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As well as using a normal type of molecular formula to describe an organic molecule, they can be represented by drawing out their structure i.e. by showing how the atoms are connected, or bonded, to each other.
In order to do this a few rules have to be followed -
(i) carbon atoms must be bonded four times;
(ii) oxygen atoms must be bonded twice;
(iii) hydrogen atoms must bond only once.
|ethanoic acid :|
A homologous series is a group of organic compounds with similar chemical properties and structural formula and a gradual change in physical properties e.g. melting point and boiling point.
Below is a graph of boiling point against number of carbon atoms for the various homologous series covered in GCSE/O level Organic Chemistry:
From the graph above, it can be seen that as the number of carbon atoms in the organic compound increases the boiling points increase.
Also, the boiling points tend to follow a straight line with the higher members of each group i.e. the difference between boiling points tends towards a single value.
The four homologous series studied at IGCSE are alkanes, alkenes, alcohols and carboxylic acids. The names and formulae of these compounds will be dealt with in separate sections.
The members of each series differ from each other by the number of carbon atoms contained in the molecule,
|carboxylic acid :|
Alkanes are the simplest homologous series of compounds and their names follow this pattern,
CH4 - methane
C2H6 - ethane
C3H8 - propane
C4H10 - butane
C5H12 - pentane
i.e. they have a prefix ( meth-, eth-, prop-, but-, etc.. ) which depends on the number of carbon atoms in the molecule and a common suffix ( -ane ).
The general chemical formula for an alkane is CnH2n+2.
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.
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) → CO2(g) + 2H2O(g)
CnH2n+2(g) + (1.5n+0.5)O2(g) → nCO2(g) + (n+1)H2O(g)
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 - Global Warming.
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".
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 °C 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 the range of carbon atoms and suggested boiling point range for the most common fractions :
|Fraction name||Number of C atoms||Boiling point (oC)|
|Refinery gases||1 to 4|
|Gasoline||5 to 9|
|Naphtha||5 to 10|
|Kerosene||10 to 16|
|Diesel oil||14 to 20|
|Fuel oil||20 to 70|
|Bitumen||more than 70|
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
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.
Alkenes all have a C=C double bond in their structure and their names follow this pattern,
C2H4 - ethene
C3H6 - propene
C4H8 - butene
C5H10 - pentene
The general chemical formula for an alkene is CnH2n.
The double bond of an alkene will undergo an addition reaction with aqueous bromine to give a dibromo compound. The orange bromine water is decolourised in the process.
e.g. ethene reacts with bromine water to give 1,2-dibromoethane,
Alkenes may be turned into alkanes by reacting the alkene with hydrogen gas at a high temperature and high pressure. A nickel catalyst is also needed to accomplish this addition reaction.
e.g. ethene reacts with hydrogen to give ethane,
This reaction is also called saturation of the double bond. In ethene the carbon atoms are said to be unsaturated. In ethane the carbon atoms have the maximum number of hydrogen atoms bonded to them, and are said to be saturated.
The carbon-carbon double bond may also be oxidised i.e. have oxygen added to it. This is accomplished by using acidified potassium manganate(VII) solution at room temperature and pressure. The purple manganate(VII) solution is decolourised during the reaction.
e.g. ethene reacts with acidified potassium manganate(VII)(aq) to give ethan-1,2-diol,
All alkenes will react with free radical initiators to form polymers by a free radical addition reaction.
Some definitions -
monomer - a single unit e.g. an alkene.
The alkene monomer has the general formula :
where R is any group of atoms, e.g. R=CH3 for propene.
free radical initiator - a compound that starts a free radical reaction by producing radicals.
e.g. benzoyl peroxide or even oxygen
The reaction progresses by the separate units joining up to form giant, long chains -
polymer- a material produced from many separate single monomer units joined up together.
An addition polymer is simply named after the monomer alkene that it is prepared from,
e.g. ethene makes poly(ethene)
propene makes poly(propene)
phenylethene makes poly(phenylethene)
chloroethene makes poly(chloroethene)
methyl acrylate makes poly(methyl acrylate)
The structure above shows just 4 separate monomer units joined together. In a real polymer, however, there could be 1000's of units joined up to form the chains. This would be extremely difficult to draw out and so the structure is often shortened to a repeat unit. There are 3 stages to think about when drawing a repeat unit for a polymer-
(1) Draw the structure of the desired monomer :
(2) Change the double bond into a single bond and draw bonds going left and right from the carbon atoms :
(3) Place large brackets around the structure and a subscript n and there is the repeat unit :
|where R||= H for ethene|
|= CH3 for propene|
|= C6H5 for phenylethene|
|= Cl for chloroethene|
|= COOCH3 for methyl acrylate|
When the individual alkene units join together to give a polymer they result in the formation of long chains of carbon atoms joined together. In any sample of a polymer there are many separate chains present. These chains will be of varying lengths, depending on the number of alkene units that make them up.
These separate chains entwine with one-another, much as cooked spaghetti does, forming weak attractions between the chains - but with no actual bonds between the chains,
This arrangement of chains enables the polymer to have great flexibility, low density and an ability to be shaped and moulded when molten.
This type of polymer structure gives what is called a thermosoftening polymer - a polymer that may be melted, shaped and cooled many different times during its life.
