4.7.2.1Structure and formulae of alkenes
Chemistry only — not assessed in Combined Science: Trilogy.
Alkenes are hydrocarbons with a double carbon-carbon bond. The general formula for the homologous series of alkenes is CnH₂ₙ.
Alkene molecules are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms. Ethene is C₂H₄ where ethane is C₂H₆ — the two missing hydrogens are the space the second bond occupies.
The first four members of the homologous series of alkenes are ethene, propene, butene and pentene.
Alkene molecules can be represented as a molecular formula such as C₃H₆, or as a displayed structural formula showing the double bond as two parallel lines between two carbon atoms.
You should be able to recognise substances that are alkenes from their names or from given formulae in these forms. From a name, the ending -ene says alkene. From a molecular formula, check whether it fits CnH₂ₙ. From a displayed formula, look for the C=C. You do not need to know the names of individual alkenes other than ethene, propene, butene and pentene.
- Alkene
- a hydrocarbon with a double carbon-carbon bond, general formula CnH₂ₙ
- Unsaturated
- describes a molecule containing a carbon-carbon double bond, so it has two fewer hydrogen atoms than the alkane with the same number of carbons
- Homologous series
- a family of compounds with the same general formula and similar chemical properties
4.7.2.2Reactions of alkenes
Chemistry only — not assessed in Combined Science: Trilogy.
Alkenes are hydrocarbons with the functional group C=C. It is the generality of reactions of functional groups that determines the reactions of organic compounds — which is why learning one reaction of the C=C bond tells you how every alkene behaves.
Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.
Alkenes react with hydrogen, water and the halogens by the addition of atoms across the carbon-carbon double bond, so that the double bond becomes a single carbon-carbon bond.
You should be able to describe the reactions and conditions for the addition of hydrogen, water and halogens to alkenes, and to draw fully displayed structural formulae of the first four members of the alkenes and the products of their addition reactions with hydrogen, water, chlorine, bromine and iodine.
Drawing an addition product is mechanical once the idea is clear. Redraw the alkene with the double bond replaced by a single bond, then attach one atom or group from the reagent to each of those two carbon atoms. Every carbon must still have four bonds when you have finished.
Worked example
Draw the displayed formula of the product when propene reacts with bromine, and give its name.
- Propene is CH₂=CH–CH₃. Identify the two carbon atoms joined by the double bond — the first and second.
- Replace the double bond with a single bond between those two carbons.
- Bromine is Br₂, so add one bromine atom to each of those two carbons.
- Check every carbon has four bonds: carbon 1 has two H, one Br and one C; carbon 2 has one H, one Br and two C; carbon 3 has three H and one C.
- Number from the end that gives the bromines the lower positions.
Answer: CH₂Br–CHBr–CH₃, drawn with every bond as a line — 1,2-dibromopropane. The orange bromine is used up, which is why the bromine water is decolourised.
Common mistake
Nothing is lost in an addition reaction — every atom of both reactants ends up in the single product. If your product has fewer atoms than the two reactants together, a hydrogen has gone missing.
- Functional group
- the group of atoms in a molecule that determines how it reacts; it is the generality of reactions of functional groups that determines the reactions of organic compounds
- Addition reaction
- a reaction in which atoms add across a carbon-carbon double bond so that the double bond becomes a single bond
4.7.2.3Alcohols
Chemistry only — not assessed in Combined Science: Trilogy.
Alcohols contain the functional group –OH. Methanol, ethanol, propanol and butanol are the first four members of a homologous series of alcohols.
Alcohols can be represented as a formula such as CH₃CH₂OH, or as a displayed structural formula in which the –OH group is drawn out. You should be able to recognise alcohols from their names or from given formulae — the ending -ol in a name, or an –OH group in a formula. You do not need to know the names of individual alcohols other than methanol, ethanol, propanol and butanol.
You should be able to describe what happens when any of the first four alcohols react with sodium, burn in air, are added to water, and react with an oxidising agent, and recall the main uses of these alcohols.
