13.1 Formulas, functional groups and nomenclature

A hydrocarbon is a compound made only from carbon and hydrogen atoms. Alkanes, alkenes and alkynes are hydrocarbons; adding oxygen, nitrogen or a halogen makes a different class of organic compound.

The definition is about composition, not whether the molecule is saturated or unsaturated. Use the molecular formula and displayed structure to check the atom types.

C₃H₈ and C₃H₆ are both hydrocarbons, while C₃H₇OH is not because it contains oxygen. The first two can belong to different homologous series.

“Organic” is broader than “hydrocarbon”. Many organic compounds contain heteroatoms and are still organic.

Alkanes are hydrocarbons containing only single C–C and C–H bonds. They are saturated because each carbon has the maximum number of hydrogen atoms allowed by four covalent bonds.

The absence of a reactive functional group helps explain their relatively limited chemistry: combustion, free-radical substitution and cracking are the key syllabus reactions. Their single bonds are still covalent bonds, not “no bonding”.

Propane, CH₃CH₂CH₃, is an alkane. It can burn in oxygen or undergo chlorination under radical conditions, but it does not decolourise aqueous bromine as an alkene does.

Saturated does not mean unreactive under every condition, and an alkane is not defined by its physical state.

A functional group is the structural feature that controls a family’s typical reactions and many of its physical properties. The carbon skeleton still affects boiling point and steric access.

Identify the functional group first, then choose a reaction type. For example, an alkene C=C undergoes electrophilic addition, an alcohol can be oxidised, and a carboxylic acid can form an ester.

Ethanol and ethanoic acid both contain oxygen but behave differently because –OH and –COOH are different functional groups. Their names and reactions should not be inferred from the element list alone.

A molecule can contain more than one functional group. Do not assume one heteroatom automatically defines the whole reaction.

A general formula describes a homologous series, a structural formula shows how atoms are connected, a displayed formula shows all bonds, and a skeletal formula abbreviates the carbon framework.

Convert between representations without changing connectivity. In skeletal formulae, every line end and vertex is a carbon unless labelled otherwise, and hydrogens on carbon are implied to complete valency four.

Propene can be written C₃H₆, CH₃CH=CH₂, a displayed structure or a zig-zag skeletal drawing. All four representations describe the same molecule.

A shorter skeletal drawing is not a different compound. Check the implied carbon and hydrogen counts before naming or calculating formulae.

Systematic naming starts with the longest continuous carbon chain containing the principal functional group. Number it to give the suffix and multiple bonds the lowest possible locant, then add substituents.

Use the syllabus scope: straight-chain examples up to six carbons, with the stated limits for esters and nitriles. The suffix identifies the main functional group; prefixes describe substituents or halogens.

CH₃CH₂CH₂OH is propan-1-ol, while CH₃CH(OH)CH₃ is propan-2-ol. The number changes because the hydroxyl group is in a different position.

Do not number from the nearest end without checking the principal group, and do not treat a branch as part of the parent chain automatically.

The molecular formula counts every atom in the displayed or skeletal structure. The empirical formula is the simplest whole-number ratio of those atoms.

Count carbons and heteroatoms directly; infer hydrogens from the bonds and carbon valency. To obtain the empirical formula, divide all subscripts by their greatest common factor.

Butan-2-ol has molecular formula C₄H₁₀O. Because the subscripts have no common factor, its empirical formula is also C₄H₁₀O; C₆H₁₂ would reduce to CH₂.

Do not simplify a molecular formula when the question asks for it, and do not count the line segments in a skeletal formula as atoms.