PHYSICAL AND CHEMICAL PROPERTIES OF CARBOXYLIC ACIDS

Carboxylic Acid

What is Carboxylic Acid?

A Carboxylic Acid is an organic compound containing a carboxyl functional group. They occur widely in nature and are also synthetically manufactured by humans. Upon deprotonation, carboxylic acids yield a carboxylate anion with the general formula R-COO–, which can form a variety of useful salts such as soaps.

The carboxylic acids are the most important functional group that present C=O. This type of organic compounds can be obtained by different routes, some carboxylic acids, such as citric acid, lactic acid or fumaric acid are produced from by fermentation most of these type of carboxylic acids are applied in the food industry.

Carboxylic Acid Structure

The general formula of a carboxylic acid is R-COOH, were COOH refers to the carboxyl group, and R refers to the rest of the molecule to which this group is attached. In this carboxyl group, there exists a carbon which shares a double bond with an oxygen atom and a single bond with a hydroxyl group.

The general structure of a carboxylic acid is illustrated below.

General Structure of Carboxylic Acid

From the illustration provided above, it can be observed that a carboxylic acid contains a hydroxyl group attached to a carbonyl carbon. Due to the electronegativity of the oxygen atom, this functional group can undergo ionization and discharge a proton.

The carboxylate ion, produced from the removal of a proton from the carboxyl group, is stabilized by the presence of two oxygen atoms (through which the negative charge can move). Some common examples of carboxylic acids include acetic acid (a component of vinegar) and Formic acid.

Nomenclature of Carboxylic Acids

Generally, these organic compounds are referred to by their trivial names, which contain the suffix “-ic acid”. An example of a trivial name for a carboxylic acid is acetic acid (CH3COOH). In the IUPAC nomenclature of these compounds, the suffix “-oic acid” is assigned.

The guidelines that must be followed in the IUPAC nomenclature of carboxylic acids are listed below.

• The suffix “e” in the name of the corresponding alkane is replaced with “oic acid”.
• When the aliphatic chain contains only one carboxyl group, the carboxylic carbon is always numbered one. For example, CH3COOH is named as ethanoic acid.
• When the aliphatic chain contains more than one carboxyl group, the total number of carbon atoms is counted and the number of carboxyl groups is represented by Greek numeral prefixes such as “di-”, “tri-“, etc.
• A carboxylic acid is named by adding these prefixes and suffixes to the parent alkyl chain. Arabic numerals are used for indicating the positions of the carboxyl group.
• The name “carboxylic acid” or “carboxy” can also be assigned for a carboxyl substituent on a carbon chain. An example of such nomenclature is the name 2-carboxyfuran for the compound 2-Furoic acid.

Carboxylic Acid Examples

Some examples describing the nomenclature of carboxylic acids as per IUPAC guidelines are provided below.

Trivial Name and Formula	IUPAC Name of the Carboxylic Acid
Formic acid, H-COOH	Methanoic acid
Crotonic acid, CH3CH=CH-COOH	But-2-enoic acid
Carbonic acid, OH-COOH	Carbonic acid
Butyric acid, CH3(CH2)2COOH	Butanoic acid

Properties of Carboxylic Acids

Most of the properties of carboxylic acids are a result of the presence of the carboxyl group. Some of the physical and chemical properties of these compounds are discussed in this subsection.

1. Physical Properties of Carboxylic Acids

• Carboxylic acid molecules are polar due to the presence of two electronegative oxygen atoms.
• They also participate in hydrogen bonding due to the presence of the carbonyl group (C=O) and the hydroxyl group.
• When placed in nonpolar solvents, these compounds form dimers via hydrogen bonding between the hydroxyl group of one carboxylic acid and the carbonyl group of the other

• The solubility of compounds containing the carboxyl functional group in water depends on the size of the compound. The smaller the compound (the shorter the R group), the higher the solubility.
• The boiling point of a carboxylic acid is generally higher than that of water.
• These compounds have the ability to donate protons and are therefore Bronsted-Lowry acids.
• They generally have a strong sour smell. However, their esters have pleasant odors and are therefore used in perfumes.

2. Chemical Properties of Carboxylic Acids

• The α-carbon belonging to a carboxylic acid can easily be halogenated via the Hell-Volhard-Zelinsky reaction.
• These compounds can be converted into amines using the Schmidt reaction.
• A carboxylic acid can be reduced to an alcohol by treating it with hydrogen to cause a hydrogenation reaction.
• Upon reaction with alcohols, these compounds yield esters.

Uses of Carboxylic Acids

• Fatty acids that are essential to human beings are made up of carboxylic acids. Examples include omega-6 and omega-3 fatty acids.
• Higher fatty acids are also used in the manufacture of soaps.
• The production of soft drinks and many other food products involve the use of many carboxylic acids.
• The manufacture of rubber involves the use of acetic acid as a coagulant.
• Hexanedioic acid is used in the manufacture of nylon-6,6.
• Carboxylic acids have numerous applications in the rubber, textile, and leather industries.
• Ethylenediaminetetraacetic acid is a widely used chelating agent.
• The synthesis of many drugs involves the use of these compounds. Therefore, carboxylic acids are very important in pharmaceuticals.
• The production of many polymers involves the use of compounds containing the carboxyl functional group.