The main uses: ethanol is the alcohol in alcoholic drinks, and is also used as a fuel — added to petrol — and methanol is used as a feedstock in the chemical industry. Both ethanol and methanol are widely used as solvents, because they dissolve substances that water will not.
You are not expected to write balanced chemical equations for the reactions of alcohols other than for combustion reactions. For example, ethanol burns as C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Aqueous solutions of ethanol are produced when sugar solutions are fermented using yeast, and you should know the conditions used.
The conditions are a warm temperature of about 25–35 °C, an aqueous solution of sugar, yeast to provide the enzymes, and the absence of oxygen — anaerobic conditions. Too cold and the reaction is very slow; too hot and the enzymes in the yeast are denatured, so fermentation stops altogether.
Common mistake
Fermentation produces an aqueous solution of ethanol, not pure ethanol. To obtain concentrated ethanol the solution has to be separated afterwards by fractional distillation.
- Alcohol
- an organic compound containing the functional group –OH; the first four are methanol, ethanol, propanol and butanol
4.7.2.4Carboxylic acids
Chemistry only — not assessed in Combined Science: Trilogy.
Carboxylic acids have the functional group –COOH. The first four members of a homologous series of carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
The structures of carboxylic acids can be represented as a formula such as CH₃COOH, or as a displayed structural formula showing the carbon double bonded to one oxygen and single bonded to an –OH.
You should be able to recognise carboxylic acids from their names or from given formulae — the ending -anoic acid, or a –COOH group. You do not need to know the names of individual carboxylic acids other than those four.
You should be able to describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, and react with alcohols. You are not expected to write balanced chemical equations for the reactions of carboxylic acids.
- Carboxylic acid
- an organic compound containing the functional group –COOH; the first four are methanoic, ethanoic, propanoic and butanoic acid
4.7.2.4Why carboxylic acids are weak acidsHigher tier
Chemistry only — not assessed in Combined Science: Trilogy.
Higher tier students should be able to explain why carboxylic acids are weak acids in terms of ionisation and pH — the ideas met under strong and weak acids.
The explanation: a carboxylic acid is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions. A strong acid such as hydrochloric acid is completely ionised, so every molecule releases its hydrogen ion. At the same concentration, the carboxylic acid therefore has a much lower hydrogen ion concentration, and so a higher pH.
Worked example
Solutions of hydrochloric acid and ethanoic acid have the same concentration, but the ethanoic acid has a pH three units higher. Explain.
- Both solutions contain the same amount of acid in the same volume — they are equally concentrated.
- Hydrochloric acid is a strong acid and is completely ionised, so almost all of it has released hydrogen ions.
- Ethanoic acid is a weak acid and is only partially ionised, so only a small fraction of its molecules have released hydrogen ions.
- A pH three units higher means a hydrogen ion concentration 1000 times lower, because each pH unit is a factor of 10.
Answer: Equal concentration but unequal ionisation: the ethanoic acid releases about a thousandth as many hydrogen ions, so its pH is three units higher. Concentration and strength are separate ideas.
Before you practise
- This sub-topic is chemistry only — it is not assessed in Combined Science: Trilogy.
- Alkenes have a C=C double bond and the general formula CnH₂ₙ: ethene, propene, butene, pentene.
- Alkenes are unsaturated — two fewer hydrogens than the alkane with the same number of carbons — and burn with smoky flames because of incomplete combustion.
- Addition across the double bond: hydrogen with a nickel catalyst gives the alkane; steam with a phosphoric acid catalyst gives an alcohol; halogens react at room temperature.
- Alcohols have the functional group –OH: methanol, ethanol, propanol, butanol.
- Alcohols react with sodium giving hydrogen, burn in air, dissolve in water to give neutral solutions, and are oxidised to carboxylic acids.
- Fermentation of sugar with yeast, warm and without oxygen, gives an aqueous solution of ethanol.
- Carboxylic acids have the functional group –COOH; with carbonates they give carbon dioxide, and with alcohols they give esters such as ethyl ethanoate.
- Higher tier: carboxylic acids are weak acids because they are only partially ionised, so the pH is higher than a strong acid of the same concentration.