Nomenclature
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Nomenclature

IUPAC Names

The naming of carboxylic acids is fairly simple. You simply find the longest carbon chain which includes the carboxylic group. Use that as the stem for the name, cross off the -e on the ending of the alkane name and replace it with -oic acid.

I think you can see how that works, if you look at this example (which is also shown in Example 1-a in your workbook). It gives you, in this case (with a three-carbon-atom chain), the name propanoic acid.
    H H    O
   | |  //
H-C-C-C
    | |  \
     H H   OH
propanoic acid
(from propan + oic acid)

 

As with aldehydes, it is not necessary to indicate where the acid functional group is because it has to be at the end of the molecule, on the #1 carbon. There is no way that this functional group can be anywhere else. Therefore, if there is any numbering to be done, it will be to show where additional alkyl groups or other groups are attached to the carbon chain. The numbering starts from the carboxylic group.

Common Names

Carboxylic acids are another example of a situation where the compounds were known and named long before anyone thought of the IUPAC method of naming compounds. Consequently, many carboxylic acids have their own common name which is distinct from the IUPAC name. The two most important of these (and the only two you will be held responsible for in this course) are shown below. They are formic acid and acetic acid. (These are also shown in Examples 1b and c in your workbook.)

Here is the structural formula for formic acid. Its IUPAC name is methanoic acid, using the meth- stem because it has one carbon atom.
  O
//
H-C  
  \
    OH
formic acid
methanoic acid
Acetic acid has two carbon atoms. Therefore it can also be called ethanoic acid.
   H  O
  | //
H-C-C
   |  \
     H   OH
acetic acid
ethanoic acid

 

Practice

As you can see, naming carboxylic acids is not terribly difficult, nor is coming up with the structural formula, given the name. But you do need to practice. To get some practice take a moment to come up with the names for the structural formulas given below (and in Exercise 2-a and b in your workbook) and come up with the structural formulas for the names that are given below (and in Exercise 2-c and d). After you have done that, check your answers below and then continue with the lesson.

      H   H   H   H     O
     |   |   |   |    //
H - C - C - C - C - C
     |   |   |   |    \
       H   H   |   H     OH
    |
    H-C-H
   |
    H-C-H
    |
    H
H    
|   
H-C-H    
O     |   H     
\\    |   |    
C - C - C - H
/    |   |    
HO     |   H      
H-C-H    
|   
H-C-H   
|   
H   
octanoic acid
2,4-dimethylpentanoic acid

 

Answers

        H   H   H   H     O
     |   |   |   |    //
H - C - C - C - C - C
     |   |   |   |    \
       H   H   |   H     OH
    |
    H-C-H
   |
    H-C-H
    |
    H		 3-ethylpentanoic acid
H    
|   
H-C-H    
O     |   H     
\\    |   |    
C - C - C - H
/    |   |    
HO     |   H      
H-C-H    
|   
H-C-H   
|   
H    		2,2-dimethylbutanoic acid
octanoic acid

 O    H   H   H   H   H   H   H
 \\   |   |   |   |   |   |   |
  C - C - C - C - C - C - C - C - H
  /   |   |   |   |   |   |   |
HO    H   H   H   H   H   H   H
2,4-dimethylpentanoic acid

     O    CH3 H   CH3 H
      \\   |   |   |   |
      C - C - C - C - C - H
      /   |   |   |   |
    HO    H   H   H   H

 

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