Nomenclature of Organic Compounds

Nomenclature means the system of naming of organic compounds. In case of aliphatic compounds, two system of nomenclature generally used are:

(1) Trivial or common system

(2) IUPAC system

Trivial or Common System

Organic compounds were named after the source from which they were first isolated.

For example : Urea got its name since the compound was first obtained from the urine of mammals.

Methyl alcohol was called wood spirit since it could be obtained by the destructive distillation of wood.

Acetic Acid got its name from the acetum since it is present in vinegar.

Formic acid was derived from formicus since it could be obtained by the destructive distillation of red ants.

Citric acid is named so because it is found in citrus fruits.

 

Common or Trivial names of some organic compounds:

Compound	Common Name
CH4	Methane
C2H2	Acetylene
CH3CH2CH2CH3	Butane
(CH3)2CHCH3	Isobutane
(CH3)4C	Neopentane
CHCl3	Chloroform
CH3CH2OCH3	Ethyl Methyl ether
HCHO	Formaldehyde
(CH3)2CO	Acetone
CH3COOH	Acetic acid
CH3CONH2	Acetamide
CH3CN	Acetonitrile
C6H6	Benzene
C6H5OH	Phenol
C6H5OCH3	Anisole
C6H5NH2	Aniline

IUPAC system

To systematize the nomenclature of organic compounds, IUPAC system of nomenclature was first introduced in 1947.

General Rules for IUPAC Nomenclature of Organic Compounds

The most important feature of this system is that any given molecular structure has only one IUPAC name and any given IUPAC name denotes only one molecular structure.

The IUPAC name of any organic compound essentially consist of three parts ,i.e.

(1) word root

(2) suffix

(3) prefix

Word root

It denotes the number of carbon atoms present in the parent chain (the longest possible continuous chain of carbon atoms including the functional group and multiple bonds) of the organic molecule.

 

Chain Length	Word Root
C1	Meth
C2	Eth
C3	Prop
C4	But
C5	Pent
C6	Hex
C7	Hept
C8	Oct
C9	Non
C10	Dec
C11	Undec
C12	Dodec

Suffix

There are two types of suffix :

(1) Primary suffix

A primary suffix is always added to the word root to indicate whether the carbon chain is saturated or unsaturated.

The three basic primary suffixes are :

Type of carbon chain	Primary suffix	General name
Saturated	-ane	Alkane
Unsaturated with one double bond	-ene	Alkene
Unsaturated with one triple bond	-yne	Alkyne

If the parent chain contains two, three, four or more double or triple bonds ,numerical prefixes such as di, tri, tetra are added to the prefect primary.

Type Of carbon chain	Primary Suffix	General name
Unsaturated with two double bonds	-diene	Alkadiene
Unsaturated with two triple bonds	-diyne	Alkadiyne

 

(2) Secondary suffix

A secondary suffix is then added to the primary suffix to indicate the nature of the functional group present in the organic compound.

 

Class of Organic compound	Functional Group	Secondary Suffix
Alcohols	-OH	-ol
Aldehydes	-CHO	-al
Ketones	-CO	-one
carboxylic acid	-COOH	-oic acid
Acid amides	-CONH2	-amide
Acid chloride	-COCl	-oyl chloride
Esters	-COOR	alkyl oate
Nitriles	-CN	nitrile
Thiols	-SH	thiol
Amines	-NH2	amine

While adding the secondary suffix to primary suffix ,the terminal e of the primary suffix is dropped if the secondary suffix begins with a vowel but is retained if the secondary suffix begins with a consonant.

 

(1) Locants are placed immediately before the part of the name to which they relate.

For Example :  But-2-ene , Propan-2-ol

(2) The locant 1 is often omitted when there is no ambiguity.

CH₃CH₂CH₂NH₂ Propane-1-amine is often named as Propamine.

ClCH₂CH₂OH 2-Chloroethan-1-ol is often named as 2-Chloroethanol

CH₃CH₂CH₂CHO Butan-1-al is often named as 2-Chloroethanol.

Prefix

There are two types of prefixes:

(1) Primary Prefix

A primary prefix is used simply to distinguish cyclic from acyclic compounds.

For example : In case of carbocyclic compounds, a primary prefix, cyclo is used immediately before the word root.

Cyclopentane

Cyclo(Primary prefix) + Pent (word root) + ane( Primary suffix)

(2) secondary prefix 

Certain groups are not considered as functional groups but instead are treated as substituents. These are called secondary prefixes and are added immediately before the word roots in alphabetical order to denote the side chain or substituent groups.

The secondary prefixes for some groups which are always treated as substituent group are given below:

Substituent Group	Secondary group	Substituent Group	Secondary Prefix
-F	Fluoro	OCH3(OMe)	Methoxy
-Cl	Chloro	-OC2H5(-OEt)	Ethoxy
-Br	Bromo	-CH3(-Me)	Methyl
-I	Iodo	-C2H5(-Et)	Ethyl
-NO2	Nitro	-CH2CH2CH3(n-Pr)	n-Propyl
	Diazo	-C(CH3)3	tert-Butyl
—NO	Nitroso	-CH(CH3)2(-iPr)	Isopropyl

 

IUPAC name of an organic compound consist of:

Secondary prefix + Primary prefix + Primary suffix + Secondary Suffix

Organic compound	Secondary Prefix	Word Root	Primary Suffix	IUPAC name
CH3CH2Br	Bromo	eth	ane	Bromoethane
CH3NO2	Nitro	meth	ane	Nitromethane
C2H5OC2H5	Ethoxy	eth	ane	Ethoxyethane