Alkane Nomenclature

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Nomenclature (Naming Compounds)

Common names

Some compounds have common names. These are historical names of compounds. We need to learn a few of them that are commonly used. Different compounds that have the same formula are called isomers. For alkanes, there are common names for the various isomers. We will only consider those that have four or five carbon atoms to get a feel for common names. There are two ways to arrange four carbon atoms alkanes. The straight chain, unbranched, form of butane is also called n-butane, which stands for normal butane. Four carbon atoms can be arranged in another way. So, we need another name for this new arrangement. We cannot call it butane since it is different. Therefore, the common name for this is isobutane (an isomer of butane). Isobutane is branched whereas n-butane is unbranched.

Isomers of butane

Five-carbon alkanes can be arranged three different ways. The normal chain is called n-pentane. Then, there is an isomer of pentane called isopentane. There is one more isomer of pentane, but the name isopentane was already taken. So, this last isomer is called neopentane for “new” pentane.

Isomers of pentane

So, how do we decide which isomer gets the iso- designation? Notice that the iso- compounds each have a “Y” shape at its end.

The "iso" designation for isomers

For six carbons, there are five isomers. So, we’ll stop looking at common names. Thankfully, a system has been devised to help us name compounds so we do not need to memorize thousands of common names. The group responsible for naming chemicals is the International Union of Pure and Applied Chemistry, IUPAC. The rules for naming compounds they have developed are called IUPAC rules.

IUPAC names for alkanes

We will look at the rules for naming alkanes by investigating why the following compound is named 3-ethyl-2,2-dimethylhexane.

Rule 1. Find the longest continuous chain of carbon atoms. This is the base chain. This name will be at the end of the name. Substituents are all of the groups attached to this main, base chain.

Watch out, the base chain may not be in a straight line!

If two pathways have the same chain length, choose the one that gives the most substituents. This keeps the substituent groups shorter making it easier to name the compound.

This is how we get the last part of the name, 3-ethyl-2,2-dimethylhexane.

Rule 2. Number the base chain so we can tell where the substituents are located.

We want the smallest numbers we can get. So, begin numbering the chain from the end closest to a substituent.

The numbers 2 and 3 are used (the first numbering system) and not 4 and 5 (the second numbering system) in the name 3-ethyl-2,2-dimethylhexane.

Rule 3. Name the alkyl group substituents. –CH3 = methyl, -CH2CH3 = ethyl, -CH2CH2CH3 = propyl, etc.

Rule 4. If there is more than one of the same kind of substituent, use di-, tri-, etc. to tell how many of them there are.

di = 2, tri = 3, tetra = 4, penta = 5, hexa = 6

In our example, we have two methyl groups. The two methyls are put together as “dimethyl” in our name, 3-ethyl-2,2-dimethylhexane. Notice, every substituent gets a number. We name the two methyl groups as 2,2-dimethyl (using “2” twice) and do not simply say, 2-dimethyl.

Rule 5. Put substituents in alphabetical order, not including the di-, tri-, tetra- prefixes.

Notice, the name is NOT 2,2-dimethyl-3-ethylhexane. Even though dimethyl starts with a “d” and ethyl with an “e”. We ignore the di- prefix and use the “m” of methyl for our alphabetical arrangement. The name we use is named 3-ethyl-2,2-dimethylhexane because “ethyl” comes before “methyl” alphabetically.

Rule 6. Use commas to separate numbers and dashes to separate numbers and letters.

2. Name the following compounds.

Nuances

If we were to name the following compound, we would run into a small issue we should look at. The longest chain is a hexane with a methyl group and an ethyl group attached.

If we number the hexane chain from left to right, we get the name 4-ethyl-3-methylhexane. If we number the hexane chain from right to left, we get the name 3-ethyl-4-methylhexane. Since there is a tie with the numbers, we choose the one with the lower number written first. So, we prefer the name 3-ethyl-4-methylhexane.

3. Name the following compounds.

But, what if a substituent is a little more complicated than a normal, n-alkyl group like methyl, ethyl, propyl, or butyl? There are some common names we should learn for isomers of propyl and butyl. We are already familiar with propyl. The only isomer of propyl that exists is isopropyl. Notice the “Y” formation in isopropyl.

Isomeric propyl groups

For butyl, there are more possible isomers. The first isomer to learn is isobutyl. This isomer again has the “Y” at the end of it. When the carbon attached to the R group is a secondary carbon, meaning it is attached to two other carbon atoms in the group, the group is called secondary-butyl or more commonly, sec-butyl. The tertiary-butyl group is so named because the carbon atom attached to the R group is a tertiary carbon atom, meaning it is attached to three other carbon atoms in the group.

Isomeric butyl groups

Sometimes these common group names are used when naming a compound. This naming system is very common for the above groups.

4. Draw the following molecules.

a) 4-isopropylheptane

b) 4-tert-butyloctane

c) isobutylcyclopentane

d) tert-butylchloride

But, sometimes, these groups will be named using the more technical IUPAC system. When these groups are named this way, they are put in parenthesis. The longest base chain of the substituent group is found and named with the -yl suffix. This alkyl base chain is always numbered starting closest to the “main” part, or the longest base chain, of the molecule. The other groups are named as substituents on this base chain.

This technique is useful for more complicated alkyl groups like:

5. Name the following molecules (one which contains an isopropyl group and one that contains a tert-butyl group) using the IUPAC name for all substituents.

a)