Nomenclature of Organic Compounds

What is 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.

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Trivial Nomenclature System

The Trivial Nomenclature system involves a non-systematic approach to the naming of organic compounds. There do 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 of 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.

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 it will be named as an 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 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 CnH2n+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 CnH2n

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 CnH2n-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:

Nomenclature of Organic Compounds

  • 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.

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