29.1 — Formulas, Functional Groups and the Naming of Organic Compounds

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand that the compounds in the A2 functional groups table each contain a functional group that controls their physical and chemical properties.
  2. Interpret and use the general, structural, displayed, and skeletal formulas of these compounds.
  3. Use systematic naming (nomenclature) for simple aliphatic (non-aromatic) organic molecules — including cyclic molecules with one ring of up to six carbons — with up to six carbons in the chain (six plus six for esters and amides; straight chains only for esters and nitriles).
  4. Use systematic naming for simple aromatic molecules containing one benzene ring with one or more substituents (e.g. 3-nitrobenzoic acid, 2,4,6-tribromophenol).

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Section 1: What Is a Functional Group?

A functional group is a specific atom or group of atoms within an organic molecule that gives the molecule its characteristic behaviour.

Think of it like this: the rest of the molecule is just a carbon-hydrogen "backbone" (called a hydrocarbon chain), but the functional group is the "active part" — it decides:

  • How the molecule reacts chemically (what reactions it undergoes)
  • What physical properties it has (boiling point, solubility in water, smell, and so on)

Two molecules can have the same number of carbon atoms but completely different properties if they have different functional groups. This is why identifying the functional group is one of the first and most important steps in organic chemistry.

Section 2: The A2 Functional Groups — Properties and Features

Below are the key functional groups you need to know at A2 level. For each one, you will learn: what it is, what its functional group looks like, and a summary of its chemical and physical properties.

2.1 Arenes

What they are: Arenes are organic molecules that contain a benzene ring. The benzene ring is a ring of six carbon atoms where the electrons are spread out (delocalised) evenly around the ring — this is shown using a hexagon with a circle drawn inside it.

The functional group: A benzene ring — drawn as a regular hexagon with a circle inside.

Chemical properties:

  • The ring has a cloud of electrons (called a π system — pronounced "pi system") spread evenly around it. This makes benzene electron-rich, meaning it attracts particles that are hungry for electrons (called electrophiles).
  • Because of this delocalised electron cloud, benzene is actually very stable — more stable than you might expect.
  • Unlike alkenes (which react readily by addition, where atoms add across a double bond), benzene resists addition reactions. Instead, it prefers electrophilic substitution reactions, where a hydrogen on the ring is swapped for another group.

Physical properties:

  • Benzene molecules are held together by van der Waals forces (weak attractions between temporary dipoles), giving it a boiling point of 80 °C.
  • Benzene is non-polar (no uneven charge distribution), so it does not dissolve in water. Water molecules are held together by strong hydrogen bonds (a type of strong intermolecular force), and it is not energetically favourable for benzene to break these bonds.

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