Part 1: Formulae, functional groups and naming

Read what a formula actually tells you, identify the reactive group and give a name that preserves the carbon skeleton.

Families and carbon skeletons

A hydrocarbon contains only carbon and hydrogen. Aliphatic compounds have straight chains, branched chains or non-aromatic rings; an alicyclic compound has a non-aromatic ring. Aromatic compounds contain aromatic rings, represented at this level by a benzene ring.

Saturated means only single carbon–carbon bonds; unsaturated compounds contain multiple carbon–carbon bonds or aromatic rings. A carbonyl C=O alone is not an unsaturated carbon skeleton in this classification.

A functional group is an atom or group responsible for characteristic reactions. A homologous series shares a functional group and general formula; successive members differ by CH₂, have similar chemistry and show gradual physical-property changes. CnH2n describes a single acyclic alkene or a saturated monocyclic hydrocarbon, so formula alone may not identify the family.

An alkyl group is derived from an acyclic alkane by removing one hydrogen and has formula CnH2n+1. The symbol R can stand for an alkyl group or another stated organic fragment; it does not represent an extra element.

Molecular, structural, displayed and skeletal formulae

A molecular formula counts each atom: C₄H₁₀O. A structural formula shows connectivity, such as CH₃CH₂CH₂CH₂OH. A displayed formula shows all atoms and bonds. A skeletal formula omits carbon symbols and attached hydrogens; every unlabelled line end or corner is a carbon. Show heteroatoms, hydrogens attached to them and any charges.

When reading a skeletal structure, count each carbon then supply enough hydrogens for carbon valency four. A double bond contributes two bonds to each carbon. In CH₃CH(OH)CH₂CH₃ the middle CH has four bonds total; writing CH₂(OH) in the same location would give too many.

Recognise the whole functional group

Number the functional group correctly

The first ten alkane stems are meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec-. Alkyl substituents end -yl: methyl, ethyl, propyl. Choose the longest appropriate chain containing the principal functional group, then number to give that group the lowest locant before considering branches.

CH₃CH(CH₃)CH₂CH₃ is 2-methylbutane. CH₃CH(OH)CH₂CH₃ is butan-2-ol. CH₂=CHCH(CH₃)CH₃ is 3-methylbut-1-ene. Use commas between numbers and hyphens between numbers and words; repeated groups use di-, tri- and so on, with substituents alphabetised ignoring those multipliers.

Aldehyde and carboxylic-acid carbons are carbon 1. For multiple groups, a carboxylic acid outranks aldehyde, ketone, alcohol and alkene for the suffix/numbering in these examples; an OH group can become hydroxy-. CH₃CH(OH)COOH is 2-hydroxypropanoic acid. In an ester, name the O-linked alkyl group first, then the acid-derived alkanoate including its carbonyl carbon.

Worked formula translation: count connections before hydrogens

Take CH₃CH(CH₃)CH₂CH₂OH. The longest chain containing OH has four carbons, but the branch adds a fifth carbon to the molecule. Count H as 3 + 1 + 3 + 2 + 2 + 1 = 12, including the hydrogen on oxygen. The molecular formula is C₅H₁₂O. Its empirical formula is also C₅H₁₂O because the three subscripts have no common factor.

Number from the OH end: HOCH₂ is carbon 1, the next CH₂ is carbon 2, and CH(CH₃) is carbon 3. The name is 3-methylbutan-1-ol. The longest chain and total carbon count answer different questions: the name can have a four-carbon stem while the molecule contains five carbons.

In a skeletal drawing, each unlabelled end or corner supplies a carbon, but a labelled O or Br replaces that implied carbon. A line ending at OH is a C–O bond to a labelled oxygen, not a hidden extra carbon followed by OH. Supply hydrogen counts only after counting single and multiple bonds at each carbon.

Use a general formula within its stated family

A general formula is a pattern, not a unique structural identification. C₅H₁₀ could describe an acyclic monoalkene or a saturated single-ring hydrocarbon. C₄H₁₀O could describe an alcohol or an ether. Decide what structural restrictions are given before applying CnH2n+2, CnH2n or CnH2n+1OH.

For saturated acyclic monohydric alcohols, CnH2n+1OH and CnH2n+2O express the same atom count. The first highlights the OH group; the second does not specify its connectivity. An empirical formula can hide even more information: C₄H₈ simplifies to CH₂, which is also the empirical formula of many different molecules.

Build a name and then reconstruct it to check

Choose the parent chain containing the principal group, number it appropriately, identify substituents and assemble their locants and names. Check a name by drawing its connectivity back: this catches a missed branch or a misplaced OH better than reading the name twice.

For CH₃CHBrCH₂CH₂OH, numbering starts at the alcohol end, giving 3-bromobutan-1-ol. For CH₃CH₂COCH₃, the carbonyl belongs within a four-carbon chain and must have the lower locant: butan-2-one. For CH₃CH₂CHO, include the CHO carbon in the chain: propanal, not ethanal.

Esters are recognised and named in the AS toolkit even though their full reaction chemistry is developed later. In CH₃CH₂COOCH₂CH₃, the oxygen-linked group is ethyl and the acid-derived group has three carbons including C=O: ethyl propanoate. Do not swap the two halves.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources