Background

The details about homologous series and functional groups covered in sub-topic 10.1 Fundamentals of organic chemistry are quite specific. The homologous series covered, where student should know the general formula (e.g. C_nH_(2n+1)OH for alcohols) are the alkanes, alkenes, alkynes, ketones, alcohols, aldehydes and carboxylic acids. The syllabus is also clear that students must be able to distinguish between class names (such as alcohol) and the functional group (e.g. hydroxyl). This means students must be able to identify the following classes of organic compounds: alkanes, alkenes, alkynes, halogenoalkanes, alcohols, ethers, aldehydes, ketones, esters, carboxylic acids, amines, amides. They must also be able to identify the functional group contained within these classes, e.g. alkyl, alkenyl, alkynyl, phenyl, hydroxyl, carbonyl, carboxyl, carboxamide, aldehyde, ester, ether, amine and nitrile. This is a significant change from previous syllabuses so not only students but also teachers will need to be careful. This is particularly the case when using past IB questions. In the past, for example, if the question had asked for the functional group present in propan-2-ol the answer would have been alcohol, now it will be hydroxyl. Similarly whereas amide might have been the correct answer to a question in the past now it would be carboxamide. As well as being able to recognise homologous series, classes of compounds and functional groups, the syllabus first published in February 2014 states that students should also be able to name compounds containing only one of the classes of functional groups: alcohols (possibly it should say hydroxyl instead of alcohol here if it is referring to functional groups), ethers, aldehydes, halogenoalkanes, ketones, esters and carboxylic acids for up to six carbon atoms.

The use of carboxamide for amide and carbonyl for a ketone functional group etc. is following what is written in the IUPAC Gold Book. However under ‘alcohols’ the Gold Book states,

“Alcohols - compounds in which a hydroxy group, –OH, is attached to a saturated carbon atom R₃COH. The term 'hydroxyl' refers to the radical species, HO^..”

This means that the functional group should probably be called hydroxy rather than hydroxyl?

When teaching functional groups and the IUPAC system of naming teachers often simply follow the syllabus and limit themselves to compounds containing up to six carbon atoms. The teaching can get rather repetitive and many of the compounds are not compounds that students can easily relate to. Consequently students often initially tend to muddle the groups up. The structures given in Sections 34, 35 and 37 in the data booklet can be really useful to reinforce identification of both classes of compounds and functional groups. Some examples to ask students are: