Key terms: Carbon Compounds

Precise vocabulary is what turns a nearly-right answer into a full-mark one. Learn each term below with its definition until you can state it in your own words, then check you can also give the general formula, functional group or condition it names. Keep this glossary beside the revision notes and practice questions for the chapter.

Series, formulae and structure

• Carbon compound, a compound in which carbon is combined with one or more other elements. Carbon’s ability to form four bonds gives an enormous number of these compounds.
• Hydrocarbon, a compound that contains only carbon and hydrogen. Alkanes and alkenes are hydrocarbons.
• Homologous series, a family of compounds with the same general formula and functional group, whose successive members differ by CH₂, that show similar chemical properties and a gradual change in physical properties.
• General formula, a formula that represents every member of a homologous series using n, for example CₙH₂ₙ₊₂ for alkanes.
• Functional group, the atom or group of atoms that gives a homologous series its characteristic chemical reactions, such as –OH in alcohols and –COOH in carboxylic acids.
• Molecular formula, a formula showing the actual number of atoms of each element in one molecule, such as C₂H₆O for ethanol.
• Structural formula, a representation that shows how the atoms in a molecule are joined together, including the position of the functional group.
• Saturated, describing a compound whose carbon atoms are joined by single bonds only, as in the alkanes.
• Unsaturated, describing a compound that contains at least one carbon–carbon double bond, as in the alkenes.

The homologous series

• Alkane, a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂; relatively unreactive, undergoing combustion and substitution.
• Alkene, an unsaturated hydrocarbon with the general formula CₙH₂ₙ and a carbon–carbon double bond; reactive, undergoing addition reactions.
• Alcohol, a compound containing the hydroxyl group –OH, general formula CₙH₂ₙ₊₁OH; ethanol is the key example.
• Carboxylic acid, a compound containing the carboxyl group –COOH; a weak acid that shows all the typical acid reactions.
• Ester, a compound with the linkage –COO–, formed from a carboxylic acid and an alcohol; esters have sweet, fruity smells.

Functional groups and naming

• Hydroxyl group, the –OH functional group that defines the alcohols.
• Carboxyl group, the –COOH functional group that defines the carboxylic acids.
• IUPAC nomenclature, the systematic way of naming carbon compounds, using a stem for the number of carbon atoms and a suffix for the functional group (for example, propan-1-ol).
• Locant, the number in a name that shows the position of a double bond or a functional group on the carbon chain, as the 1 in but-1-ene.
• Alkyl group, a branch derived from an alkane, such as methyl (CH₃–) or ethyl (C₂H₅–), named in a compound as a substituent.

Isomerism

• Isomerism, the existence of two or more compounds with the same molecular formula but different arrangements of atoms.
• Structural isomer, one of a set of compounds with the same molecular formula but a different structural formula, such as butane and 2-methylpropane.

Named reactions and conditions

• Combustion, the burning of a compound in oxygen; complete combustion gives carbon dioxide and water, while incomplete combustion (limited oxygen) gives carbon monoxide and soot.
• Substitution reaction, a reaction in which one atom or group replaces another; alkanes react with halogens by substitution, and only in ultraviolet light.
• Addition reaction, a reaction in which atoms add across a carbon–carbon double bond, opening it; the characteristic reaction of alkenes.
• Hydrogenation, the addition of hydrogen to an alkene using a nickel catalyst, giving an alkane; used to harden oils into fats.
• Polymerisation, the joining of many small molecules (monomers) into a long-chain molecule (polymer), as ethene forms poly(ethene) by addition polymerisation.
• Fermentation, the conversion of glucose to ethanol and carbon dioxide by enzymes in yeast, in the absence of air.
• Oxidation (of an alcohol), the addition of oxygen to ethanol, using acidified potassium dichromate(VI) (orange to green) or acidified potassium manganate(VII) (purple to colourless), giving a carboxylic acid.
• Dehydration, the removal of water from an alcohol using hot aluminium oxide or concentrated sulfuric acid, giving an alkene.
• Esterification, the reaction of a carboxylic acid with an alcohol, catalysed by concentrated sulfuric acid, to form an ester and water.
• Fats and oils, natural esters of glycerol and long-chain fatty acids; fats are saturated and solid at room temperature, oils are unsaturated and liquid.

More useful terms

• Monomer, a small molecule that joins with many others to form a polymer, such as ethene.
• Polymer, a long-chain molecule made of many repeating monomer units, such as poly(ethene).
• Catalyst, a substance that speeds up a reaction without being used up, such as nickel in hydrogenation or concentrated sulfuric acid in esterification.
• Empirical formula, the simplest whole-number ratio of atoms of each element in a compound.
• Volatility, how readily a substance evaporates; within a homologous series, volatility decreases as the molecules get larger.
• Fatty acid, a long-chain carboxylic acid that, with glycerol, forms a fat or an oil.

How to use this glossary

Test yourself by covering the definition and stating each term aloud, then by working backwards, given a general formula or a functional group, name the term. When a practice question or a marking scheme uses one of these words, make sure your own answer uses it too, and correctly. A one-to-one teacher can quiz you on the terms you confuse most, which is a quick way to tighten your written answers before the SPM Chemistry papers.