Carbon and its Compounds

The One Thing

Carbon forms diverse covalent compounds because tetravalency enables four covalent bonds, while catenation enables carbon atoms to form chains, branches, and rings. Their properties and reactions depend mainly on carbon–carbon bonding, functional groups, and interactions with other substances.

Reactions, Processes and Experiments

Key Terms

• Tetravalency: The ability of a carbon atom to form four covalent bonds with other atoms.
• Catenation: The ability of carbon atoms to bond with one another to form straight chains, branched chains, and rings.
• Covalent bond: A chemical bond formed by the sharing of electrons between atoms.
• Hydrocarbon: A compound made only of carbon and hydrogen.
• Saturated hydrocarbon: A hydrocarbon containing only single carbon–carbon bonds; these are called alkanes.
• Unsaturated hydrocarbon: A hydrocarbon containing one or more double or triple carbon–carbon bonds.
• Alkane: A saturated hydrocarbon with the general formula CnH2n+2.
• Alkene: An unsaturated hydrocarbon containing at least one carbon–carbon double bond, with the general formula CnH2n for one double bond.
• Alkyne: An unsaturated hydrocarbon containing at least one carbon–carbon triple bond, with the general formula CnH2n-2 for one triple bond.
• Homologous series: A family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by –CH2–.
• Functional group: An atom or group of atoms that gives an organic compound its characteristic chemical properties.
• Alcohol: An organic compound containing the hydroxyl group, –OH; for example, ethanol, CH3CH2OH.
• Aldehyde: An organic compound containing the –CHO group at the end of a carbon chain.
• Ketone: An organic compound containing the carbonyl group, >C=O, within a carbon chain.
• Carboxylic acid: An organic compound containing the carboxyl group, –COOH; for example, ethanoic acid, CH3COOH.
• Ester: A sweet-smelling compound formed when an alcohol reacts with a carboxylic acid.
• Ethanol: An alcohol with formula C2H5OH; it is used as a solvent, fuel, and in some sanitising products.
• Ethanoic acid: A carboxylic acid with formula CH3COOH, commonly present in vinegar in dilute form.
• Esterification: The reaction of an alcohol with a carboxylic acid in the presence of concentrated sulphuric acid to form an ester and water.
• Saponification: The reaction of an ester or fat with a strong base such as sodium hydroxide to produce soap and an alcohol.
• Soap: The sodium or potassium salt of a long-chain fatty acid.
• Detergent: A synthetic cleansing agent whose molecules have a water-attracting part and a long hydrocarbon part.
• Micelle: A spherical arrangement of soap or detergent molecules in which the hydrocarbon tails trap oil and grease while the ionic ends remain in water.
• Addition reaction: A reaction in which atoms add across a double or triple bond, converting an unsaturated compound into a more saturated compound.
• Substitution reaction: A reaction in which one atom or group in a compound is replaced by another atom or group.
• Combustion: The burning of a carbon compound in oxygen, usually producing carbon dioxide, water, heat, and light.
• Oxidation: A reaction involving the addition of oxygen or removal of hydrogen; ethanol can oxidise to ethanoic acid.

Easily Confused

• Saturated and unsaturated hydrocarbons: Saturated hydrocarbons contain only single carbon–carbon bonds, whereas unsaturated hydrocarbons contain one or more double or triple bonds.
• Alkanes, alkenes and alkynes: The suffix -ane indicates a single bond, -ene a double bond, and -yne a triple bond.
• Addition and substitution reactions: Addition occurs across a double or triple bond, whereas substitution replaces one atom or group with another.
• Soap and detergent: Soap forms insoluble scum with calcium and magnesium ions in hard water, whereas detergents generally form soluble compounds and therefore work more effectively.
• Esterification and saponification: Esterification forms an ester from an alcohol and a carboxylic acid, whereas saponification uses a strong base to produce soap and an alcohol.
• Complete and incomplete combustion: Complete combustion usually produces carbon dioxide and water, whereas incomplete combustion may produce carbon monoxide and soot.
• Aldehydes and ketones: An aldehyde contains a terminal –CHO group, whereas a ketone contains a >C=O group within a carbon chain.

What Gets Asked

• Writing general formulae: Questions may require the formulae of alkanes, alkenes, or alkynes. The relevant formulae are CnH2n+2, CnH2n, and CnH2n-2, respectively; confusing the three costs marks.
• Identifying structures and names: Questions may test methane, ethane, ethene, ethyne, propane, and butane, as well as the prefixes meth-, eth-, prop-, but-, and pent-. The bond type must match the suffix -ane, -ene, or -yne.
• Classifying reactions: Combustion, oxidation, addition, substitution, esterification, and saponification may need to be identified from an equation or description. In particular, hydrogenation of ethene is an addition reaction, while chlorination of methane in sunlight is substitution.
• Completing or interpreting equations: Questions may include CH4 + 2O2 → CO2 + 2H2O + heat, oxidation of ethanol, esterification, the reaction of ethanoic acid with sodium hydrogen carbonate, or the reaction of ethanol with sodium. Omitting products such as water, carbon dioxide, or hydrogen loses marks.
• Explaining soap and detergent action: A complete explanation should refer to the negatively charged, water-attracting ionic end, the water-repelling hydrocarbon tail, micelle formation, and the difference between insoluble soap scum and soluble detergent compounds in hard water.
• Explaining physical properties: Questions may ask why carbon compounds generally have low melting and boiling points or do not conduct electricity. The required explanations are relatively weak intermolecular forces and the absence of free ions or free electrons, respectively.