Nomenclature and Structure of Carboxylic Acids

We all know that ant and wasp stings contain formic acid, which causes our bodies to burn. But did you know that formic acid can also be referred to as methanoic acid? Because the Latin term for ant is Formica, methanoic acid is referred to as Formic acid, referring to its early isolation via distillation of ant bodies. Vinegar, also known as acetic acid or Ethanoic acid, is a typical preservative in our kitchen. What is the origin of vinegar’s name? Let’s learn everything about the naming of carboxylic acids in this article.

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Carboxylic Acid

Carboxylic acids consist of a carboxyl functional group \(\left( { – {\rm{COOH}}} \right)\) in which a carbon atom is bonded to an oxygen atom by a double bond \(\left( {{\rm{C}} = {\rm{O}}} \right)\) and a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) through a single bond. The carboxyl \(\left( {{\rm{COOH}}} \right)\) group is so-named because of the carbonyl group \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and a hydroxyl group.

The name carboxyl comes from the fact that a carbonyl group \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) are attached to the same carbon atom.

\({\rm{Carbonyl group}}{\mkern 1mu} ( > {\rm{C}} = {\rm{O}}) + {\rm{Hydroxyl }}{\mkern 1mu} {\rm{group }}\left( { – {\rm{OH}}} \right) \to {\rm{Carboxyl}}\;{\rm{group }}\left( { – {\rm{COOH}}} \right)\)

A carboxylic acid group always lies at the end of a carbon chain. The general molecular formula of the carboxylic acid homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{{\rm{O}}_2}\), where \({\rm{n}} = 1,\,2,\,3 \ldots \)

The general structural formula of the carboxylic acid is \({\rm{RCOOH}}\), where \({\rm{R}}\) stands for any side group comprising \({\rm{R}}\) or an alkyl group or another \( – {\rm{C}}\) bonded to a certain chain.

Carboxylic acids occur widely in nature and in everyday household items.

1. Vinegar contains acetic acid,
2. aspirin is acetylsalicylic acid,
3. vitamin C is ascorbic acid,
4. lemons contain citric acid, and
5. spinach contains oxalic acid.

The Naming of Carboxylic acids

Many carboxylic acids are called by the common names. Chemists assign these names to describe the source from where the compound is obtained. This naming is done by adding \( – {\rm{ic}}\) to the source name of the compound. Some of these compounds are listed below:

When substituted carboxylic acids are named by common names, the carbon positions are often designated with Greek letters. The carbon next to the \(\ – {\rm{COOH}}\) group is called the \(\alpha \) carbon, followed by \(\beta \) (beta), \(\gamma \) (gamma), \(\delta \) (delta), etc.

For example:

IUPAC System of Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system assigns a characteristic suffix to this class of compounds. The parent hydrocarbon chain’s -ane suffix is changed to “oic acid,” dropping the “\({\rm{e}}\)” of the parent alkane name. Some basic IUPAC rules for naming alcohols are:

1. The longest carbon chain of carbon atoms containing the \(\ – {\rm{COOH}}\) group is chosen as the parent compound.

2. The carboxyl \(\left( { – {\rm{COOH}}} \right)\) functional group is always present at the end of the hydrocarbon chain; hence it occupies the \(# 1\) locant in numbering, and it is not necessary to include it in the name.

3. The carboxyl group has the highest priority over aldehydes, alkyl groups and halogen substituents, and double and triple bonds in the numbering of the parent chain.

The order of decreasing priority for some functional groups is:

\([ – {\rm{COOH}},{\mkern 1mu} – {\rm{S}}{{\rm{O}}_3}{\rm{H}},{\mkern 1mu} – {\rm{COOR}}({\rm{R}} = {\rm{alkyl}}{\mkern 1mu} {\rm{group}}),{\mkern 1mu} {\rm{COCl}},{\mkern 1mu} – {\rm{CON}}{{\rm{H}}_2},{\mkern 1mu} – {\rm{CN}},{\mkern 1mu} – {\rm{HC}} = {\rm{O}},{\mkern 1mu} > {\rm{C}} = {\rm{O}},{\mkern 1mu} – {\rm{OH}},{\mkern 1mu} – {\rm{N}}{{\rm{H}}_2},{\mkern 1mu} > {\rm{C}} = {\rm{C}} < ,{\mkern 1mu} – {\rm{C}} \equiv {\rm{C}} – \)

4. The -e suffix of the parent hydrocarbon alkane chain is replaced by -oic, and the number indicating the \(\ – {\rm{COOH}}\) group’s position is not mentioned in the name of the parent hydrocarbon.

