Nomenclature of Organic Compounds - IUPAC

What is IUPAC Nomenclature?

IUPAC nomenclature of organic compounds refers to the systematic approach taken for the nomenclature of organic compounds as per the recommendation of the International Union of Pure and Applied Chemistry (often abbreviated to IUPAC).

The necessity for such a systematic approach arose due to the sheer quantity of new discoveries of organic compounds which made the trivial nomenclature of organic compounds highly inconvenient.

However, the IUPAC nomenclature guidelines are not always followed by chemists since some compounds have very long and extremely tedious names as per the IUPAC nomenclature guidelines. These compounds are assigned more trivial names.

Table of Contents

• Trivial Nomenclature System
• Drawbacks of the Trivial Nomenclature System
• IUPAC Nomenclature
• IUPAC Nomenclature Methods
• IUPAC Nomenclature of a Few Important Aliphatic Compounds
• Example of IUPAC Nomenclature

IUPAC Nomenclature of Organic Chemistry

In chemical nomenclature, the organic chemistry nomenclature IUPAC is a method of organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the Organic Chemistry Nomenclature.

Ideally, any conceivable organic compound should have a name from which to construct an unmistakable structural formula. Inorganic chemistry also has an IUPAC nomenclature.

In order to avoid long and tedious names in normal communication, the official recommendations for naming IUPAC are not always followed in practice, except when a compound has to be given an unambiguous and absolute definition. Sometimes, IUPAC names may be simpler than older names, as with ethanol, rather than ethyl alcohol.

They can be more easily understood for relatively simple molecules than non-systematic names which have to be learned or looked over. The common or trivial name, however, is often considerably shorter and clearer, and so preferred. Such non-systematic names are often derived from an original compound source. Furthermore, very long names may be less obvious than structural formulas.

Organic Chemistry Nomenclature

Each and every object in this universe is identified by its name. When organic chemistry was in its infancy and not many compounds were known, the compounds were named based on their sources or the name of their discoverer. These names are called common or trivial names.

A set of rules formulated by IUPAC (the International Union of Pure and Applied Chemistry) for systematic nomenclature of organic compounds which is revised from time to time. The IUPAC nomenclature of organic compounds essentially consists of three parts which are stem name, prefix and suffix.

It is interesting to note that the existence of preferred IUPAC names does not prevent the use of other names to take into account a specific context or to emphasize structural features common to a series of compounds. Preferred IUPAC names belong to a “preferred IUPAC nomenclature”. Any name other than a preferred IUPAC name (as long as it is unambiguous and follows the principles of the IUPAC recommendations herein) is acceptable as a general IUPAC name in the context of a “general IUPAC nomenclature”.

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Trivial Nomenclature System

The Trivial Nomenclature system involves a non-systematic approach to the naming of organic compounds. There does not exist any particular collection of rules for writing the trivial naming of compounds. Names of organic compounds are simplified via this system. Examples of this include phenol, acetic acid, and toluene.

The compounds named via trivial nomenclature often have much shorter and simpler names than the corresponding IUPAC nomenclature of the same compounds. For this reason, this system is still not obsolete today.

An example of this relative ease of naming compounds can be seen in the following example – A type of carboxylic acid which is generally found in tamarind is referred to as tartaric acid as per the trivial system. The corresponding IUPAC nomenclature of tartaric acid would be 2,3-dihydroxy-1,4-Butanedioic acid.

Drawbacks of the Trivial Nomenclature System

A few shortcomings of the trivial system for naming organic compounds are listed below.

• Several trivial names can exist for one specific compound. An example of this can be observed in the alternate names of Phenol, for which names such as hydroxybenzene and carbolic acid also exist.
• The Trivial nomenclature system is limited to only a few compounds in each specific group. An example of this is: the first two members belonging to the carboxylic acid group have the trivial names formic acid and acetic acid. However, no trivial names exist for carboxylic acids with a greater number of atoms.
• There exist no particular set of guidelines for the nomenclature of complex compounds in the trivial system.

