What Are the Acid-Base Properties of Carboxylic Acids

4 What Are the Acid-Base Properties of Carboxylic Acids  

Carboxylic acids are weak acids. Values of for most unsubstituted aliphatic and aromatic carboxylic acids fall within the range from lO to 10 . The value of for acetic acid, for example, is 1.74 X 10 , and the of acetic acid is 4.76  

As we discussed in Section 2.5B, carboxylic acids (p.J 4-5) are stronger acids than alcohols 16-18) because resonance stabilizes the carboxylate anion by delocalizing its negative charge. No comparable resonance stabilization exists in alkoxide ions. 

Substitution at the a-carbon of an atom or a group of atoms of higher electronegativity than carbon increases the acidity of carboxylic acids, often by several orders of magnitude (Section 2.5C). Compare, for example, the acidities of acetic acid (pii 4.76) and chloroacetic acid (p7 2.86). A single chlorine substituent on the a-carbon increases acid strength by nearly 100 Both dichloroacetic acid and trichloroacetic acid are stronger acids than phosphoric acid (p7i 2.1)  

The acid-strengthening effect of halogen substitution falls off rather rapidly with increasing distance from the carboxyl group. Although the acid ionization constant for 

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