Carboxylic Acids
A carboxylic acid is an organic compound that contains a carboxyl group, –COOH.
Carboxyl group - a carbon atom that is double-bonded to 1 oxygen atom and single-bonded to a hydroxyl group.
When a carboxylic acid is in aqueous solution, a hydrogen ion may be removed from the carboxyl group relatively easily.
A solution that contains hydrogen ions is acidic. These compounds are therefore weak acids.
Naming Carboxylic Acids
The functional group of carboxylic acids, –COOH, combines two smaller groups:
- a hydroxyl group and
- a carbonyl group.
To name organic compounds containing a carboxylic group, start with the alkane name for the longest chain, including the carbon atom in the carboxyl group.
Drop the -e suffix of the root compound and replace it with the suffix -oic, followed by the word acid.
For example, a carboxylic acid with 3 carbon atoms is propanoic acid.
In a compound that combines an alcohol and a carboxylic acid, the alcohol is indicated as a substituent group: hydroxy.
Some carboxylic acids have more than one carboxyl group.
When naming an acid with two carboxyl groups, use the suffix dicarboxylic acid.
Citric acid is an example of a compound with three carboxylic groups.
Formic acid, acetic acid, and benzoic acid are acceptable IUPAC (International Union of Pure and Applied Chemistry) names that do not follow the systematic rules for naming acids.
Properties of Carboxylic Acids
Since carboxylic acids have two polar groups—a carbonyl and a hydroxyl — located close together, their molecules are very polar.
The carboxyl groups form hydrogen bonds with one another and with solvents such as water.
Due to this hydrogen bonding, carboxylic acids with 5 or fewer carbon atoms are very soluble in water.
Larger carboxylic acids have decreasing solubility due to the large non-polar hydrocarbon group. They are, however, soluble in polar organic solvents, such as alcohols.
Carboxylic acids share many properties with other acids.
Esters
Ester - an organic compound that contains a carbonyl group bonded to a second oxygen atom which is bonded to another carbon atom.
R - CO - O - R
Naming Esters
Esters are formed by the condensation reaction of a carboxylic acid and an alcohol.
To name the resulting ester, determine which part of the molecule was contributed by the alcohol. This is the part that does not include the carbonyl group. This gives the ester the first part of its name.
The second part of the name comes from the carboxylic acid.
The -oic acid part of the name is dropped and replaced with -oate.
Properties of Esters
The functional group of an ester is similar to the carboxyl group of an acid, but without the hydroxyl group.
As a result, esters are less polar than carboxylic acids and do not form hydrogen bonds.
Small esters are soluble in water due to the polarity of their carbon–oxygen bonds. Esters are less soluble in water than carboxylic acids and have lower boiling points.
Reactions Involving Carboxylic Acids and Esters
Formation of Carboxylic Acids
Carboxylic acids can be formed by the oxidation of aldehydes in the presence of an oxidizing agent.
Formation of Esters: Esterification
Esterification is a condensation reaction in which an alcohol and carboxylic acid react to form an ester and water.
Reaction of Esters: Hydrolysis
When esters are treated with an acid or a base, the esterification process can be reversed. The ester splits into the carboxylic acid and alcohol components with the addition of a molecule of water. This reaction is called hydrolysis.
Fats and Oils
Fats and oils are large ester molecules known as lipids.
The long-chain carboxylic acid component is called a fatty acid.
The alcohol component is glycerol.
Glycerol is a 3-carbon alcohol with three hydroxyl groups, so it can bond with three fatty acids at once. The ester that is formed is called a triglyceride. So, fats and oils are triglycerides.
Saponification
When these long-chain esters are heated with a strong base, such as sodium hydroxide, the hydrolysis reaction occurs.
A triglyceride can be split to produce glycerol and the sodium salt of the fatty acid when sodium hydroxide (NaOH) is added to the triglyceride.
This resulting sodium salt is commonly called soap, so the reaction is called saponification.
Structure and Properties of Fats and oils
Lipids are not water soluble because of the non-polar nature of the long fatty acid chains in the triglyceride molecules. These long, non-polar chains overcome the polarity of the carbon–oxygen bonds in the ester groups.
The hydrocarbon chains in fatty acids affect the physical state of the lipid.
