Nomenclature of Acyclic Alkanes: Prefix

I hope you have the other component of naming alkanes down, because I am never reviewing it again (just kidding). Now for the prefix. While the parent name identifies the longest carbon chain, the prefix tells us where on that chain branches occur and what the branches look like. Depending on how much branching (and what kind) is going on, the prefix may be anywhere from nonexistent (no branches, which we sometimes denote by using "n" as a prefix) to ridiculously long.

Firstly, the location of a branch is denoted using Arabic numerals. A branch at the second carbon in the longest carbon chain gets a "2" and a branch at the third carbon gets a "3" and so on. Some carbons in the longest carbon chain might have two branches. When that happens, its number gets used twice.

Often, there are multiple possible places to start from. With alkanes, the correct starting carbon is the one which, when started from, yields the lowest possible number being named first. If we start counting on one end of a chain and the first number that comes up is for a branch at the fourth carbon, but counting from the other end of the chain would make our first branch be at the second carbon, then it is the end that would make the first branch be at the second carbon that is the correct starting point.

Also, numerals are separated from each other by commas and from the rest of the name by hyphens. That's not just for alkanes. That's a universal rule. Commit it to memory, slave.

Anyway, to specify how long a branch is, we use the wonderful numerical prefixes I introduced in my last post. You know, the ones that are mostly Greek, but not really. A branch that is only one carbon is a "methyl" group. Two carbons is an "ethyl" group, etc. A branch that is seven carbons long is a "heptyl" group (and since it's not part of the longest carbon chain, that means the longest carbon chain must be really long). This all works nicely for branches that are themselves straight. But what about branches that have branches of their own? That's the hard part. Kind of. In order for considerable branching to occur, the molecule itself has to be pretty big. I've never had to deal with such compounds myself. The textbook is covering substituents with up to four carbons and that's always been good enough for what I've had to do. There are not very many. Here we go...

Methyl group: R—CH₃
Ethyl group: R—CH₂CH₃
Propyl group: R—CH₂CH₂CH₃
Isopropyl group: R—CH(CH₃)₂
Butyl group: R—CH₂CH₂CH₃
sec-Butyl group: R—CH(CH₃)CH₂CH₃
Isobutyl group: R—CH₂CH(CH₃)₂
tert-Butyl group: R—C(CH₃)₃

If you find the condensed structures confusing for those four-carbon groups, here are some links to pictures (off-site) for the butyl variations...

Butyl, sec-butyl, isobutyl, and tert-butyl.

And that's all. Now you know how to name acyclic alkanes. Oh, one more thing. If two or more of the same type of branch exists in a molecule, those branches get named together and get a Greek numerical prefix just to confuse you even more. But really, that's it. Stay tuned for next time, where I'll do a follow-up post with some examples of naming alkanes using problems from the textbook. Oh wait, this isn't a radio. You can't tune anything. Whatever.