Hydrocarbon frameworks rarely consist of single rings or chains, but are often branched. Rings, chains, and branches are all combined in structures like that of the marine toxin palytoxin that we met at the beginning of the chapter, polystyrene, a polymer made of six membered rings dangling from linear carbon chains, or of β-carotene, the compound that makes carrots orange.

Just like some short straight carbon chains, some short-branched carbon chains are given

names and organic element symbols. The most common is the isopropyl group. Lithium

diisopropylamide (also called LDA) is a strong base commonly used in organic synthesis.

Notice how the ‘propyl’ part of ‘isopropyl’ still indicates three carbon atoms; they are just joined together in a different way—in other words, as an isomer of the straight chain propyl group. Sometimes, to avoid confusion, the straight chain alkyl groups are called ‘n-alkyl’ (for example, n-Pr, n-Bu)—n for ‘normal’—to distinguish them from their branched counterparts. Iproniazid is an antidepressant drug with i-Pr in both structure and name. ‘Isopropyl’ may be abbreviated to i-Pr, ⁱPr, or Prⁱ. We shall use the fi rst in this book, but you may see the others used elsewhere.

The isobutyl (i-Bu) group is a CH₂ group joined to an i-Pr group. It is i-PrCH₂–. Two isobutyl groups are present in the reducing agent diisobutyl aluminium hydride (DIBAL). The pain killer ibuprofen (marketed as Nurofen) contains an isobutyl group. Notice how the invented name ibuprofen is a medley of ‘ibu’ (from i-Bu for isobutyl) + ‘pro’ (for propyl, the three- carbon unit shown in brown) + ‘fen’ (for the phenyl ring). We will talk about the way in which compounds are named later in this chapter.

There are two more isomers of the butyl group, both of which have common names and abbreviations. The sec-butyl group (s-butyl or s-Bu) has a methyl and an ethyl group joined to the same carbon atom. It appears in an organolithium compound, sec-butyl lithium, used to introduce lithium atoms into organic molecules.

The tert-butyl group (t-butyl or t-Bu) group has three methyl groups joined to the same carbon atom. Two t-Bu groups are found in butylated hydroxy toluene (BHT E321), an anti-oxidant added to some processed foods.

This quick architectural tour of some of the molecular edifices built by nature and by humans serves just as an introduction to some of the hydrocarbon frameworks you will meet in the rest of this chapter and this book. Yet, fortunately for us, however complicated the hydrocarbon framework might be, it serves only as a support for the functional groups. And, by and large, a functional group in one molecule behaves in much the same way as it does in another molecule. What we now need to do, and we start in the next section, is to introduce you to some functional groups and explain why it is that their attributes are the key to understanding organic chemistry.