Nomenclature of Organic Compounds

The formation of a systematic name for an organic compound requires the first selection and then naming of a parent structure. This basic name may then be modified by prefixes, infixes and in the case of a parent hydride, suffixes which convey precisely the structural changes required to generate the compound in question from the parent structure. In contrast to such systematic names, there are traditional names which are widely used both in industry and academic circles. Examples are acetic acid, benzene and pyridine. Therefore when they meet the requirements of utility and when they fit into the general pattern of systematic nomenclature, these traditional names are retained.

A major new principle is elaborated and the concept of preferred IUPAC names is developed and systematically applied. Up to now, the nomenclature developed and recommended by IUPAC has emphasized the generation of unambiguous names in accord with the historical development of the subject. In 1993 due to the explosion in the circulation of information and the globalization of human activities it was deemed necessary to have a common language that would prove important in legal situations with manifestations in patents, export import regulations, environmental health and safety information etc.

Scope of Nomenclature for Organic Compounds

For nomenclature purposes all compounds containing carbon as the principal element to be organic compounds are qualified. Oxygen, hydrogen and nitrogen are the three elements usually associated with carbon to form the system of functional or characteristics groups. Other elements, among them halogens and sulfur complete the basic core of elements found in organic compounds. Substitutive nomenclature was first applied to compounds containing this set of atoms. The success of this type of nomenclature was such that it was extended to all elements of Groups 14, 15, 16, 17 and in Group 13 to boron; it could be extended to all elements of Group 13.

IUPAC Nomenclature

According to the Guidelines set by IUPAC, the nomenclature of compounds must follow these steps:

1. The Longest Chain Rule: The parent hydrocarbon must be identified and subsequently named. The parent chain belonging to the compound in question is generally the longest chain of carbon atoms, be it in the form of a straight chain or a chain of any other shape.
2. The Lowest Set of Locants: The carbon atoms belonging to the parent hydrocarbon chain must be numbered using natural numbers and beginning from the end in which the lowest number is assigned to the carbon atom which carries the substituents.
3. Multiple instances of the same substituent: Prefixes which indicate the total number of the same substituent in the given organic compounds are given, such as di, tri, etc.
4. Naming of different substituents: In the organic compounds containing multiple substituents, the corresponding substituents are arranged in alphabetical order of names in the IUPAC nomenclature of organic compounds in question.
5. The naming of different substituents present at the same positions: In the scenario wherein two differing substituent groups are present at the same position of the organic compound, the substituents are named in ascending alphabetical order.
6. Naming Complex Substituents: Complex substituents of organic compounds having branched structures must be named as substituted alkyl groups whereas the carbon which is attached to the substituent group is numbered as one. These branched and complex substituents must be written in brackets in the IUPAC nomenclature of the corresponding compounds.

The format of the IUPAC Name of the Compound can be written as: Locant + Prefix + Root + Locant + Suffix

1. Root

The Word root indicates the total number of carbon atoms present in the longest carbon chain belonging to the compound. For example, ‘Meth’ refers to a chain with 1 carbon atom and ‘Pent’ refers to a chain with 5 carbon atoms.

2. Suffix

The suffix in IUPAC nomenclature is usually a functional group belonging to the molecule which follows the root of the name. It can be further divided into the following types.

• A Primary Suffix, which is written immediately after the word root as in the case of alkanes, where the suffix is ‘ane’.
• A Secondary Suffix, which is generally written after the primary suffix is written. For example, compounds having an alkane and alcohol group attached to them will be named as alkanol, with ‘ol’ being the secondary suffix for the alcohol group.

In accordance with these norms, the suffix of a compound can be written as a part of the IUPAC name of the given compound.

3. Prefix

Prefixes are added prior to the root of the compounds IUPAC nomenclature. Prefixes are very useful since they indicate the presence of side chains or substituent groups in the given organic molecule. These prefixes also offer insight into the cyclic or acyclic natures of the compounds in question.

• Primary Prefixes Indicate the cyclic or acyclic nature of the given compound. The prefix ‘cyclo’ is used for cyclic compounds, for example.
• Secondary Prefixes Indicate the presence of side chains or substituent groups. An example of these types of prefixes would be the ‘CH₃’ group, which is called the methyl group.

Thus, prefixes in IUPAC nomenclature can be broadly classified into primary prefixes and secondary prefixes.

