Carboxylic acids and Esters

Carboxylic acids Carboxylic acids are organic compounds that contain the carboxyl group (COOH). The carboxyl group is always on a terminal carbon atom. Carboxylic acids are weak acids, since only a small fraction of acid molecules ionize when dissolved in water. They give up the hydrogen on the carboxyl group to bases and establish an equilibrium. + H2O + H3O+

Properties of carboxylic acids Like alcohols, carboxylic acids form hydrogen bonds with each other, so they have higher boiling points than related alkanes, aldehydes and ketones. Because each molecule can form two hydrogen bonds, carboxylic acids also have higher boiling points than alcohols.

Properties of carboxylic acids Because they can also hydrogen bond with water, carboxylic acids with 4 carbons or less are water soluble, however, they become less soluble as the carbon chain becomes longer.

Nomenclature of carboxylic acids Number the parent chain, giving the carbon with the carboxyl group position number one. Change the “e” ending in the alkane name to “oic acid.” Because the carboxyl group is always on carbon one, don’t write a one in front of the name of the parent chain. If there are substituents, identify the number of the carbon to which it is attached and write them alphabetically. If there are two or more identical branches, use prefixes di, tri, etc.

Naming carboxylic acids Ethanoic acid Propanoic acid 3-methylbutanoic acid

Naming carboxylic acids 2-hydroxypropanoic acid (the carboxyl group takes precedence over the hydroxyl group, so it is named as a carboxylic acid). This is also known as lactic acid. Benzoic acid

Naming carboxylic acids Dicarboxylic acids- contain two COOH groups. The are named by adding dioic acid to the alkane name. Ex. ethanedioic acid propanedioic acid

Naming Carboxylic acids Benzene dicarboxylic acids – a benzene ring with two carboxyl groups. Name them as benzene followed by the number on the carbons to which the carboxyl groups are attached, followed by dicarboxylic acid. Carboxyl groups should have the lowest possible numbers. Ex. Benzene-1,2-dicarboxylic acid Benzene-1,4-dicarboxylic acid

Preparation of carboxylic acids Carboxylic acids are usually prepared by oxidizing aldehydes. It is also possible, as previously stated, to start with a primary alcohol and oxidize it to an aldehyde and then to a carboxylic acid, using reflux apparatus.

Reactions of carboxylic acids Neutralization reactions – since they are weak acids, carboxylic acids are capable of reacting with bases to produce salt and water. For example: + NaOH → Na+ + H2O Ethanoic acid sodium hydroxide sodium ethanoate water

Reactions of carboxylic acids Esterification reactions – the reaction of a carboxylic acid with an alcohol to produce an ester and water. In the reaction, the –OH group of the carboxylic acid is replaced by the O-R group of the alcohol.

Reactions of carboxylic acids The ester is named by naming the alkyl portion of the alcohol followed by the name of the carboxylic acid ending in “oate.” Example:

Reactions of carboxylic acids This reaction is an equilibrium reaction and is catalyzed by concentrated sulfuric acid. Because the ester has the lowest boiling point in the mixture, it can be separated by distillation. Unlike the carboxylic acid and alcohols, esters don’t form hydrogen bonds and are mostly insoluble.

esters Esters can be considered derivatives of carboxylic acids in which an organic group replaces the acidic hydrogen of the carboxyl group. The most common physical property of esters is their pleasant odors. The aromas of many flowers, fruits, and perfumes are due to mixtures of esters. Esters are also widely used as food additives to impart specific flavors and aromas to processed foods. Methyl butanoate (apple) Ethyl butanoate (pineapple)

Reactions of esters 1. Acid hydrolysis – the most important reaction of esters is their hydrolysis to a carboxylic acid and alcohol. Hydrolysis is the reverse reaction of an esterification reaction. The reaction is catalyzed by strong acids. Example: ester + water carboxylic acid + alcohol + H2O + ethanoic acid ethanol

Reactions of esters 2. Base hydrolysis (Saponification) – the base-catalyzed hydrolysis of an ester. Example: ester + strong base → carboxylate salt + alcohol + NaOH → + Methyl ethanoate sodium ethanoate methanol