Organic Chemistry

Nomenclature of Organic Compounds

How did you get your name? Easy. Maybe your parents or your relatives decided. But, who gave names to the organic compounds? Why are they called what they are called? Have you ever wondered? In this chapter, we will look at the concept of nomenclature of organic compounds. We will see how these compounds get their names. Let’s begin.

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Nomenclature of Organic Compounds

In earlier days, people knew organic compounds by their common names. For example, methane was ‘marsh gas’. This is because we found it in marshy places. With the evolution of so many organic compounds and continuous addition of new compounds, dealing with trivial names became a difficulty.

Therefore, scientists introduced a proper method in order to name the organic compounds. This uniform system for naming the compounds is the IUPAC system, which is the International Union of Pure and Applied Chemistry.

Nomenclature of organic compounds

Features of the Trivial System

The name of an organic compound, when in a non-systematic manner or vernacular name is what is known as a trivial system. There are no particular set of rules for the trivial name of the compound. In this system, names are usually simple like acetic acid, toluene, and phenol etc. For example, tartaric acid is a carbolic acid that we usually find in tamarind. But in IUPAC, it is 2,3-dihydroxy-1,4-Butanedioic acid.

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Drawbacks of this System

  • Many trivial names are present for a single compound. For example, Phenol has different names like hydroxybenzene, carbolic acid, and phenol.
  • This system is limited to few compounds in each group. For example, the first two members of the carbolic acid family have trivial names, formic acid, and acetic acid respectively but carbolic acid with more atoms does not have any trivial names.
  • There are no particular guidelines for naming complex compounds.

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Organic Compounds

Chemical Nomenclature: IUPAC Rules

According to the IUPAC system, the nomenclature of organic compounds consists of the following parts:

1) Steps Involved

  • Longest Chain Rule: In this step, all we have to do is identify the parent hydrocarbon and give the name to it. The parent chain of the compound is usually the longest chain of carbon atoms. This chain could be straight or of a different shape.
  • Lowest number of Locants: We start the numbering of the carbon atoms in the longest chain from the end that gives the lowest number to the carbon atoms carrying the substituents.
  • Multiple Presence of the same substituent: Prefixes such as di, tri, etc. are added to the substituents that are present twice, thrice respectively in the parent chain.
  • Naming the various substituents: If more than one substituent is present, then we need to arrange the substituents in an alphabetical order of their names.
  • Naming different substituents at equivalent positions: If we find the presence of two different substituents on the same position from the two ends, what do we do? In such cases, the substituent first in the alphabetical order gets the lowest number.
  • The Naming of Complex Substituents: We name the complex substituent when the substituent on the parent chain has a branched structure (i.e complex structure). We name these substituents as a substituted alkyl group. It is also important to note that the carbon atom of this substituent gets the number 1. We write the name of these type of substituents in brackets.

The final name will be in format : Locant + Prefix + Root + Locant + Suffix. Now, we will look at some more details of nomenclature of organic compounds.

2) Word root

It indicates the number of carbon atoms in the longest selected carbon chain. For example, C1 is ‘Meth’ and C5 is ‘Pent’.

3) Suffix

A suffix is generally a functional group in the molecule which follows the word root. We can divide it into:

  • Primary suffix: We write it immediately after the word root. For example, in alkanes the suffix is ane.
  • Secondary suffix: We write it after the primary suffix. For instance, if a compound has alkane and alcohol group attached to it, the naming will be alkanol, -ol being the suffix for alcohol.

4) Prefix

We add the prefix to the word root while naming the compound. It indicates the presence of substituent groups or side chains in the organic molecule. It reveals the cyclic and acyclic nature of the compound.

  • Primary prefix: Indicates whether the molecule is cyclic or not. For example, for cyclic compounds the prefix used is cyclo.
  • Secondary prefix: Indicates the presence of substituent groups or any side chain. For example –CH3is known as Methyl and –Br is Bromo.

Types of Chemical Nomenclature

1) Compositional Nomenclature of Organic Compounds

This term denotes the named constructions based on the composition of species or substances being named, against the systems that involve structural composition or information. One among them is the generalised stoichiometric name. Substances or the elements are named with multiple prefixes in order to give the overall stoichiometry of an element or a compound.

When there are more components, then we divide them into 2 classes namely, electropositive and electronegative components. These names will sound like salt names and this does not imply the chemical nature and behaviour of those species. Examples: Sodium Chloride – NaCl, Trioxygen – O3, Phosphorous trichloride – PCl3

2) Substitutive Nomenclature of Organic Compounds

It is based on the approach where parent hydride is changed by replacement of hydrogen atoms with other atoms or a group of atoms. It is a system where we name the organic compounds using functional groups as the suffix or prefix to the name of the parent compound. We use this system in naming compounds derived from hydrides of specific group elements in the periodic table.

Similar to that of carbon, these elements may form rings and chains that will have many derivatives. Rules come in handy in naming the parent or main compounds and their substituents. Hydrides belonging to group 13-17 of the periodic table get the suffix – ane. For example – Borane, Phosphane, and oxidane etc. Examples: 1, 1-difluoro trisilane (SiH3.SiH2.Si.HF2), Trichlorophosphine(PCl3)

Solved Example for You

Q: Write a note on the additive nomenclature of organic compounds.

Ans: We use this method for the coordination compounds even though it has wide applications. An example for its application is pentaamminechlorocobalt (III) chloride – [CoCl(NH3)5]Cl2.

Chloride will have the prefix ‘chloro’ while ligand will have ‘chlorido’.
For example: PCl3 – trichloridophosphorus, [CoCl3 (NH3)3] – tri-ammine-trichloridocobalt.

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