CH3
displayed formula :
skeletal formula :

and for 3-ethylpentane the skeletal formula would look like this -


Alkanes - Reactions

(1) Combustion :

Alkanes, along with all other types of hydrocarbon, will burn in an excess of oxygen to give carbon dioxide and water only as the products,

e.g. CH4(g) + 2O2(g) arrow CO2(g) + 2H2O(g)

in general,

CnH2n+2(g) + (1.5n+0.5)O2(g) arrow nCO2(g) + (n+1)H2O(g)

(2) Halogenation -

The only other reaction that an alkane will undergo is a reaction with a halogen ( chlorine or bromine typically ) with UV light present as an initiator of the reaction,

e.g. CH4(g) + Br2(g) CH3Br(g) + HBr(g)

The UV light causes the formation of free radical halogen atoms by providing enough energy for the bond between the two halogen atoms to break.

A halogen atom attacks the alkane, substituting itself for a hydrogen atom. This substitution may occur many times in an alkane before the reaction is finished.

A similar process occurs high up in the earth's atmosphere when CFC's and other organic solvents react with intense sunlight to produce free radicals, chlorine atoms in this case. These attack molecules of ozone ( O3 ) depleting ozone's concentration and leading to the "holes".


Alkanes - Reaction Mechanism

The main reaction of alkanes involves the UV light catalysed halogenation of an alkane, e.g. methane with chlorine.

This reaction is called a free radical substitution reaction, since it involves replacing a hydrogen atom in the alkane with a halogen atom (the free radical, i.e. a species with an unpaired electron).

The reaction has three different parts to it :

(i) Initiation Cl2 -----> 2Cl.
(ii) Propagation CH4 + Cl. CH3. + HCl
CH3. + Cl2 CH3Cl + Cl.
(iii) Termination CH3. + CH3. CH3CH3
CH3. + Cl. CH3Cl
Cl. + Cl. Cl2

At the start of the propagation there is a molecule of methane and a chlorine atom and at the end of the propagation there is a molecule of chloromethane and another chlorine atom. This atom may be recycled to the beginning of the propagation step. This is a so-called chain reaction.

The termination steps are simply the combination of any two free radicals in the reaction mixture. These reactions stop the reaction as there are no free radicals produced to continue a reaction.

This reaction, as all free radical reactions are, is very hard to control. When actually tried, the chloromethane product will also have its hydrogen atoms slowly replaced by chlorine atoms, to form tetrachloromethane eventually.


Alkanes - Crude oil

(1) Fractional distillation :

Crude oil is a mixture of many different hydrocarbon compounds, some of them liquid and some of them gases. These compounds can be separated because the different length of alkanes will have different boiling points.

The crude oil is heated up to about 350 oC and is fed into a fractionating column, as in the diagram below,

The vapours with the lowest boiling points pass all the way up the column and come off as gases, e.g. methane, ethane and propane. The temperature of the column gradually decreases the higher up the vapours go, and so various fractions will condense to liquids at different heights.

The fractions with the highest boiling points do not vaporize and are collected at the bottom of the fractionating column, e.g. bitumen

Here is a table with some boiling points for the commonest fractions :

Fraction name Boiling point (oC)
Refinery gases
Gasoline
Naphtha
Kerosene
Diesel oil
Fuel oil
Bitumen
(2) Cracking :

In industry the fractions obtained from the fractional distillation of crude oil are heated at high pressure in the presence of a catalyst to produce shorter chain alkanes and alkenes.

e.g. C10H22 C5H12 + C5H10

(3) Reforming :

This is a process where straight chain alkanes are turned into branched alkanes and cyclic alkanes are turned into aromatic compounds.

Both these reactions result in the formation of chemicals that improve the performance of fuels as well as enable more exotic compounds to be made.


Alkanes - s-Bonds

Carbon forms covalent bonds in organic compounds. A single covalent bond is the sharing of two electrons between two atoms, with one electron coming from each atom in the bond.

Looking at the electron configurations for carbon and hydrogen,

H = 1s1

C = 1s22s22p2

These are the ground state configurations for the elements i.e. the configuration that has the lowest energy.


Alkanes - Uses

(2) Environmental effects :

If there is not enough oxygen present then instead of carbon dioxide, carbon monoxide, CO, is produced. Carbon monoxide is particularly toxic and absorbed into blood, through respiration, very easily. For domestic heating systems it is particularly important that enough air can get to the flame to avoid carbon monoxide being generated in the home. Car engines also require a lot of air and there is a lot of research going on to make the internal combustion engine more efficient, and so put out less carbon monoxide.

Note also that both alkanes and carbon dioxide are green house gases, i.e. they trap infra-red (i.-r.) radiation inside the Earth's atmosphere, gradually increasing global temperatures.


written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Friday, 4th February, 2011

Created with the aid of ChemWindow5.1


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