Acidity
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Acidity

On this page we'll look at the type of reaction that gives carboxylic acids their name -- acid-base reactions.

Because of the presence of two oxygen atoms (both at the same end of the molecule) carboxylic acids have very polar molecules. Not only are they polar, they also contain hydrogen bonded to an oxygen atom which is  bonded to carbon, a nonmetal.

64str02.JPG (5125 bytes)

 

Because of this they will dissociate to a small extent in water to give hydronium ion plus the remaining ion from the acid. This property, of course, is what makes them acids.

Equation for the reaction of a carboxylic acid and water. [64rxn02.JPG]

 

As acids, these compounds can be neutralized by reaction with a base which results in the formation of a salt and water. Such salts are made up of a positive ion from the base and the negative ion from the acid.

Equation for the reaction of a carboxylic acid and a base. [64rxn03.JPG]

 

As you know from your study of acids and bases in the previous course, the reaction of an acid can be represented in several ways. (By the way, carboxylate ions are named by replacing the -ic ending of the acid with an -ate ending. We will deal with that formally in the section on carboxylate salts.)

The basic idea is shown here (and in Example 6-a in your workbook) -- an acid is an acid because it can give off a hydrogen ion, a proton. This equation has some shortcomings. First, the proton doesn't just come off of an acid molecule. It goes somewhere. It attaches to a base (changing it to its conjugate acid). Second, since carboxylic acids are weak acids, the equation should have arrows going in both directions to indicate that this is an equilibrium reaction.
     O
   //
R-C    
    \
        OH

carboxylic
acid

®

 

      O
   //
R-C    
   \
       O-

carboxylate
ion

 + H+

 
     O
   //
R-C    
    \
        OH 		®
¬		      O
   //
R-C    
   \
       O-		 + H+
This equation (also shown in Example 6-b in your workbook) shows what happens when an acid dissociates in water. Notice that I said "dissociates" rather than "dissolves" in water. Only a small portion of the molecules that dissolve in water actually dissociate. In this case acetic acid dissociates to form acetate ion.
    H     O
  |   //
H-C-C   
  |     \
       H      OH

acetic
acid

+ H2O

 

 ®
¬

 

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

acetate
ion

  

+ H3O+

 

 
The equations below (also shown in Example 6-c) show the reaction between butanoic acid and the base sodium hydroxide to form sodium butanoate and water. If this reaction were to take place in solution in water, the sodium hydroxide would be dissociated as would the sodium butanoate and the actual reaction would really be more like what is shown in the second equation.
H H H O
| | | || 
H-C-C-C-C-OH
| | |  
H H H  

butanoic acid

+ NaOH

 

 ®
¬

 

 H H H O 
| | | || 
H-C-C-C-C-ONa
| | |   
H H H   

sodium butanoate

 + H2O

 
H H H O
| | | || 
H-C-C-C-C-OH
| | |  
H H H   		+ OH- ®
¬		 H H H O 
| | | || 
H-C-C-C-C-O-
| | |  
H H H   		 + H2O

 

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Clackamas Community College
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