Formulas, functional groups and the naming of organic compounds — common mistakes

Examiners frequently test the difference between formula types. Be precise! A 'structural formula' like $CH_3CH(Cl)CH_3$ is not a 'displayed formula'. For displayed formulae, you must draw out every single bond, including all the C-H bonds. Forgetting one is a common way to lose a mark.

With millions of organic compounds, a common or trivial name for each would be impossible to remember and would not describe the structure. The IUPAC system provides a logical, unambiguous set of rules so that any chemist worldwide can draw the exact structure of a compound from its name, and vice versa.

A structural formula shows the connectivity of atoms in a condensed way, for example, $CH_3CH_2COOH$. It tells you the structure without drawing it. A displayed formula is a full 2D drawing showing every single atom and every single bond connecting them. It is explicit and leaves no ambiguity about the bonding.

You apply a priority system. The principal functional group (e.g., -OH, -COOH) must be given the lowest possible number. If there's a choice after that, or if there is no principal functional group, you then number to give any other substituents (like alkyl groups or halogens) the lowest possible numbers.

For AS Level, you will typically encounter molecules with only one main functional group, which automatically becomes the principal group and determines the suffix. The full priority list (Carboxylic Acid > Aldehyde > Ketone > Alcohol > Alkene > Halogenoalkane) becomes more important in the A2 syllabus when dealing with polyfunctional compounds.