a. 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.
b. 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
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.
CH3 COOH + CH3 CH2 OH ------> CH3 C O CH2 CH3 + H2O