Nomenclature
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Nomenclature

Now let's consider naming alcohols. There are at least two ways to name alcohols, depending on how you look at the structural formula of the alcohol. The IUPAC approach is to see the -OH functional group attached to a chain of carbon atoms. Another approach is to see a group of carbon atoms attached to the -OH functional group.

Example 1

This is the structural formula for one alcohol. (It is also shown in Example 1 in your workbook). You can see it has the functional group -OH attached to one carbon atom. This particular alcohol goes by several names. Its two chemical names are methyl alcohol and methanol. It also goes by the common names of wood alcohol and completely denatured alcohol.

H  

H-C-O-H
|  

methyl alcohol
(wood alcohol)

IUPAC: methanol
(from methan + ol)

 

In naming this compound, you can take (as I said) two approaches. You can either look at it as a methyl group attached to the -OH or you can look at it as an -OH group attached to a carbon chain consisting of one carbon atom.

You could say that it is a methyl group attached to the alcohol functional group and therefore call it methyl alcohol.

You could also look at this as a one-carbon-atom chain with an -OH attached to it and therefore start with the name for the carbon chain as methane and then drop the -e and add an -ol ending to indicate that it is an alcohol. This method is the IUPAC method and it is the preferred method for naming alcohols.

Example 2

This is the structural formula for the most common of the alcohols (and is also shown in Example 2 in your workbook). It is ethyl alcohol or ethanol, again depending on how you choose to name the compound.

H H 
| |
H-C-C-OH
| |
H H 

ethyl alcohol
(grain alcohol)

IUPAC: ethanol
(from ethan + ol)

The IUPAC method of naming the compound is the preferred name. It is derived from the name ethane because of the two-carbon-atom chain. The -e at the end of the name is replaced with -ol because of the alcohol functional group. 
The other names, however, are well ingrained in use and will undoubtedly continue to be used. Grain alcohol is one such name. Also, the name "alcohol" by itself usually refers to ethanol.

 

In these first two examples there has been no need for the use of numbers to indicate where the -OH group is attached because there is only one place that it can be attached. In methyl alcohol there is only one carbon. In ethyl alcohol the -OH can go on either carbon but either way, that carbon will be the number one carbon, so there is no need for the number.

Example 3

However, in the structural formula shown here (and in Example 3 in your workbook), this is not the case. Here it is necessary to indicate where the hydroxyl group is attached to the carbon chain.
H   H   H
|   |   |
H-C - C - C-H
|   |   |
H   OH  H
isopropyl alcohol

IUPAC: 2-propanol

The IUPAC name for this compound is 2-propanol. In this name the 2- indicates a group attached to the second carbon. The propan- part indicates that the carbon chain is three carbon atoms long. The -ol on the end indicates the presence of an -OH functional group at that second carbon.
The other approach to naming this compound is to look at it as an alkyl group attached to -OH. Since that alkyl group is an isopropyl group, this alcohol can be named isopropyl alcohol.

 

Sometimes the two methods of nomenclature get mixed together and you may hear of this compound being called isopropanol, using the iso- from one name and the -ol from the other name. Using this kind of mixed nomenclature is undesirable but at times you may hear it.

I should make one additional comment about the numbering that is used in the IUPAC method of naming alcohols--that is that the number is set up so that the -OH group has the lowest number possible. That is, you would start numbering the carbon chain from the end closest to the -OH group.

Practice

I have given you only a minimal introduction to naming alcohols from their structural formula, and I haven't said anything about going from the name to the structural formula. However, with all the work that we did on relating names and structural formulas in the previous lessons combined with this additional bit of information from this lesson, you should be able to name an alcohol from the structural formula or deduce the structural formula from the name of an alcohol at this time. I would like you to try doing this by coming up with the proper names and formulas for the following compounds (also shown in Exercise 4 in your workbook). Please take some time to do that now. Then check your answers below before you continue.

H   H   H   H   H
|   |   |   |   |
H-C - C - C - C - C-H
|   |   |   |   |
H   |   H   OH  H
|       
H-C-H       
|       
H-C-H       
|       
H       
    H
    |
    H-C-H
    |
H   H   |   H
|   |   |   |
H-C - C - C - C-H
|   |   |   |
OH  H   H   H
H           
|           
H-C-H           
|           
|   H   H   H
|   |   |   |
H-C - C - C - C-H
|   |   |   |
OH  H   H   H
3-pentanol
4-ethyl-2-methyl-3-hexanol
What is the correct name for a compound mistakenly named "4,5-diethyl-3-pentanol"?

 

Answers

H   H   H   H   H
|   |   |   |   |
H-C - C - C - C - C-H
|   |   |   |   |
H   |   H   OH  H
|       
H-C-H       
|       
H-C-H       
|       
H       
4-methyl-2-hexanol
    H
    |
    H-C-H
    |
H   H   |   H
|   |   |   |
H-C - C - C - C-H
|   |   |   |
OH  H   H   H
3-methyl-1-butanol
H           
|           
H-C-H           
|           
|   H   H   H
|   |   |   |
H-C - C - C - C-H
|   |   |   |
OH  H   H   H
2-pentanol
3-pentanol H   H   H   H   H
|   |   |   |   |
H-C - C - C - C - C-H
|   |   |   |   |
H   H   OH  H   H
4-ethyl-2-methyl-3-hexanol     H
    |
H     H-C-H 
|       |    
H-C-H   H-C-H    
|       |    
H   |   H   |   H   H
|   |   |   |   |   |
H-C - C - C - C - C - C-H
|   |   |   |   |   |
H   H   OH  H   H   H
What is the correct name for a compound mistakenly named "4,5-diethyl-3-pentanol"?

The key here is to pick the longest carbon chain as the starting point for the name.

            C   C
            |   |
            C   C
            |   |
C - C - C - C - C
|
OH

4-ethyl-3-heptanol

If you had any difficulty naming or drawing structural formulas for these compounds, you should work with the instructor for whatever period of time is necessary to master the art of naming alcohols and deriving their structure from the IUPAC name. Take some time now, if you need it, to do that. When you are ready, continue with the lesson.

 

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E-mail instructor: Eden Francis

Clackamas Community College
2001, 2003 Clackamas Community College, Hal Bender