3.2 Aliphatic Chains

A large number of names of comounds are based on names given to straight chains of carbon atoms. Open chains of only carbon and hydrogen are called aliphatic groups, the word aliphatic meaning resembling fats and oils. Starting with a single carbon atom and counting up, the names for the first 4 saturated straight-chain aliphatic groups are methyl, ethyl, propyl, butyl.

We can recognize methyl as the name of the CH₃- groups we saw in section II, and ethyl as part of the name of ethyl alcohol. Propane and butane are familiar as fuels; indeed the saturated hydrocarbons with these numbers of carbon atoms are called methane, ethane, propane, and butane.

After these, we start counting carbon atoms using mostly Greek numbers: pentyl, hexyl, heptyl, octyl, nonyl, decyl. The pentyl group is sometimes called an amyl group, as in amyl alcohol.

After the decyl group we have undecyl (11 carbons), dodecyl (12 carbons), and so on.

There are also two accepted names of aliphatic groups that contain a double bond: vinyl or CH₂=CH- and allyl or CH₂=CHCH₂-. Later we will see how the plastic known as "vinyl" derives from the vinyl group.

All of these are called alkyl groups, and their hydrocarbons (the ones without double bonds) are called alkanes. Despite the similarity of the name, alkanes have nothing to do with alkali; many of the names of chemicals originally come from Arabic, so that's why so many of them start with al-, which just means "the".

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