The naming of carboxyl groups in the presence of other functional groups

5. Carboxylic acids have a higher priority than all other functional groups, and therefore, they define the parent chain. They give the corresponding suffix and specify the compound’s name. All the other groups standing below in the functional group priority table are added as a prefix.

If there are other substituents present in the parent chain, then the numbering is done to give the substituents the lowest number at the first point of difference.

1. In molecules containing carboxylic acid and alcohol functional groups, the \(\ – {\rm{OH}}\) group is a substituent (low priority than \(\ – {\rm{COOH}}\)). It is named as a hydroxy substituent, as shown in the above table.

2. In molecules containing carboxylic acid and aldehydes, the prefix ‘Formyl’ is used.

3. In molecules containing carboxylic acid and ketones functional groups, the carbonyl is named as an “oxo” substituent.

4. In molecules containing a carboxylic acid and amine functional group, the amine is named an “amino” substituent.

5. In the case of molecules containing carboxylic acids linked to an alkene, the following order is followed:

(Location number of the alkene)-(Prefix name for the longest carbon chain, replace \(‘ – {\rm{ane}}’\) with \(‘ – {\rm{en}}’\) ending)\(-\)(Locant number)\(-\)(add \( – {\rm{oic}}\) acid ending to indicate the presence of an alkene and carboxylic acid)

As shown below, the \(‘{\rm{en}}’\) and \(‘{\rm{oic}}’\) parts are generally written together as \(‘{\rm{enoic}}’\) The carboxylic acid has a higher priority, so it should get the lowest possible locant number. In the case of alkenes, \({\rm{cis}}/{\rm{trans}}\) or \({\rm{E}}/{\rm{Z}}\) nomenclature should be included if required. It is unnecessary to specify the carboxyl group’s location because it will automatically be carbon \(# 1\).

Naming carboxyl groups added to a ring

6. When a carboxyl group is added to a ring, the suffix -carboxylic acid is added to the name of the cyclic compound. The ring carbon attached to the carboxyl group is given the \(#1\) location number.

7. If substituents are also present, the numbering starts from the carbon connected to the \({\rm{COOH}}\) group. It goes either in a clockwise or anticlockwise direction that minimizes the numbering of the substituents:

8. The compound is named benzoic acid if the carboxyl group is directly attached to an aromatic ring. In a substituted aromatic carboxylic compound, the name of the compound is derived from benzoic acid. The aromatic ring carbon that carries the carboxyl group is marked by the position number \(“1″\).

Naming carboxylates

9. Salts of carboxylic acids are named by writing the name of the cation followed by the name of the acid with an \( – {\rm{ate}}\) ending. The \( – {\rm{ic}}\) acid ending of the acid is replaced by an \(- {\rm{ate}}\) ending. This is true for both the IUPAC and Common nomenclature systems.

Naming dicarboxylic acids

10. For dicarboxylic acids, the location numbers for both carboxyl groups are omitted because both functional groups are expected to occupy the ends of the parent chain. The ending \(- {\rm{dioic}}\) acid is added to the end of the parent chain.

Trivial names of dicarboxylic acids are also used. These names are often derived from the source from which they were initially isolated. Some of the dicarboxylic acids, along with their common names and IUPAC names, are listed below:

Structure	Common name	IUPAC name
\({\rm{COOH}} – {\rm{COOH}}\)	Oxalic acid	Ethanedioic acid
\({\rm{COOH}} – {\rm{C}}{{\rm{H}}_2} – {\rm{COOH}}\)	Malonic acid	Propanedioic acid
\({\rm{COOH}} – {\left( {{\rm{C}}{{\rm{H}}_2}} \right)_2} – {\rm{COOH}}\)	Succinic acid	butanedioic acid
\({\rm{COOH}} – {\left( {{\rm{C}}{{\rm{H}}_2}} \right)_3} – {\rm{COOH}}\)	Glutaric acid	pentanedioic acid
\({\rm{COOH}} – {\left( {{\rm{C}}{{\rm{H}}_2}} \right)_4} – {\rm{COOH}}\)	Adipic acid	hexanedioic acid
	Maleic acid	cis-\(2\)-butenedioic acid
	Fumaric acid	trans-\(2\)-butenedioic acid

