Learn to read the information in an organic formula, count hidden atoms and identify the part of a molecule that controls its characteristic reactions.
Carbon structures and the meaning of a hydrocarbon
A hydrocarbon contains carbon and hydrogen only. An alcohol contains oxygen, so it is an organic compound but not a hydrocarbon. Carbon commonly forms four covalent bonds in neutral organic molecules; count a double bond as two and a triple bond as three when checking its valency.
A saturated hydrocarbon has only carbon–carbon single bonds. Unsaturation includes carbon–carbon double or triple bonds; the first examples in this course are alkenes with C=C. Rings do not automatically mean unsaturation: cyclopentane has only single bonds. Around a carbon with four single bonds the arrangement is approximately tetrahedral, with bond angles near 109.5°.
Six ways to describe the same molecule
Use butan-2-ol, CH₃CH(OH)CH₂CH₃, to compare what each representation tells you. A molecular formula cannot tell you where the OH group is attached.
| Representation | Meaning | Butan-2-ol example or drawing rule |
|---|---|---|
| Molecular | Actual number of each type of atom in one molecule | C₄H₁₀O |
| Empirical | Simplest whole-number atom ratio | C₄H₁₀O; the oxygen count prevents further simplification |
| General | Algebraic formula for a defined family | CₙH₂ₙ₊₂O for saturated, acyclic monohydric alcohols |
| Structural (condensed) | Shows how groups are connected | CH₃CH(OH)CH₂CH₃ |
| Displayed | Shows every atom and every bond | Draw all C–H and O–H bonds explicitly |
| Skeletal | Carbon framework with most C and C-bound H labels omitted | Line ends/corners are C; show O and the H of OH |
Counting atoms in a skeletal formula
Count every unlabelled corner and line end once. A labelled O, N or halogen at an end replaces a carbon at that position. Supply enough hydrogen atoms to give each neutral carbon four bonds. Hydrogens on heteroatoms such as oxygen must be shown.
For a double bond, each of the two parallel strokes joins the same pair of carbons: do not count it as two extra carbon atoms. Ring vertices count as carbons unless labelled otherwise. A plain hexagon with single edges therefore represents C₆H₁₂, not C₆H₁₄.
Diagram placeholder
Butan-2-ol in displayed and skeletal form
Labels to include:
- four-carbon chain
- OH attached to carbon 2
- all ten H atoms in displayed form
- O–H shown in skeletal form
- line ends and corners
Draw a four-carbon zigzag, then attach OH to the second carbon. The carbon-bound hydrogen counts are 3, 1, 2 and 3; the O–H hydrogen gives ten overall. In the displayed version replace each implicit hydrogen with an explicit C–H bond. This description supplies the complete structure while the labelled diagram is pending.
Homologous series and functional groups
Members of a homologous series share a functional group and a general formula. Neighbouring members differ by CH₂. Their characteristic reactions are similar, while physical properties such as boiling temperature usually change gradually as molecular size changes. A functional group is an atom or group of atoms responsible for characteristic chemical reactions.
A general formula needs a scope. CₙH₂ₙ fits both acyclic monoalkenes and saturated monocyclic hydrocarbons; it does not by itself prove that a C=C bond is present.
| Family | Structural feature | Example |
|---|---|---|
| Alkane | Only C–C and C–H single bonds; acyclic formula CₙH₂ₙ₊₂ | CH₃CH₂CH₃ |
| Alkene | C=C | CH₂=CHCH₃ |
| Halogenoalkane | C–X, with X a halogen | CH₃CH₂Cl |
| Alcohol | OH on a saturated carbon | CH₃CH₂OH |
| Aldehyde | Terminal –CHO, a carbonyl bonded to H | CH₃CHO |
| Ketone | C=O between two carbons | CH₃COCH₃ |
| Carboxylic acid | –COOH | CH₃COOH |
| Ester | –COO– connecting carbon groups | CH₃COOCH₂CH₃ |
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Give the molecular and empirical formulae of CH₃CH₂CH₂CH₂CH₂CH₃.Show answer
The molecular formula is C₆H₁₄. Divide both subscripts by two to obtain the empirical formula C₃H₇.
Q2. A saturated, acyclic alcohol contains five carbons and one OH group. Deduce its molecular formula.Show answer
Use CₙH₂ₙ₊₂O with n = 5: C₅H₁₂O. This formula does not specify the OH position or the carbon skeleton.
Q3. A skeletal chain has five unlabelled carbon positions joined by single bonds and a terminal –OH. How many hydrogens does it contain?Show answer
It represents pentan-1-ol. Carbon-bound hydrogens total 3 + 2 + 2 + 2 + 2 = 11; the OH adds one, giving C₅H₁₂O.
Q4. Identify the functional groups in CH₃CH(OH)CH₂COOH. Is it a hydrocarbon?Show answer
It contains an alcohol OH and a carboxylic acid group. The OH inside COOH belongs to the acid group; do not count it as a second alcohol. It is not a hydrocarbon because it contains oxygen.
Q5. A compound has empirical formula CH₂ and Mᵣ = 84.0. Find its molecular formula and explain whether this proves it is an alkene.Show answer
Empirical formula mass = 14.0; multiplier = 84.0 ÷ 14.0 = 6. Molecular formula = C₆H₁₂. It could also be a saturated monocyclic hydrocarbon, so the formula alone does not establish an alkene.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Introduction to organic chemistry — Coverage checklist, pp1–9. Explanations and practice on this site are original.
- AQA 7405 specification — 3.3.1: nomenclature, reaction mechanisms and isomerism.
- AQA June 2022 AS Paper 2 mark scheme — Q03, pp15–18: formula representations and isomers.
- AQA June 2022 AS Paper 2 examiner report — Q03, pp3–4: drawing and naming isomers.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.