4.1-4.3 - Organic formulae and homologous series

Hydrocarbons

A hydrocarbon is a compound made from hydrogen and carbon only. The word only is the trap: a compound can be organic without being a hydrocarbon if it also contains another element.

For example, C3H8 is a hydrocarbon because it contains carbon and hydrogen atoms and no other elements. CH3OH is not a hydrocarbon because it contains oxygen as well as carbon and hydrogen. C2H5Cl is not a hydrocarbon because it contains chlorine.

When a Pearson question asks whether a substance is a hydrocarbon, inspect the elements in the formula rather than the size of the molecule or whether it looks carbon-based.

Five formula types

The same organic molecule can be represented in different ways. Each type of formula hides some information and shows other information clearly.

Use an empirical formula when the ratio is the point, not the actual size of the molecule.

Use a molecular formula when you need the real atom count for one molecule.

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For a molecule with molecular formula C2H4, the empirical formula is CH2 because the ratio 2:4 simplifies to 1:2. The molecular formula stays as C2H4 because that is the actual number of carbon and hydrogen atoms in one molecule.

A general formula does not represent one particular molecule until a value of n is chosen. If a question gives the general formula $\ce{C_nH_{2n}}$ and says n = 3, the molecular formula is C3H6. In this cluster, the key skill is understanding what a general formula does; later lessons teach the named homologous series that use particular general formulae.

Structural and displayed formulae

Structural formulae and displayed formulae both show how atoms are connected. The difference is how much bond detail they show.

A structural formula is compact, so it is useful when the connectivity matters more than drawing every single bond.

Pearson exam diagrams use displayed formulae when every bond matters. A displayed formula must show bonds from carbon to hydrogen as well as bonds between carbon atoms. If a displayed formula is missing hydrogens or gives a carbon three or five bonds, it is chemically wrong.

A structural formula is more compact. For example, CH3CH2CH3 shows three carbon atoms connected in a chain without drawing every C-H bond. Branches may be written with brackets or drawn as a short side branch, but the important idea is the same: a structural formula tells you the atom connectivity.

When converting between these two forms, use carbon's usual four bonds as a check. A carbon in a displayed formula should have four bond lines around it, counting a double bond as two.