Solved Example on Naming of Carboxlyic Acid

Name the following compound:

The parent chain consists of seven carbon atoms and is heptane. So, we have heptanoic acid. Halogens and nitrile groups rank very low in the priority list of functional groups. Hence, they are considered substituents and get a prefix. For fluorine, the prefix “fluoro” is used along with its position, and for nitrile, the prefix ‘cyano’ is used along with its position in alphabetical order.

Hence, the IUPAC name of the compound is :

Summary

Carboxylic acids are an essential class of compounds that are widely used in our daily lives. The sourness of lemon is due to the presence of a carboxylic acid called citric acid. One of the most common preservatives available in household items is vinegar which is nothing else but a carboxylic acid known as ethanoic acid. We must learn the methods and rules of naming these common acids. In this article, we learned the IUPAC nomenclature as well as common names of some carboxylic acids.

FAQs on Nomenclature of Carboxylic acid

Q.1. What are the trivial names of some carboxylic acid?

Q.2. Is ethanoic acid and vinegar the same?
Ans: Vinegar is the common name of ethanoic acid. It is also known as acetic acid. Carboxylic acids derive their common names from their source of isolation. The Latin name of vinegar is acetum; hence it is called acetic acid.

Q.3. What is the structure of carboxylic acid?
Ans: The general molecular formula of the carboxylic acid homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{{\rm{O}}_2}\), where \({\rm{n}} = 1,\,2,\,3 \ldots \). The general structural formula of the carboxylic acid is \({\rm{RCOOH}}\), where \({\rm{R}}\) stands for any side group comprising \({\rm{R}}\) or an alkyl group or another \( – {\rm{C}}\) bonded to a certain chain. In a carboxyl group, a carbon atom is bonded to an oxygen atom by a double bond \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and to a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) through a single bond. The name carboxyl comes from the fact that a carbonyl group \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) are attached to the same carbon atom.

\({\rm{Carbonyl}}{\mkern 1mu} {\rm{group}}( > {\rm{C}} = {\rm{O}}) + {\rm{Hydroxyl}}{\mkern 1mu} {\rm{group}}( – {\rm{OH}}) \to {\rm{Carboxyl}}\;{\rm{group}}( – {\rm{COOH}})\)

Q.4. How do you name a carboxylic acid?
Ans: In IUPAC nomenclature, the longest carbon chain consisting of the carboxylic group is chosen as the parent chain. All other functional groups are numbered accordingly with the lowest position (always 1) occupied by the carboxyl group. These functional groups act as substituents with their names arranged in alphabetical order. The -ane of the parent carbon chain is replaced by -anoic. The prefixes used for the substituents is followed by adding -oic to the name of the parent chain. However, the common names of carboxylic acids are derived from their source of isolation.

Q.5. What is the structural formula of the carboxylic acid?
Ans: The general structural formula of the carboxylic acid is \({\rm{RCOOH}}\), where \({\rm{R}}\) stands for any side group comprising \({\rm{H}}\) or an alkyl group or another \( – {\rm{C}}\) bonded to a certain chain. In a carboxyl group, a carbon atom is bonded to an oxygen atom by a double bond \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and to a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) through a single bond. The name carboxyl comes from the fact that a carbonyl group \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) and a hydroxyl group \(\left( { – {\rm{OH}}} \right)\) are attached to the same carbon atom.

\({\rm{Carbonyl}}{\mkern 1mu} {\rm{group}}( > {\rm{C}} = {\rm{O}}) + {\rm{Hydroxyl }}{\mkern 1mu} {\rm{group}}( – {\rm{OH}}) \to {\rm{Carboxyl}}\;{\rm{group}}( – {\rm{COOH}})\)

Study Physical Properties of Carboxylic Acids Here

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