Organic chemistry

Formulae, functional groups and the homologous series

Organic chemistry is the chemistry of carbon compounds, and there are millions of them because carbon forms four strong covalent bonds and can join into chains of any length. A displayed formula draws every atom and every bond, so you can see exactly how the molecule is put together. A structural formula is an unambiguous written description of the same arrangement, such as CH2=CH2 for ethene, CH3CH2OH for ethanol and CH3COOCH3 for methyl ethanoate. Before you hand in a drawing, count the bonds on every carbon: each one must have exactly four.

Each family of organic compounds has its own general formula. The alkanes are CnH2n+2, the alkenes are CnH2n, the alcohols are CnH2n+1OH and the carboxylic acids are CnH2n+1COOH. Putting a number into the general formula gives the molecular formula of any member of the series, so an alcohol with four carbon atoms is C4H9OH, not C4H8OH.

A functional group is an atom or group of atoms that determines the chemical properties of a homologous series. The C=C double bond is what makes alkenes react by addition, the -OH group is what makes alcohols react with sodium and burn in air, and the -COOH group is what makes carboxylic acids react with metals, bases and carbonates. A homologous series is a family of similar compounds with similar chemical properties because they all contain the same functional group. Every member has the same general formula, each member differs from the next by a -CH2- unit, and there is a trend in physical properties: as the chain lengthens the boiling point rises and the molecules become less volatile and more viscous.

Two more definitions are tested every year. A saturated compound has molecules in which all the carbon-carbon bonds are single bonds, so alkanes are saturated. An unsaturated compound has one or more carbon-carbon bonds that are not single bonds, so alkenes, with their C=C, are unsaturated. Structural isomers are compounds with the same molecular formula but different structural formulae: C4H10 can be butane, CH3CH2CH2CH3, or methylpropane, CH3CH(CH3)CH3, and C4H8 can be but-1-ene, CH3CH2CH=CH2, or but-2-ene, CH3CH=CHCH3.

• Displayed formula: every atom and every bond drawn out, so the shape of the molecule is clear.
• Structural formula: an unambiguous written arrangement, for example CH3CH2OH for ethanol.
• General formulae: alkanes CnH2n+2, alkenes CnH2n, alcohols CnH2n+1OH, carboxylic acids CnH2n+1COOH.
• Homologous series: same functional group, same general formula, each member one -CH2- unit bigger than the last.
• Saturated means all carbon-carbon bonds are single; unsaturated means at least one carbon-carbon bond is not single.
• Structural isomers have the same molecular formula but different structural formulae.

Fuels: petroleum and fractional distillation

The fossil fuels are coal, natural gas and petroleum. Natural gas is mainly methane, and petroleum (crude oil) is a mixture of hydrocarbons - compounds that contain hydrogen and carbon only. Petroleum is not one substance but hundreds of different hydrocarbons mixed together, and the mixture has to be separated before it is any use.

The separation is done by fractional distillation. The petroleum is heated until it vaporises, and the vapour is passed into a fractionating column that is hot at the bottom and progressively cooler towards the top. As the vapour rises it cools, and each hydrocarbon condenses when the temperature falls to its boiling point. Because different hydrocarbons have different boiling points they condense at different heights, and each group collected at a particular height is called a fraction.

The trend from the bottom of the column to the top is worth memorising, because examiners ask for it directly. Going up the column the hydrocarbon chains get shorter, the boiling points get lower, the fractions become more volatile (they evaporate more easily) and they become less viscous (they flow more easily and are thinner). The short-chain fractions at the top are also the most flammable, which is why they are used as fuels.

Each fraction has a job. Refinery gas is used as a gas for heating and cooking, gasoline or petrol fuels cars, naphtha is a chemical feedstock, kerosene or paraffin is jet fuel, diesel oil or gas oil fuels diesel engines, fuel oil fuels ships and home heating systems, lubricating oil makes lubricants, waxes and polishes, and bitumen is used for making roads. Remember that a fraction is still a mixture, not a pure compound, and that the demand for short chains such as petrol is greater than distillation alone can supply - which is exactly why cracking is needed.

Alkanes: combustion and substitution by chlorine

Alkanes are saturated hydrocarbons: the bonding is single covalent, with each carbon joined to its neighbours by single bonds and to hydrogen by single bonds. Their general formula is CnH2n+2, so the first four are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Alkanes are generally unreactive, because the C-C and C-H bonds are strong and there is no functional group for other substances to attack.

