Organic chemists, with carbon chemistry as their subject, have developed all kinds of shorthand phrases to describe structures and phenomena that might otherwise take a sentence of two to explain. Here’s today’s example: the terminology of carbon-containing functional groups: primary, secondary, tertiary, quaternary.
- Primary carbons, are carbons attached to one other carbon. (Hydrogens – although usually 3 in number in this case – are ignored in this terminology, as we shall see).
- Secondary carbons are attached to two other carbons.
- Tertiary carbons are attached to three other carbons.
- Finally, quaternary carbons are attached to four other carbons.
You can’t go higher than that. To have five substitutents, you’d need 10 electrons around carbon, a clear violation of the octet rule. When people do write 5 covalent bonds around carbon, it’s a mistake. (In the trade, these are often called Texas carbons – 1) because it resembles a star, 2) because everything is bigger in Texas, and 3) because the only man who can put five bonds on carbon is also known as Walker, Texas Ranger.)
We use the same terminology for carbocations. A primary carbocation is attached to one other carbon, a secondary to two, and a tertiary to three. You can’t have a quaternary carbocation without violating the octet rule either (you’d need an extra empty p orbital for that, bringing the total to 5).
Alcohols also follow the primary/secondary/tertiary nomenclature. The rule for alcohols is that they are named according to the number of carbons attached to the carbon bearing the hydroxyl group: in other words, whether the hydroxyl bound to a primary, secondary, or tertiary carbon. You can’t have a quaternary alcohol – again, that would involve breaking the octet rule. [A bit of non-essential nomenclature: the carbon attached to the OH is sometimes referred to as the “carbinol” carbon].
A related category of compounds are the alkyl halides, which we encounter in SN1/SN2/E1/E2 reactions (among many others). The naming for alkyl halides is similar to that for alcohols: they are named according to the number of carbons attached to the halogen, where halogen is fluorine, chlorine, bromine, or iodine.
Next, we come to amines, which are a little bit different. They are named according to the number of carbons attached to nitrogen. Primary, secondary, and tertiary amines are nitrogens bound to one, two and three carbons, respectively. Since the nitrogen has a lone pair, it is still possible to form another bond to carbon. These are called quaternary amines, although they bear a positive charge on nitrogen and are not at all basic. They are often referred to as quaternary ammonium salts. You’ll see the -ium ending quite a bit – it designates a positively charged species.
Finally, amides also fall into this category. A primary amide is bound to one carbon – the carbonyl carbon. Successive substitutions of hydrogen for carbon turn the amide into secondary and tertiary amides. [You might ask – can you have quaternary amides? Well, yes. Except nobody calls them that – they’re quite unstable, and go by another name. Why? Because. Why is the plural of goose, “geese”, but the plural of moose is “moose”? There is no satisfying answer to nomenclature questions]