Carboxyl Group: Carboxylic Acid, Definition, and Structure

Carboxyl group is defined as carbonyl and hydroxyl attached to a carbon atom, which means carbon atom is double bonded with oxygen and single bonded with the hydroxyl. And Carboxylic acid is organic compound which consists of a carboxyl group. In this topic, we are going to learn about the carboxyl group and carboxylic acid structure. Let’s begin.

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Carboxylic Acid

It is one of the very important classes of organic compounds. The general formula of the class is R-C(O)OH. In this formula, R is the alkyl or aryl group. Carboxylic acids occur widely in nature. However, the majority of the members of this group is manufactured synthetically. The double bond presents in the structure of carboxylic acids play a very important role in properties of the different compounds of carboxylic acids. Refer to the diagram below.

Diagram of Carboxylic Acid (Source: Wikipedia)

When a carbon compound is attached to the functional group –COOH (carboxyl group) then the compound refers to as carboxylic acids. However, the formation of a carboxyl group is possible by the attachment of a hydroxyl group to a carbonyl group, thus the name “carboxyl group.” Carboxylic acids can be either aliphatic or aromatic on the basis of the group present. If an alkyl group is present (RCOOH) and if an aryl group is present (ArCOOH).

The higher members of the aliphatic carboxylic acids, from C12-C18, are known as fatty acids. They are found in nature as natural fats or esters of glycerol. Moreover, this group is the starting material for many essential organic compounds like esters, acid chlorides, anhydrides, amides etc.

There are many natural compounds containing carboxylic acid. For instance formic acid is present in insect sting, butyric acid is present in butter, carbonic acid is present in the bicarbonate system of blood and tissues, lauric acid is present in coconut oil, palmitic acid is present in palm oil, arachidic acid is present in peanut oil, and stearic acid is present in chocolate, waxes, soaps, and oils. In this topic, we will discuss how the properties and structure of the carboxyl group affect the properties of the compounds in the carboxylic acid group.

Browse more Topics under Aldehydes Ketones Carboxylic Acids

• Chemical Reactions and Uses of Carboxylic Acids
• Methods of Preparation of Carboxylic Acids
• Nomenclature and Structure of Carbonyl Group
• Nomenclature and Structure of Carboxyl Group
• Nucleophilic Addition Reaction
• Oxidation
• Physical properties of Aldehydes, Ketones and Carboxylic Acids
• Preparation of Aldehydes
• Preparation of Aldehydes and Ketones
• Preparation of Ketones
• Reactions due to Alpha-Hydrogen
• Reduction
• Uses of Aldehydes and Ketones

Carboxylic acid structure

Carboxyl group is a functional organic compound. In this structure of a carboxyl group, a carbon atom is attached to an oxygen atom with the help of a double bond. It also has a single bond to a hydroxyl group. Carboxylic acids are compound containing carboxyl structure. There are many members in this class of organic acids such as acetic acid and amino acid.

Carboxyl group is generally present on the sides of the molecules. The carboxyl group ionizes and releases the H atom present in the hydroxyl group part as a free H⁺ ion or a proton. However, the rest of part, this is O, convey a negative charge. The charge moves in between the two oxygen molecules forward and backward thereby making the state of ionization relatively steady.

3-D structure of Carboxyl Group (Source: Wikipedia)

Resonance Structure of Carboxylic Group

The bonds present in the carboxyl carbon lie in one plane in carboxylic acids. The bonds angles of carboxyl carbon are approximately 120°. The resonance structure of the carboxyl carbon makes it less electrophilic in comparison to a carbonyl carbon.

Learn more about the Structure of Carbonyl Group.

Nomenclature of Carboxylic Acids

This class is among one of the earliest organic compounds that were isolated from nature. This is the reason many of the compounds of this class of compounds have common names more in use.

Common Names

The origin of the common names of many members of the carboxylic acids group is from their respective Latin or Greek names of the natural sources. The common names generally end with the suffix –ic acid. For instance, HCOOH is formic acid. The origin of the name is from the Latin word “formica” which means ant. The source of formic acid is red ants.  Vinegar is the source of acetic acid (CH3COOH) and the Latin word for vinegar is “acetum.” Similarly, rancid butter is the source of butyric acid which in Latin refers to butyrum.

IUPAC Name

According to the IUPAC system, it is easy to name aliphatic carboxylic acids by replacing the “e” of the respective alkane with –oic acid. The numbering of the carbon atom chain in a carboxylic acid starts with the carboxylic carbon. This means carboxylic carbon will always be the first carbon in the parent chain.

It is important to number the alkyl chain if more than one carboxyl group is present in a compound. You can indicate the number of carboxylic acids present in the compound by addition of the multiplicative prefix to the name of the parent alkyl chain such as dicarboxylic acid, tricarboxylic acid, and so on. Additionally, it is necessary to write the Arabic numerals prior to the multiplicative prefix to indicate the position of –COOH groups in a particular compound.

The simple members of this class of compound have names such as methanoic acid, next is ethanoic acid, propanoic acid, butanoic acid and so on. However, the nomenclature of aromatic carboxylic acids is not usually is in the standard form. In fact, they have the special IUPAC-approved special name such as benzoic acid.

We can also name the compounds on the basis of the position and alphabetical order to the subject compound.

If a particular compound contains more than one functional group, the nomenclature is on the basis of carboxyl group instead of other functional groups. Importance is given to Carboxylic group over other functional groups. In the below example the name of the molecule is carboxylic acid instead of alcohol.

By now we know that the carboxyl carbon atom will always be the carbon-1 during the naming of a compound. Therefore, we do not require to use a locant while naming the carboxyl group.

Learn more about the Nomenclature of Carbonyl Group.

Solved Question for You

Q. Name the common name and IUPAC name of the following structures:

1. CH₃(CH₂)₂COOH
2. CH₃(CH₂)₈ COOH

Solution:

1. The common name of CH₃(CH₂)₂COOH is Butyric acid and the IUPAC name is Butanoic Acid.
2. The common name of CH₃(CH₂)₈COOH is Capric acid and the IUPAC name is Decanoic Acid.