There are two reactions you must know. The first is combustion. In a good supply of air alkanes burn completely to give carbon dioxide and water, releasing a great deal of energy, which is why they are such valuable fuels: CH4 + 2O2 -> CO2 + 2H2O. In a limited supply of air the combustion is incomplete, and the products include carbon monoxide - a colourless, odourless and poisonous gas - and carbon, which appears as soot.

The second is substitution by chlorine. In the dark, chlorine and methane do nothing at all. In ultraviolet light a photochemical reaction begins, because the ultraviolet light provides the activation energy, Ea, needed to break the chlorine molecules into reactive atoms. A chlorine atom then replaces a hydrogen atom in the methane: CH4 + Cl2 -> CH3Cl + HCl. The organic product is chloromethane and the other product is hydrogen chloride.

A substitution reaction is one in which one atom or group of atoms is replaced by another atom or group of atoms. Notice the difference from an addition reaction: here a hydrogen atom is lost as the chlorine joins, so two products are formed, and the alkane stays saturated. At this level you only need the monosubstitution product, but further substitution can happen, replacing more of the hydrogens one at a time.

• Alkanes are saturated hydrocarbons with single covalent bonds and the general formula CnH2n+2.
• They are unreactive except in combustion and in substitution by chlorine.
• Complete combustion gives carbon dioxide and water; incomplete combustion gives carbon monoxide and soot.
• Substitution: one atom or group of atoms is replaced by another atom or group of atoms.
• The reaction with chlorine is photochemical - ultraviolet light supplies the activation energy, Ea.

Alkenes: cracking, the bromine water test and addition reactions

Alkenes are unsaturated hydrocarbons. Their bonding includes a carbon-carbon double bond, C=C, and their general formula is CnH2n, so ethene is C2H4. The double bond is the functional group, and it is the reason alkenes are far more reactive than alkanes: the second pair of electrons in the double bond can open up to let new atoms join.

Most alkenes are made by cracking. Large alkane molecules from the longer fractions are broken down by a high temperature and a catalyst into smaller, more useful molecules, and some of the products are alkenes. Hydrogen is often produced as well: C10H22 -> C8H18 + C2H4. Cracking is carried out for two reasons: there is more demand for short-chain fractions such as petrol than fractional distillation alone can supply, and alkenes such as ethene are needed as a feedstock for making plastics, ethanol and many other chemicals.

The test that separates a saturated hydrocarbon from an unsaturated one is the reaction with aqueous bromine. Add bromine water, which is orange, to the unknown liquid or gas and shake. An unsaturated hydrocarbon such as ethene decolourises it, turning it from orange to colourless, because bromine adds across the double bond: C2H4 + Br2 -> C2H4Br2. A saturated hydrocarbon such as hexane leaves the bromine water orange, because it has no double bond for the bromine to add to.

The bromine reaction is an example of addition, in which only one product is formed. Two other addition reactions of ethene matter: with hydrogen in the presence of a nickel catalyst it forms ethane, C2H4 + H2 -> C2H6, and with steam in the presence of an acid catalyst it forms ethanol, C2H4 + H2O -> C2H5OH. In every case the double bond opens and the two new atoms attach themselves to the two carbons, so nothing is lost.

Alcohols: making ethanol and what it does

Alcohols contain the hydroxyl group, -OH, and have the general formula CnH2n+1OH. The one you must know is ethanol, C2H5OH. There are two ways to make it, and the exam expects you to know the conditions for both.

Fermentation. Aqueous glucose is mixed with yeast and kept at 25-35 °C with no oxygen present. The yeast contains enzymes that break the glucose down into ethanol and carbon dioxide: C6H12O6 -> 2C2H5OH + 2CO2. The conditions are not decoration - they are the reason the process works. Oxygen must be excluded because it would let the yeast respire aerobically and would oxidise the ethanol to ethanoic acid, and the temperature must be kept below about 40 °C because above that the enzymes in the yeast are denatured and fermentation stops.

Catalytic addition of steam to ethene. Ethene from cracking is mixed with steam and passed over an acid catalyst at 300 °C and 6000 kPa (about 60 atm): C2H4 + H2O -> C2H5OH. This is a continuous process that gives pure ethanol, but it depends on ethene made from crude oil and it needs a costly plant that can withstand high pressure.

