310

Unit 3
Elements of Organic Chemistry

3-10. CARBOXYLIC ACIDS

Carboxylic acids are formed by the two-step oxidation of alcohols as stated previously and have the general structural formula   O    or R –COOH.

Some examples of carboxylic acids are:

Properties of Carboxylic Acids. Carboxylic acids are very polar compounds due to the two oxygen atoms and can form two hydrogen bonds between themselves as shown below.

They have the highest melting points of any of the classes of compounds in table 3-3; a carboxylic acid has a higher melting point than a different type of organic compound with a similar molecular weight. Consequently, they are all solids under normal conditions. The compounds with four carbons or less are miscible with water; those with five carbons are slightly soluble, and those with more than five carbons are generally insoluble in water.

Reactions of Carboxylic Acids. As their name implies, carboxylic acids are the most acidic of all organic compounds but are still weak acids when compared to inorganic acids.

Carboxylic acids will form salts with inorganic bases, and as with the basic amines, this property is often used to make insoluble organic acids soluble in water as their salt. This pair, ethanoic acid (acetic acid) and its salt sodium ethanoate (sodium acetate), is used as a buffer system.

Carboxylic acids undergo three other important chemical reactions: reduction, ester formation, and amide formation.

  1. Reduction in organic chemistry is the opposite of oxidation and is the addition of hydrogen to or the elimination of oxygen from a compound. In the case of carboxylic acids, the removal of oxygen first results in an aldehyde, which may be reduced further by the addition of hydrogen to form an alcohol.
  2. Ester formation, as illustrated by the reaction below, is the reaction of a carboxylic acid with an alcohol to yield a new class of compound called an ester. O

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Table 3-3. Summary of properties for functional groups

 

(3) Amide formation, as illustrated by the reaction below, is the reaction of a carboxylic acid with an amine to yield a new class called an amide.

803fig0316.jpg (11773 bytes)

Uses of Carboxylic Acids. Many acids, such as acetic, salicylic, and lactic, are used topically to treat local conditions. Others are used systemically. Still others, like citric acid, which is found naturally in lemons, are used to flavor syrups for administration of other drugs. They are also used in many analytical procedures in the clinical laboratory.