Revision notes: Carbon Compounds

These notes work through Carbon Compounds one content standard at a time so you can revise each series, check the definitions the marking scheme expects, and practise naming and equation-writing. Keep them beside past-paper questions and use them to confirm your answers. This chapter rewards precision: the right general formula, the right conditions, and a balanced equation every time.

10.1–10.2 Carbon compounds and the homologous series

A carbon compound contains carbon combined with other elements; a hydrocarbon contains only carbon and hydrogen. Most fuels, petroleum and natural gas, are mixtures of hydrocarbons. A homologous series is a family of compounds that:

• share the same general formula;
• have the same functional group and similar chemical properties;
• show a gradual change in physical properties (boiling point, melting point and density rise, while volatility falls, as molecular size increases);
• differ from the next member by a CH₂ unit; and
• can be prepared by similar methods.

Learn the general formulae. Alkanes CₙH₂ₙ₊₂; alkenes CₙH₂ₙ; alcohols CₙH₂ₙ₊₁OH; carboxylic acids CₙH₂ₙ₊₁COOH. Knowing these lets you deduce a molecular formula from the number of carbon atoms.

10.2 IUPAC nomenclature

An IUPAC name has a stem for the number of carbon atoms in the longest chain and a suffix for the functional group.

• Stems: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
• Suffixes: -ane (alkane), -ene (alkene, with a number showing where the C=C double bond starts), -ol (alcohol), -oic acid (carboxylic acid).
• Branches are named as alkyl groups (methyl, ethyl) with a locant, e.g. 2-methylpropane.

Practise naming from a structure and drawing a structure from a name, both directions are examined.

10.3 Alkanes

Alkanes are saturated hydrocarbons, every carbon–carbon bond is a single bond. They are relatively unreactive. Two reactions matter:

• Combustion. Complete combustion in plenty of air gives carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O. In limited air, incomplete combustion gives carbon monoxide and soot (carbon).
• Substitution. With a halogen in the presence of ultraviolet light or sunlight, a hydrogen atom is replaced: CH₄ + Cl₂ → CH₃Cl + HCl. Light is the essential condition.

10.4 Alkenes

Alkenes are unsaturated hydrocarbons, they contain a carbon–carbon double bond (C=C), which is the functional group and the reactive site. Their typical reactions are addition reactions, in which the double bond opens and atoms add across it:

• Combustion, burns with a sootier, yellow flame because of a higher carbon content.
• Hydrogenation, adds hydrogen with a nickel catalyst at about 180 °C to give an alkane.
• Halogenation, adds bromine; ethene + Br₂ → 1,2-dibromoethane. This decolourises brown bromine water at room temperature.
• Addition of hydrogen halide, e.g. ethene + HBr → bromoethane.
• Hydration, adds steam with a phosphoric acid catalyst to form an alcohol.
• Polymerisation, many ethene molecules join to form poly(ethene).

Distinguishing an alkane from an alkene

This is a favourite question. Shake each gas or liquid with bromine water or with acidified potassium manganate(VII):

• The alkene decolourises brown bromine water quickly at room temperature, and decolourises purple acidified potassium manganate(VII).
• The alkane gives no change with either reagent (bromine reacts with an alkane only in ultraviolet light).

10.5 Isomerism

Structural isomers have the same molecular formula but a different structural formula, so their atoms are joined in a different order. For example, C₄H₁₀ exists as butane (a straight chain) and 2-methylpropane (a branched chain), and C₅H₁₂ has three isomers. When a question asks you to draw isomers:

• keep the molecular formula fixed;
• rearrange the carbon skeleton (straight versus branched) or, for alkenes and alcohols, move the position of the double bond or the –OH group;
• give the correct IUPAC name for each; and
• check every carbon has four bonds and no two structures are the same molecule drawn differently.

10.6 Alcohols

Alcohols contain the hydroxyl group –OH. Ethanol, C₂H₅OH, is the key example. It is made in two ways:

• Fermentation, glucose is converted by enzymes in yeast, in the absence of air, to ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
• Industrial hydration, steam is added to ethene with a phosphoric acid catalyst.

Reactions of alcohols to know:

• Combustion, burns completely to carbon dioxide and water.
• Oxidation, acidified potassium dichromate(VI) (orange → green) or acidified potassium manganate(VII) (purple → colourless) oxidises ethanol to ethanoic acid.
• Dehydration, passing ethanol vapour over hot aluminium oxide, or heating with concentrated sulfuric acid, removes water to give ethene.

10.7 Carboxylic acids

Carboxylic acids contain the carboxyl group –COOH. Ethanoic acid, CH₃COOH, is a weak acid but shows all the typical acid reactions:

• with a reactive metal → salt + hydrogen;
• with a carbonate → salt + water + carbon dioxide;
• with a base or alkali → salt + water; and
• with an alcohol → an ester (esterification).

10.8 Esters, fats and oils

An ester is formed when a carboxylic acid reacts with an alcohol, using concentrated sulfuric acid as catalyst, this is esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Esters are named alkyl alkanoate (here, ethyl ethanoate). They have sweet, fruity smells and are used in flavourings and perfumes. Fats and oils are natural esters of glycerol and long-chain fatty acids: fats are saturated and solid at room temperature, oils are unsaturated and liquid. Adding hydrogen to an oil (hydrogenation) raises its melting point and is used to make margarine.

How to use these notes

Revise one standard at a time and quiz yourself out loud. Write out each general formula, name five structures and draw them back from the name, and rehearse the reaction conditions until they are automatic, the catalyst, the reagent, the colour change. Then attempt the practice questions for this chapter and mark yourself against the equations above. A one-to-one teacher can check that your isomers are genuinely different and your equations are balanced, the two places students most often drop marks. Because Carbon Compounds threads naming, functional groups and organic reactions together, securing it strengthens your whole SPM Chemistry preparation.