Nomenclature
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Nomenclature

To name an ester you treat it as a derivative of an alcohol and an acid, which it is. (You can also refer back to Example 7 in your workbook to see a similar equation.)

Equation showing formation of ester. [64rxn09.JPG]
First you name the part from the alcohol and then the part from the acid using an -ate ending. The resulting name should be written as two words. This diagram shows an ester that is made from methyl alcohol and acetic acid. So its name is methyl acetate. Methyl from the methyl alcohol. Acetate from the acetic acid.

Equation showing formation of ester and derivation of its name. [64rxn10.JPG]

 

Notice that as far as naming is concerned, it is somewhat similar to naming a salt. In sodium acetate, we have a sodium ion bonded to the acetate ion that is the remainder of the compound. In methyl acetate, we have a methyl group bonded to the acetate group that is the remainder of the compound.
  H   O 
|  //  
H-C-C    
|  \ 
     H  O- Na+

sodium acetate
(a salt)

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

methyl acetate
(an ester)

In both cases the acetate is bonded to the other part. In the case of a salt, however, it is an ionic bond between the acetate and the sodium. In the case of the ester it is a covalent bond between the acetate and the methyl. So there is quite a difference in that you are dealing with a molecular compound instead of an ionic compound. But as far as naming esters, you can approach the naming of esters the same way you approach naming a salt. You name the group that is attached to the oxygen of what was the acid. Then you name what remains of the acid by using the -ate ending.

There is one disadvantage to the naming of esters as though they were similar to salts, and that is that it may lead you to the feeling that the oxygen between the two groups originally started out with the acid. This is not true. In forming esters, the oxygen that was originally bonded to the carboxylic acid becomes part of the water. The oxygen that was in the alcohol ends up being the oxygen that is the link between the methyl and the acetate in methyl acetate. The general reaction is shown below.

O 
||
R¾C¾OH

carboxylic acid

  

+     R'OH

          alcohol

 H+
¾¾¾®		 O     
||  
R¾C¾O¾R'

ester

 +     HOH

 

Guidelines

Before turning you loose on the names and formulas of esters, let me point out that it is very important to make sure that you get correct which part of the ester came from the alcohol and which part came from the acid, because the -ate ending goes with the part that was part of the acid. Two examples are shown here, one in the left series of boxes and one in the right. The simplest way to identify the -ate part is to find the oxygen atom that is part of the continuous chain (second box). It is bonded to carbon on both sides. On one side of that oxygen there will be a carbonyl group (third box). On the other side there won't be. Instead there will be an alkyl group. The side of the molecule that has the carbonyl group is the side that came from the acid (fourth box) and that part will have an -ate ending (fifth box). The other part, the alkyl group without the carbonyl, will simply have a -yl ending.
O  
||     
C-C-C-O-C-C		       O
      ||
C-C-C-O-C-C
O     
||  
C-C-C-O-C-C		       O
      ||
C-C-C-O-C-C
O    
||      
C-C-C-O-C-C		       O
      ||
C-C-C-O-C-C
 O   
||      
C-C-C-O-C-C		       O
      ||
C-C-C-O-C-C
ethyl propanoate propyl ethanoate

As shown above, the formulas for these molecules can be written in either direction. That is, the part from the acid may be on the left or it may be on the right. You have to look at the structural formula to determine which part is from the acid (and gets the -ate ending) and which part is from the alcohol (and gets the -yl ending).

 

If there are side groups on either portion of the molecule, you will, of course, need to use numbers to show where they are attached. Number each portion starting from the carbon atom that is attached to the oxygen in the carboxylate group. In a sense this means numbering the atoms from the middle of the molecule out to the ends. However, if you think about an ester being made from a carboxylic acid and an alcohol, the numbering goes from the oxygen ends of those molecules. The subscripts here are to number the carbon atoms rather than indicate how many there are.
 O
  ||
C2-C1-O-C1-C2-C3
|       |
C       C
1-methylethyl 2-methylpropanoate
or
isopropyl 2-methylpropanoate
In the example above, a methyl group is attached to the first carbon atom of the two-carbon ethyl group chain (this can also be called an isopropyl group) and a methyl group is also attached to the second carbon atom of the three-carbon propanoate chain.

 

Practice

Now get some practice working on the structural formulas and the names shown below (and in Exercise 13 in your workbook). Check your answers below and then continue with the lesson.

O          
||           
CH3C-O-CH2CH2CH3
O  
||       
CH3CH2CH2-C-O-CH2CH2CH3
 
 
 
     


  
ethyl acetate
isopropyl formate
2-methylbutyl propanoate
butyl 2-methylpropanoate

 

Answers

 O          
 ||          
CH3C-O-CH2CH2CH3
 propyl acetate
or
propyl ethanoate
 O    
 ||    
CH3CH2CH2-C-O-CH2CH2CH3
 propyl butanoate
 O      
||       
C-C-C-O-C-C-C
|     |  
C     C  		 1-methylpropyl 2-methylpropanoate
or
sec-butyl 2-methylpropanoate
      O
      ||
C-C-C-O-C-C
| |     
C C    		 1,2-dimethylpropyl ethanoate
or
1,2-dimethylpropyl acetate
ethyl acetate O       
||       
CH3-C-O-CH2-CH3
isopropyl formate  O         
||        
H-C-O-CH-CH3
 |
    CH3
2-methylbutyl propanoate O          
||          
CH3-CH2-C-O-CH2-CH-CH2-CH3
           |     
         CH3
butyl 2-methylpropanoate O            
 ||            
CH3-CH-C-O-CH2-CH2-CH2-CH3
|                 
CH3               

Note that your answers to the last four might be reversed left-to-right or top-to-bottom and still be correct. If you have questions about that, please check with your instructor.

 

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Clackamas Community College
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