Now that we’ve covered basic substituents and alkane, alkene, and alkyne families in the previous posts, we will put it all together. Further functional families, such as ethers, carboxylic acids, amides and more, will be discussed in their own blog post. I haven’t introduced them yet to keep it simple, but they will mostly follow the same rules we have here.
There are many different rules that will arise when we name compounds. The following list describes when to use which rule, and then an example with it. At the end, we will put all of these rules together for some more trickier examples.
(a) Priority Groups
Sometimes, we will have different substituents in the same compound, and you may have the option of numbering your main chain from one direction or the other. The direction you number the main chain should be in order of the priority groups, where higher priority groups have the lower numbers. Here are the priority of common substituent groups, with the highest priority at the top and decreasing as you go down. The first column has the pictures, the second column contains the names of the groups if they were substituents, the third column is the ending of the name if that group is the main functional family, and the last column is an example.
For example, if we have this compound, with an alcohol AND a double bond group on it, we will number with the lowest number starting on the alcohol because it is higher in priority using the chart above. Additionally, this group would be named as an alcohol rather than a “hydroxy” substituent on an alkene. The name of the double bond will come in the end of the chain still, but see below on what I mean. This is how to (and how not to) name this shape:
(b) Substituents with more substituents
Sometimes, your main chain will have a substituent that is a straight chain, but suddenly branches off into another substituent!? In a previous lesson (click here), we discussed how to name longer carbon substituents that branch off, such as sec-butyl and isopropyl. However, we can also manually name these without the shortcut name when substituents branch off. An example of this is shown below. We name this 4-(1-methylethyl)nonane because we have all our substituents coming off the carbon 4, so the 4 is outside the brackets. Inside the brackets we have a substituent on a substituent, aka the blue methyl group on the 1st carbon of the green ethyl group, so it’s a 1-methylethyl on the 4 carbon. The final chain is a nonane so it’s at the end of the name.
(c) Having two of the same substituent or family in the shape
Sometimes, we will have two double bonds in an alkene, or two methyls, or three ethyls. We do not write out the same substituent twice – instead we put the number twice in front of the prefix, like when we name this picture below. These two green highlighted methyl (CH3) groups coming off the 2nd carbon are named as 2,2-dimethyl, instead of 2-methyl if we had just one methyl group.
(d) Alphabetical order
Substituents must be put in alphabetical order, but there are exceptions.
(1) Multiplier prefixes, such as “di” in the dimethyl above, or “tri”, “tetra”, etc, are ignored when determining alphabetization. You need to look at the name of the main chain connected to that prefix.
(2) Other prefixes such as “neo” (eg. neopentane), “sec” (eg. sec-butyl), “tert” (eg. tert-butyl), “cyclo” (eg. cyclopentane) are all ignored in determining alphabetization. You need to look at the name of the main chain connected to that prefix. The only exception is the “i” in isobutyl and isopropyl – this counts in the alphabetization!
(3) If you have a substituent on a substituent, such as the 4-(1-methylethyl) or 2-(3,3-dimethylethyl), these need to be alphabetized under the very first letter in the brackets, as I put in bold. Respectively, that’s under “m” for 1-methylethyl, or under “d” for dimethylethyl.