Formulae (empirical, molecular, general, structural, displayed and skeletal), homologous series and functional groups, IUPAC nomenclature, and isomerism (structural and E/Z stereoisomerism with Cahn-Ingold-Prelog priority).

What this topic is asking

OCR specification point 4.1.1 wants you to use the different types of formula, explain what a homologous series and a functional group are, name organic compounds by IUPAC rules, and recognise the two families of isomerism: structural (chain, position and functional group) and stereoisomerism (E/Z, decided by Cahn-Ingold-Prelog priority). This is the language that every later organic topic uses.

Types of formula

Skeletal formulae are the workhorse of organic chemistry because they are quick to draw and make functional groups stand out. Each line end or vertex is a carbon, and enough hydrogens are assumed to give each carbon four bonds.

Homologous series and functional groups

The Module 4 series are the alkanes, alkenes, alcohols and haloalkanes. An alkyl group has the general formula $\text{C}_n\text{H}_{2n+1}$ (methyl, ethyl, propyl) and appears as a branch.

IUPAC nomenclature

To name a compound:

• Find the longest carbon chain containing the functional group; this gives the stem (meth, eth, prop, but, pent).
• Add the suffix for the main functional group (-ane, -ene, -ol).
• Add prefixes for substituents (methyl, chloro) with locant numbers, chosen to give the lowest set of locants.
• Use di, tri, tetra for repeated groups and list substituents alphabetically.

So $\text{CH}_3\text{CH(CH}_3)\text{CH}_3$ is 2-methylpropane and $\text{CH}_3\text{CH=CHCH}_3$ is but-2-ene.

Isomerism

E/Z isomerism needs two conditions: a $\text{C=C}$ double bond that restricts rotation, and two different groups on each carbon of the double bond.

Examples in context

Example 1. Why nomenclature matters in industry. A single molecular formula such as $\text{C}_4\text{H}_{10}$ covers butane and 2-methylpropane (isobutane), which have different boiling points and uses (isobutane is a refrigerant). Systematic naming removes ambiguity so chemists worldwide describe the same molecule.

Example 2. E/Z isomerism in vision and fats. Restricted rotation about $\text{C=C}$ bonds underlies the cis-to-trans switch of retinal in the eye and the difference between cis (healthier) and trans (less healthy) fats, both direct consequences of the geometric isomerism introduced here.

Try this

Q1. State what is meant by a homologous series. [2 marks]

• Cue. A series of compounds with the same functional group and general formula, differing by $\text{CH}_2$, with similar chemical properties and a gradual change in physical properties.

Q2. Give the molecular formula of the alkene with empirical formula $\text{CH}_2$ and three carbon atoms, and name it. [2 marks]

• Cue. $\text{C}_3\text{H}_6$, propene.