IUPAC Nomenclature Methods

1. Compositional Nomenclature

Compositional Nomenclature is used to name compounds based on the composition of the species or substances against systems involving structural information or composition, in accordance with IUPAC nomenclature.

• The generalized stoichiometric name is involved in the compositional nomenclature of compounds. Substances are named using multiple prefixes so that the overall stoichiometry of the compound is made clear from the name.
• In cases wherein multiple components are present, the components are divided into two classes – electronegative components and electropositive components.
• These names are similar to the names of salts. However, this does not predict or suggest the behaviour or chemical nature of the species named in such a way.
• The usage of multiple prefixes along with the ordering of components and the usage of the more electronegative component towards the end of the name must be kept in mind while naming these compounds via IUPAC norms. An example of such a name for a compound would be the name phosphorus trichloride used to refer to the compound PCl₃

2. Substitutive Nomenclature

Substitutive Nomenclature is used in the IUPAC nomenclature of compounds wherein the parent hydride is altered by the replacement of hydrogen atoms with a substituent group.

• In this nomenclature system, organic compounds are named with the use of functional groups as the prefix or suffix to the parent compounds name.
• Compounds derived from the hydrides of specific elements can also be named using this method. These elements may also from ring structured or chain structured compounds
• Hydrides that belong to groups 13 to 17 of the modern periodic table are named with the suffix ‘ane’. Examples of this include Borane, Phosphane, and oxidane, etc.

An example of the use of substitutive nomenclature can be observed in the usage of the name Trichlorophosphine to refer to the compound PCl₃

3. Additive Nomenclature

This method was formulated primarily for its applications in the nomenclature of coordination compounds. It has a wide range of applications. An example for such nomenclature can be observed in the name Penta-ammine-chloro-cobalt(III) chloride used to describe the coordination compound given by the chemical formula [CoCl(NH₃)₅]Cl₂.

“The prefix ‘chloro’ corresponds to a Chloride, whereas the prefix ‘chlorido’ corresponds to the ligand.”

An example of this nomenclature can be observed in the name tri-chlorido-phosphorus which is used to describe the compound with the formula PCl₃

IUPAC Nomenclature of a Few Important Aliphatic Compounds

The IUPAC nomenclature of alkanes, alkenes, and alkynes are discussed in the subsections below.

1. Alkanes

The General formula of alkanes corresponds to C_nH_2n+2

The suffix ‘ane’ is generally used to describe alkanes. Examples for the nomenclature of alkanes as per IUPAC guidelines include methane for the compound CH₄ and Butane for the compound C₄H₁₀

2. Alkenes

The General formula of alkenes is described as C_nH_2n

The suffix ‘ene’ is used to describe alkenes via IUPAC norms. Examples for the nomenclature of alkenes include the name ethene used to describe the compound given by C₂H₄ and Propene used to describe the compound given by C₃H₆

3. Alkynes

The General formula of alkynes is C_nH_2n-2

The suffix ‘yne’ is generally used to describe alkynes. An example of the IUPAC nomenclature of alkynes is: ethyne used to describe the compound given by C₂H₂

Example of IUPAC Nomenclature

Considering the following Example:

• There exist 9 carbon atoms on the straight chain and the 5th carbon atom (from both ends of the chain) consists of a substituent group which in turn has 3 carbon atoms in a chain.
• Furthermore, there the first and second carbons of this substituent chain have an additional CH group attached to them.
• In the nomenclature of this compound, the 9 membered carbon chain is identified as the parent chain and is numbered.
• The substituent chain attached to position 5 of the parent chain is 3 members long, with 2 methyl groups attached at positions 1 and 2.
• Thus the carbon chain substituent group on the parent chain can be called 1,2 dimethyl propane. The name for the substituent chain containing this compound would be 1,2 dimethyl propyl.
• Substituting this name on the parent chain, the IUPAC name of the compound in question is found to be: 5-(1,2 dimethyl propyl) nonane.

Thus, the IUPAC nomenclature of the given carbon chain based compound is achieved. To Learn more about organic chemistry, Install BYJU’s – The Learning App from the Google Play Store for Free.