Concepts: IUPAC nomenclature and isomerism

3.1 Components of an IUPAC name

An IUPAC name has three parts: a word root for the length and type of the carbon chain, a suffix for the principal functional group and prefixes for substituents. The longest continuous chain containing the principal group gives the root, and the chain is numbered from the end that gives the principal group the lowest number. If there is a choice, the lower locant set for all substituents decides the numbering.

Functional groups are ranked in a fixed priority order, and only the highest priority group present is named as the suffix. The others are named as prefixes in alphabetical order. Alcohols outrank halides as suffixes, but a carboxylic acid outranks an alcohol, and the suffix changes with the group: -ol, -al, -one, -oic acid and so on. This priority order is one of the most examined items in the chapter.

When the same substituent appears more than once, prefixes di, tri and tetra are used without changing the alphabetical order of the parent substituent names. Prefixes such as cyclo, iso and neo are also ignored in alphabetisation, while sec and tert are considered part of the name. Numbers are separated from letters by hyphens and the locants within a set by commas, and the complete name is written as one word for the root part.

3.2 Isomerism

Isomers have the same molecular formula but different structures. Structural isomers differ in the way the atoms are connected: chain isomers have different carbon skeletons, position isomers have the functional group or multiple bond at different positions, and functional group isomers belong to different families such as alcohols and ethers. The number of structural isomers grows quickly with the number of carbon atoms, which is why butane has two and pentane has three chain isomers.

Stereoisomers have the same connectivity but differ in the spatial arrangement of atoms. Geometrical isomers arise when rotation is restricted, as around a carbon-carbon double bond, and are labelled cis and trans when two identical groups are on the same side or on opposite sides. Optical isomers arise when a molecule is chiral, which is usually the case for a carbon bonded to four different groups, and they rotate the plane of polarised light in opposite directions.

A useful check is to count the different groups attached to each carbon. A carbon with four different groups is a stereocentre, and a molecule with one stereocentre has two optical isomers. A double bond can show geometrical isomerism only when each carbon of the double bond carries two different groups; if any carbon has two identical groups, the two arrangements are the same molecule.

3.3 Worked examples

Name the molecule CH3-CH(CH3)-CH2-CH2-OH. The longest chain has four carbons and carries the hydroxyl group at position one, so the root is butan-1-ol. The methyl group is at position three, giving 3-methylbutan-1-ol. The hydroxyl gets the lowest number even though the methyl would get a lower number from the other end, which is the rule for the principal group.

Name the compound with a five carbon chain, a double bond between carbons two and three and a methyl group on carbon four. The root is pentene, the double bond gets the lowest number from the end nearest it, so the name is 4-methylpent-2-ene. The position of the double bond is decided before numbering substituents, and the double bond takes priority over the methyl group for locant choice.

For isomerism, consider butane and 2-methylpropane. Both have the formula C4H10 but differ in the carbon skeleton, so they are chain isomers. Consider pentanoic acid and ethyl propanoate, both C5H10O2, one a carboxylic acid and the other an ester; they are functional group isomers. Write the condensed structures side by side and compare both the skeleton and the functional group before deciding the category.

3.4 Common mistakes to avoid

The most frequent error is to choose the longest chain containing the functional group even when a longer chain exists that includes it, or to number from the wrong end. Another is to alphabetise di and tri prefixes along with the substituent names, which is wrong: the prefixes are ignored when ordering. A third is to confuse position isomerism with chain isomerism when both the skeleton and the position change, in which case the question should specify which feature is compared.

In stereochemistry the common error is to call a compound cis when the two identical groups are on different carbons of the double bond but not the ones being compared. Always mark the two groups being compared, and check whether they are on the same or opposite sides of the double bond. For optical isomerism, check that the carbon really has four different groups; a carbon with two identical groups is not a stereocentre even if it looks asymmetric in a drawing.