IUPAC Naming

From the beginning of documented history, the human race has sought to classify and label chemical compounds, even before these compounds were properly understood. Urea is one example of a complex chemical compound named after where it was originally found (in urine). Naming compounds in this manner was acceptable when the number of known compounds was small. When it became apparent that there existed an infinite number of compounds, a new naming system had to be developed in order to distinguish the compounds from each other. Nomenclature of alkanes, or saturated hydrocarbons, is no exception.

            Straight-chain alkanes are named according to the number of carbon atoms they contain. The prefixes below make the basic part of naming fairly easy. Once the number of carbons in the chain has been determined, the appropriate prefix can be picked and combined with the suffix -ane.

Number of Carbons (n)	Name	Formula (CnH2n+2)
1	Methane	CH4
2	Ethane	C2H6
3	Propane	C3H8
4	Butane	C4H10
5	Pentane	C5H12
6	Hexane	C6H14
7	Heptane	C7H16
8	Octane	C8H18
9	Nonane	C9H20
10	Decane	C10H22
11	Undecane	C11H24
12	Dodecane	C12H26

If you remove a hydrogen from an alkane, the partial structure becomes an alkyl group. Alkyl groups are named by replacing the –ane ending with an –yl ending. For example if you remove a hydrogen atom from methane the partial structure becomes methyl. Alkyl groups are not stable compounds by themselves, they are parts of larger compounds. Hence methyl is only one part of 4-methylhexane.

            The name of an alkane consists of  four parts: locant—prefix—parent--suffix. Locant refers to the location of the substituents (the smaller groups branching off from the main carbon chain), the prefix denotes the substituents in the molecule, parent refers to the number of carbons in the longest chain, and the suffix identifies the primary functional group within the molecule.

            To examine how these components are found, take a look at the following skeletal structure.

            Step 1: find the longest continuous carbon chain in the molecule. It is acceptable to “round corners” to find the longest chain. In this molecule's case, there are several options, all yielding the same carbon chain length.

            There are two different ways to obtain a 7-carbon chain. By convention, the chain with the most branching should be selected. Both options in this case yield the same number of branches off the parent chain. Either numbered chain can be used. For simplicity, the red numbered chain will be selected as the parent chain for discussion. There are seven carbons. Our “parent” is heptane.

            Step 2: number the chain, placing “1” at the end of the chain that is closet to a substituent.  This particular molecule's main carbon chain can be numbered two different ways.

            Notice that if we start from the left, the first substituent is at “2”. If we start from the top, the first substituent reached is at carbon “4”. The blue-numbered chain is the correct way to number this alkane because the first substituent reached has the lowest number. If the first substituent reached has the same number regardless of which end of the chain was used as a starting point, the substituent locants are added together and the chain with the lowest number is the correct chain. For example, an alkane could be named 2,4,5-trimethylhexane, or 2,3,5-trimethylhexane. The second option is preferred, as 2,3,5 has a lower sum than 2,4,5.

            Step 3: identify substituents and provide each with a locant, and a prefix if necessary. The “locant” is merely the number on our properly numbered carbon chain where the substituent is located.

           

            There are two methyl groups on the main chain, at “2” and “3”. Our locant is thus 2,3-, and our multiplier prefix is “di” because there are two methyl groups. All together, this is denoted as: 2,3-dimethyl. Take note that if there were two methyl groups at the same locant (for example, two methyl groups at “2”) the proper way to denote the groups would not be 2-dimethyl. It would be 2,2-dimethyl because we have to account for both methyl groups.

           

            Other prefixes include “tri” and “tetra” for 3 groups and 4 identical groups, respectively.

            Step 4: name complex substituents. In this molecule there is a complex substituent at carbon 4. This substituent is not a simple chain. It is structured as a molecule in itself. Therefore it must be named as if it were another molecule and should be numbered and named as such, but with an -yl ending. This complex substituent is 4-(1,1-dimethylmethane). This is also more commonly known as 4-isopropyl.

            Step 5: put the locants, prefixes, and parent together to create the entire name. From the previous sections the following components were found:

-        the parent is heptane.

-        there are two methyl groups, denoted as “2,3-dimethyl.”

-        there is one complex substituent named “4-isopropyl.”

-        the primary functional group is an alkane, so the suffix will be “ane.”

            To arrange all of these pieces into one name, they must be alphabetized. The multiplier prefixes do not count when alphabetizing, so the “di” in “2,3-dimethyl” does not count, but the “methyl” component does. In a complex substituent, however, the first letter counts even if there is a prefix. Thus “isopropyl” comes before “dimethyl” because “i” comes before “m”. This results in the following name:

           

            4-isopropyl-2,3-dimethylheptane

           

            There are a few exceptions to the naming scheme. There are several simpler-branched alkyl groups that have common names that are not systematic.

            These common names are acceptable to use and can be alphabetized by their names. There is one exception. Those common group names that begin with an italicized “tert” or “sec” do not count towards alphabetization. For example, in tert-Pentyl, the “tert” would not matter when alphabetizing, but “Pentyl” would. The prefixes “tert” and “sec” merely refer to whether the carbon connected to the rest of the molecule is a secondary or tertiary carbon. A secondary carbon has two other carbons attached to it; a tertiary carbon has three other carbons attached to it.

            A final evaluation should be made regarding the substituents of the molecule. The substituents may not all be alkyl groups, but an alcohol or a halogen. Many of these extra “things” in the carbon chain are considered “functional groups.” Functional groups are groups of atoms, in a specific pattern, that have characteristic chemical behavior. Below are the most common functional groups.