There is another type of polymer structure though. If the individual chains are actually joined to one-another by a few covalent bonds. This gives greater strength and durability to the material,
____ = crosslink between chains
This rigidity means that once this type of polymer has been formed, the structure prevents the material from being melted. This is called a thermosetting polymer and will only char or burn when heated.
|e.g.||vulcanised rubber for car tyres|
|resins for glueing|
Alcohols all have an -OH group and their names follow this pattern,
CH3OH - methanol
C2H5OH - ethanol
C3H7OH - propanol
C4H9OH - butanol
C5H11OH - pentanol
The general chemical formula for an alcohol is CnH2n+1OH.
Ethanol is prepared in the laboratory and in the alcoholic drinks industry, by the process of fermentation. This involves the use of an enzyme (yeast) that changes a carbohydrate, e.g. sucrose, into ethanol and carbon dioxide gas,
C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g)
The yeast used requires a certain temperature to be active - somewhere between 15 and 37 °C. Too high a temperature and the yeast "dies" and too low a temperature causes the yeast to become dormant.
The production of carbon dioxide gas can be monitored by bubbling any gases produced during the reaction through limewater (calcium hydroxide(aq)). The formation of a white precipitate (calcium carbonate) in the limewater shows that carbon dioxide has been given off.
To obtain pure ethanol from the fermentation mixture, the process of fractional distillation must be carried out on the resulting solution. The equipment is shown below,
In a process similar to that of crude oil, the ethanol/water mixture can be separated by fractional distillation because of the difference in boiling points. Ethanol boils at 79 °C and water boils at 100 °C, so that ethanol boils first and therefore comes over through the condenser first. The fractionating column allows the vapours to condense and drop back down into the round-bottom flask, stopping water vapour from passing through into the condenser
Experimental sheet for the dehydration of ethanol.
All alcohols contain hydrogen and oxygen (as well as carbon) and these atoms can be removed from an alcohol as a molecule of water (H2O). This type of reaction is called dehydration. It can be accomplished by passing alcohol vapour over a heated aluminium oxide catalyst.
e.g. ethanol can be turned into ethene,
CH3CH2OH(g) → CH2=CH2(g) + H2O(g)
Experimental sheet for the oxidation of ethanol.
Oxidation can be defined as the addition of oxygen to a substance. This can be accomplished with alcohols by the use of acidified potassium dichromate(VI)(aq). This turns the alcohol into a carboxylic acid.
e.g. ethanol can be turned into ethanoic acid,
Ethanol is used in alcoholic drinks; as a solvent, e.g. methylated spirits; and as an alternate to petrol or diesel, especially in California.
Carboxylic acids all have the -COOH structural group in them and their names follow this pattern,
HCOOH - methanoic acid
CH3COOH - ethanoic acid
C2H5COOH - propanoic acid
C3H7COOH - butanoic acid
C4H9COOH - pentanoic acid
The general chemical formula for a carboxylic acid is CnH2n-1OOH.
Experimental sheet for the esterification of ethanol.
Carboxylic acids will react with alcohols to produce organic compounds called esters.
e.g. ethanoic acid and ethanol will produce ethyl ethanoate,
Some concentrated sulphuric acid is added to act as a catalyst for the reaction. It removes the water produced in the reaction, thus helping the reaction to produce more products.
Esters are used as flavourings and perfumes in all sorts of materials.
As well as the addition polymers formed from alkenes and free radical initiators already mentioned, there is another method of preparing long chain polymers.
This second method of polymerisation relies on the reaction between a dicarboxylic acid and an dialcohol ( or a diamine ) and is called condensation polymerisation since water is released during the formation of the polymer chains.
A monocarboxylic acid will react with an alcohol to give an ester ( see equation above).
If a molecule had two carboxylic acid groups on it, one at each end, and it reacted with a molecule with two -OH groups on it then many ester groups, i.e. a polyester, would be formed and long chains produced -
where the boxes represent any group of atoms.
If the dialcohol is replaced by a diamine then a polyamide or nylon is formed -
The above picture encompasses only the synthetic part of the organic work. There are a number of natural polymers required. These are :
These natural materials contain the ester link found in the synthetic polyesters shown above.
They may be hydrolysed ( broken down ) by a reaction with sodium hydroxide (a strong base) and heat.
Once hydrolysed they form soaps (sodium salts of carboxylic acids) and glycerol (propan-1,2,3-triol).
These naturally occurring materials contain the amide link found in the synthetic polyamides shown above.
These compounds may also be hydrolysed by a reaction with enzymes and/or aqueous acid. Proteins in the food we ingest are broken down by stomach acids and enzymes which work at body temperature.
Once hydrolysed they form amino acids which can then be used by the human body to prepare vital chemicals needed to sustain life.
Saponification means "soap-making" and is a reaction in which a fat, or oil, is turned into a salt of a carboxylic acid.
The oil is heated with a concentrated solution of a caustic base, such as sodium hydroxide. The base breaks down the ester links, forming alcohol groups and carboxylate ion groups on different molecules.
This picture summarises all the chemistry mentioned above and links back to the relevant sections. Simply click over a particular chemical or reaction to transfer to it.
Written by Dr Richard Clarkson : © Saturday, 1 November 1997
Updated : Saturday, 3rd March, 2012
Created with the aid of and