Ethanol burns in air to give carbon dioxide and water, C2H5OH + 3O2 -> 2CO2 + 3H2O, and it is used as a fuel, often blended with petrol as bioethanol, and as a solvent in perfumes, cosmetics and methylated spirits, because it dissolves substances that water cannot. Ethanol can also be oxidised to ethanoic acid, either with acidified aqueous potassium manganate(VII), which changes from purple to colourless as it oxidises the alcohol, or slowly by bacterial oxidation in the air - the reaction that turns wine into vinegar.

Carboxylic acids and esters

Carboxylic acids contain the carboxyl group, -COOH, and have the general formula CnH2n+1COOH. The simplest one you need is ethanoic acid, CH3COOH, the acid in vinegar. Carboxylic acids are weak acids: they are only partly ionised in water, so a solution of ethanoic acid has a higher pH than a solution of a strong acid such as hydrochloric acid of the same concentration.

Ethanoic acid shows all the usual acid reactions. With a metal it gives a salt and hydrogen, for example 2CH3COOH + 2Na -> 2CH3COONa + H2. With a base it is neutralised to give a salt and water, for example CH3COOH + NaOH -> CH3COONa + H2O. With a carbonate it gives a salt, water and carbon dioxide, for example 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2. Because the acid is ethanoic acid, every one of these salts is an ethanoate, and you should be able to name them: sodium ethanoate, magnesium ethanoate and so on.

Ethanoic acid is made by the oxidation of ethanol. Acidified aqueous potassium manganate(VII) will do it in the laboratory, and the purple colour of the manganate(VII) disappears as the ethanol is oxidised. The same change happens slowly in air, because bacteria oxidise the ethanol in wine or beer into ethanoic acid - this bacterial oxidation is how vinegar is produced.

Carboxylic acids also react with alcohols. Warm the acid with an alcohol and a few drops of an acid catalyst and an ester is formed, together with water: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O, which is ethyl ethanoate. The ester name is built from the two partners, alcohol first and acid second. Esters are volatile and many have sweet, fruity smells, which is why they are used as flavourings and in perfumes and solvents.

• 2CH3COOH + 2Na -> 2CH3COONa + H2 - acid plus metal gives a salt and hydrogen.
• CH3COOH + NaOH -> CH3COONa + H2O - acid plus base gives a salt and water.
• 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2 - acid plus carbonate gives a salt, water and carbon dioxide.
• CH3COOH + C2H5OH -> CH3COOC2H5 + H2O - acid plus alcohol gives an ester and water, with an acid catalyst.

Polymers: addition, condensation and the plastics problem

Polymers are large molecules built up from many smaller molecules called monomers. Plastics are made from polymers. There are two ways the monomers can be joined together, and the difference between them is one of the most common exam questions in this section.

In addition polymerisation unsaturated monomers join together and nothing else is formed. The C=C double bond of each monomer opens, and the monomers link into a long chain. Poly(ethene) is made from ethene monomers: n CH2=CH2 -> (-CH2-CH2-)n. Because nothing is lost, the repeat unit contains exactly the same atoms as the monomer, so to draw it you take the monomer, open the double bond and show a free bond on each side going out to the neighbouring units.

In condensation polymerisation two different monomers react, each carrying two functional groups, and a small molecule - usually water - is eliminated at every link. A dicarboxylic acid reacting with a diamine gives a polyamide with amide linkages, -CO-NH-; this is how nylon is made. A dicarboxylic acid reacting with a diol gives a polyester with ester linkages, -CO-O-; this is how PET is made. Proteins are natural polyamides: their monomers are amino acids with the general structure H2N-CHR-COOH, where R is a different side chain in each amino acid, and they are joined by amide links. Because the linkages in a condensation polymer can be broken by hydrolysis, PET can be converted back into its monomers and re-polymerised, which is one way of recycling it.

The disposal of plastics is a serious environmental problem because most polymers are unreactive and not biodegradable, so they do not rot away. Burying them in landfill uses up land and the plastic may remain for centuries. Plastic that escapes into rivers and the sea accumulates in the oceans, where animals swallow it or become tangled in it. Burning plastics is not a simple answer either: incomplete combustion can release toxic gases such as carbon monoxide and other harmful products. The practical response is to reduce, reuse and recycle, and to remember that condensation polymers such as PET can be broken back down into monomers while addition polymers are much harder to deal with.