Organic chemistry gets a bad rap. It really isn’t that rough of a subject if you put the time in. Now, sadly, not all alkanes (simple organic compounds) are straight-chain alkanes. That would be too easy. Many alkanes are so-called branched alkanes. Branched alkanes differ from continuous-chain alkanes in that carbon chains substitute for a few hydrogen atoms along the chain. Atoms or other groups (like carbon chains) that substitute for hydrogen in an alkane are called substituents.
Naming branched alkanes is slightly more complicated than naming the continuous chain ones, but you need only to follow a simple set of steps to arrive at a proper (and often lengthy) name:
Count the longest continuous chain of carbons.
Tricky chemistry teachers often draw branched alkanes with the longest chain snaking through a few branches instead of obviously lined up in a row. Consider the two carbon structures below. The two are actually the same structure, drawn differently! Yikes. In either case, the longest continuous chain in this structure has eight carbons.One carbon structure drawn two different ways.
Number the carbons in the chain starting with the end that’s closest to a branch.
To make sure you’ve done this step correctly, you can check your work by numbering the carbon chain from the opposite end as well. The correct numbering sequence is the one in which the substituent branches extend from the lowest-numbered carbons. For example, as it’s drawn and numbered above, the alkane has substituent groups branching off of its third, fourth, and fifth carbons. If the carbon chain had been numbered backwards, these would be the fourth, fifth, and sixth carbons in the chain. Because the first set of numbers is lower, the chain is numbered properly. The longest chain in a branched alkane is called the parent chain.
Count the number of carbons in each branch.
These groups are called alkyl groups and are named by adding the suffix -yl to the appropriate alkane prefix. The three most common alkyl groups are the methyl (one carbon), ethyl (two carbons), and propyl (three carbons) groups. Figure 1 has two methyl groups, one ethyl group, and no propyl groups.
Be careful when you find yourself dealing with alkyl groups made up of more than just a few carbons. A tricky drawing may cause you to misnumber the parent chain!
Attach the number of the carbon from which each substituent branches to the front of the alkyl group name.
For example, if a group of two carbons is attached to the third carbon in a chain, like it is in Figure 1, the group is called 3-ethyl.
Check for repeated alkyl groups.
If multiple groups with the same number of carbons branch off the parent chain, don’t repeat the name. Rather, include multiple numbers, separated by commas, before the alkyl group name. Also, specify the number of instances of the alkyl group by using the prefixes di-, tri-, tetra-, and so on. For example, if one-carbon groups (in other words, methyl groups) branch off carbons four and five of the parent chain, the two methyl groups appear together as “4,5-dimethyl.”
Place the names of the substituent groups in front of the name of the parent chain in alphabetical order.
Prefixes like di-, tri-, and tetra- don’t figure into the alphabetizing, so the proper name of the organic molecule in Figure 1 is 3-ethyl-4,5-dimethyloctane.
Note that hyphens are used to connect all the naming elements except for in the last connection to the parent chain (that is, dimethyl-octane would be wrong).