Formulae, functional groups and terminology
Homologous series, functional groups, general formulae and isomerism.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.1.1Define a homologous series and describe its general characteristics.
- 11.1.2Draw and interpret displayed and structural formulae of organic molecules.
6 minute read
Formulae, functional groups and terminology
Organic chemistry can feel like learning a new language, but almost the whole topic rests on one idea: molecules with the same functional group behave the same way chemically, no matter how long their carbon chain is.
Homologous series
A homologous series is a family of compounds that: share the same general formula, differ from neighbour to neighbour by a CH₂ unit, show a gradual change in physical properties (like boiling point, which rises as the chain gets longer, since bigger molecules have stronger intermolecular forces), and have similar chemical properties because they share the same functional group. Alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols and carboxylic acids are all examples.
Functional groups
A functional group is the specific atom or group of atoms responsible for a molecule's characteristic reactions — for example the C=C double bond in alkenes, or the −OH group in alcohols. The rest of the carbon chain is comparatively unreactive, which is why all alcohols undergo similar reactions regardless of chain length.
Types of formula
- Molecular formula shows the number of each atom, e.g. C₂H₆O.
- General formula describes an entire homologous series, e.g. CₙH₂ₙ₊₂ for alkanes.
- Structural formula shows how atoms are grouped, e.g. CH₃CH₂OH.
- Displayed formula shows every atom and every bond drawn out explicitly.
Isomerism
Structural isomers are compounds with the same molecular formula but a different structural arrangement of atoms — e.g. butane and methylpropane are both C₄H₁₀ but have different carbon skeletons.
Think of it like this
A homologous series is like a family of siblings: they all share the family "surname" (functional group) that determines how they behave in the world, while just growing taller (longer chain) as you go up the family.
Worked examples
Method, step by step
C₄H₁₀ has two structural isomers. Explain what is meant by "structural isomers" and name the two isomers of C₄H₁₀.
- 1Structural isomers share the same molecular formula but have atoms arranged differently.
- 2One isomer has an unbranched 4-carbon chain: butane.
- 3The other has a branched 3-carbon chain with a methyl branch: methylpropane (2-methylpropane).
Structural isomers have the same molecular formula but different structural arrangements of atoms. The two isomers of C₄H₁₀ are butane and methylpropane.
Common misconceptions
- Thinking a longer carbon chain changes the *type* of chemical reaction a molecule undergoes — chain length mainly affects physical properties (like boiling point); the functional group controls the chemistry.
- Confusing molecular formula (same for isomers) with structural formula (different for isomers) — two isomers always share a molecular formula but never a structural one.
In the exam
- If asked to define a homologous series, you need at least two of: same general formula, same functional group, differ by CH₂, gradual change in physical properties — a one-word answer will not get full marks.
- When drawing displayed formulae, every bond (including every C–H bond) must be shown as a explicit line — omitting hydrogens is a common way to lose marks.