In earlier days, organic compounds were known by their common names for example methane was known as marsh gas as it was found 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 a proper method was introduced in order to name the organic compounds. This uniform system for naming the compounds is called as IUPAC system, which is the International Union of Pure and Applied Chemistry.
Organic Chemistry Nomenclature – Features of the Trivial system
A trivial system is a vernacular name or non-systematic name of an organic compound. There are no particular set of rules for the trivial name of the compound.
In this system, names will be simple like acetic acid, toluene, and phenol etc. For example, a carbolic acid which is generally found in tamarind is named as tartaric acid. But in IUPAC, it is named as 2,3-dihydroxy-1,4-Butanedioic acid.
- 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.
Chemical Nomenclature – IUPAC Rules:
According to the IUPAC system, the nomenclature of organic compounds consists of the following parts:
- Longest Chain Rule: Identify the parent hydrocarbon and name it. The parent chain of the compound is considered as the longest chain of carbon atoms. This chain can either be straight or of any other shape.
- Lowest Set of Locants: The numbering of the carbon atoms in the longest chain starts from the end which gives the lowest number to the carbon atoms carrying the substituents.
- Presence of same substituent more than once: Prefixes such as di, tri, etc. are given to the substituents which are present twice, thrice respectively in the parent chain.
- Naming different substituents: If more than one substituent is present then the substituents are arranged in alphabetical order of their names.
- Naming different substituents at equivalent positions: If two different substituents are present on the same position from the two ends then the substituents are named such that the substituent which comes first in the alphabetical order gets the lowest number.
- Naming The Complex Substituents: Naming of a complex substituent is done when the substituent on the parent chain has a branched structure (i.e complex structure). These substituents are named as a substituted alkyl group and the carbon atom of this substituent attached to the parent chain is numbered as 1. Name of these type of substituents is written in brackets.
The final name will be in format : Locant + Prefix + Root + Locant + Suffix
Word root: It indicates the number of carbon atoms in the longest carbon chain that is selected. For example, C1 is Meth and C5 is Pent.
The suffix – primary (1°) and secondary (2°): A suffix is generally a functional group in the molecule which follows the word root. It is further divided into two types.
- Primary suffix: It is written immediately after the word root. For example, in alkanes the suffix is ane.
- Secondary suffix: It is written after the primary suffix. For instance, if a compound has an alkane and alcohol group attached to it, the naming will be alkanol, -ol being the suffix for alcohol.
Prefix – primary (1°) and secondary (2°): It is added before 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 -CH3 is known as Methyl and -Br is Bromo.
Chemical Nomenclature- Types
- The term is used to denote 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 generalized 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 they are divided into 2 classes namely electropositive and electronegative components.
- These names will sound like salt names and this does not imply the chemical nature and behavior of species that are named.
- The rules ordered are required to the using up of multiple prefixes, ordering of components and proper endings in names for electronegative components.
Examples– Sodium Chloride – NaCl, Trioxygen – O3, Phosphorous trichloride – PCl3
- It is based on the approach where parent hydride is changed by replacement of hydrogen atoms with atoms or a group of atoms.
- It is a system where organic compounds are named using functional groups as the suffix or prefix to the name of the parent compound.
- This system is also used 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 belong to group 13-17 of the periodic table are named suffix – ane. For example – Borane, Phosphane, and oxidane etc.
Examples – 1, 1-difluoro trisilane (SiH3 SiH2 Si HF2), Trichlorophosphine, PCl3
- This method is mainly formulated 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.
Examples– PCl3 – trichloridophosphorus, [CoCl3 (NH3)3] – tri-ammine-trichloridocobalt.
Names of some important aliphatic compounds
- Alkanes: General formula = CnH2n+2
Suffix – ane
Examples: CH4 – methane and C4H10 – Butane
- Alkenes: General formula = CnH2n
Suffix – ene
Examples: C2H4 – ethene and C3H6 – Propene
- Alkynes: General formula = CnH2n-2
Suffix – yne
Examples: C2H2 – Ethyne and C4H6 – But-1-yne
Let us understand it with the help of an example:
In this case, we have 9 carbon atoms in the straight chain. 5th Carbon atom from both the ends of the straight chain consists of a substituent having 3 carbon chains. On the first two carbon atoms of the substituent group, there is one additional carbon atom attached.
Now if we consider this as a new parent chain, it has a substituent which has one additional carbon each. For naming them we will first number the parent chain. In this case, we have 9 carbon atoms in the straight chain which is also the parent chain. Then we find that the substituent is on the fifth carbon atom.
Now we have 3 substituent carbons and out of these three, two substituents have additional carbons atoms attached to them. We find that the longest chain, in this case, can be the first four carbon atom chain but this is wrong as the last carbon is not attached to the parent chain.
So we will consider only three carbon atom chain as the main chain. Thus it can be named as propane and on the first and second position, we have methyl group. We can write the name as 1-2 Dimethyl propane, but it will be written as 1-2 Dimethyl propyl as it is a substituent group.
Now taking the substituent with the parent chain we will get 5-(1-2-Dimethyl Propyl) and as the parent chain has 9 carbon atoms so, it will be named as nonane. Thus the final name of the compound will be 5-(1-2-Dimethyl Propyl)nonane.
This article covers the basics of naming the organic compounds. For any further query on organic chemistry, install, Byju’s the learning app.