General rules for IUPAC nomenclature

16 Feb

According to IUPAC system, the systematic name of the organic compound consists of three parts:

  1. Root word
  2. Suffix
  3. Prefix

Root word:

A root word indicates the nature of the basic carbon skeleton. For chains containing upto four carbon atoms special root words are used and for those with more than four carbon atoms, Greek numerals are used.

Chain Length Root word Chain
Length
Root word
C1 Meth– C16 Hexadec–
C2 Eth– C17 Heptadec–
C3 Prop– C18 Octadec–
C4 But– C19 Nonadec–
C5 Pent– C20 Icos–
C6 Hex– C21 Heneicos–
C7 Hept– C22 Dcos–
C8 Oct– C23 Tricos–
C9 Non– C24 Tetracos–
C10 Dec– C25 Pentacos–
C11 Undec– C26 Hexacos–
C12 Dodec– C27 Heptacos–
C13 Tridec– C28 Octacos–
C14 TetraDec– C29 Nonacos–
C15 Pentadec– C30 Triacont–
Suffix:
The root word is linked to the suffix. Suffix may be
(i) Primary and (ii) Secondary
Primary Suffix:
Primary suffix indicates the degree of saturation(single bond) or unsaturation (double or triple bond) in the carbon chain. For single bonded carbon atoms, the primary suffix is “ane“. For the double bonded carbon atoms, it is “ene” and for the triple bonded carbon atoms, the primary suffix is “yne“.
Chain Primary suffix Generic name
C–C –ane Alkane
C=C –ene Alkene
CºC –yne Alkyne
If the parent chain contains two, three or more double or triple bonds, then the following suffixes are used :
Bond Two Three etc.
Double bond –diene –triene …etc.
Triple bond –diyne –triyne …etc.
The root word and the primary suffix together are known as Base Name or Hydrocarbon name.
Formula IUPAC name( Base name)
CH4 Methane (Meth+ane)
CH3(CH2)3CH3 Pentane (Pent+ane)
CH3(CH2)23CH3 Pentacosane (Pentacos+ane)
CH2 = CH2 Ethene (Eth+ene)
CH3CH2CH = CH2 Butene (But + ene)
CH3CH2CH=CH2 Butene (But+ene)
CH3(CH2)12CH=CH2 Pentadecene (Pentadec+ene)
CH º CH Ethyne (Eth+yne)
CH3(CH2)6º CH Nonyne (Non+yne)
CH3º CH Propyne (Prop+yne)
Secondary suffix:
Secondary suffix indicates the functional group present in the molecule. It is added after the primary suffix. Depending on the number of occurrences of the same functional group in a compound, the words like di, tri, tetra etc., are prefixed to the secondary suffix.
Note:While adding the secondary suffix to the primary suffix, the terminal ‘e’ of the primary suffix (i.e., ane, eneor yne) is dropped if the complete secondary suffix ( di, tri, tetra etc., (if any) + secondary suffix ) begins with a vowel. However, the terminal ‘e’ is retained if the complete secondary suffix begins with a consonant.
When an organic compound contains two or more functional groups, one group is treated as the principal functional group and is regarded as secondary suffix whereas the other functional groups are regarded assubstituents and are indicated by prefixes.
The order for the preference of principal functional group is as given below .
Sulphonic acids > Carboxylic acids > Acid anhydrides > Esters > Acyl
halides > Amides > Aldehydes > Nitriles > Ketones > Alcohols > Amines > Ethers
Class of organic compounds Functional group Secondary suffix
Sulphonic acids – SO3H –sulphonic acid
Carboxylic acids Carboxylic acids –oic acid
Acid anhydrides Acid anhydrides –oic anhydride
Esters Esters –alkyl…oate
Acyl halides Acyl halides –oyl halide
Amides Amides –amide
Aldehydes Aldehydes –al
Class of organic compounds Functional group Secondary suffix
Nitriles (Cyanides) Nitriles –nitrile
Isonitriles (Isocyanides) Isonitriles –isocyanide
Cyanates Cyanates –cyanate
Isocyanates Isocyanates –isocyanate
Ketones Ketones –one
Alcohols Alcohols –ol
Thiols –S –thiol
Class of organic compounds Functional group Secondary suffix
Amines Amines
Amine
Suffix
–amine
Ethers Ethers
Alkyl halides Alkyl halides
Alkyl nitrites Alkyl nitrites –nitrite
Alkenes Alkenes –ene
Alkynes Alkynes –ol
Alkyl group(Alkane with one hydrogen less) R–
Note2: If double and triple bond comes together in the same chain, then ‘e’ of ene or yne coming in between is dropped.
Prefixes:
Prefixes indicate the substitution of other groups ( not regarded as functional groups) in place of hydrogen atoms in the compound. These are regarded as substituents or side chains and the names of these are prefixed to the root word in alphabetical order. These names may be
  1. Alkyl groups (formed by the removal of hydrogen atom from alkanes)
  2. some functional groups which are not considered as Principal functional groups.

A few important prefixes are given below

Substituent Prefix
Alkyl groups
– CH3 Methyl
– C2H5 Ethyl
– C3H7 Propyl
– CH(CH3)2 Iso–propyl
– C6H5 Phenyl
– C(CH3)3 t–Butyl
Substituent Prefix
Functional groups as Prefixes
– F Fluoro
– Cl Chloro
– Br Bromo
– I Iodo
– NO2 Nitro
– NO Nitroso
– OCH3 Methoxy
– OC2H5 Ethoxy
– OR R–oxy
– COOH Carboxy
–COOR Carbonyl or Carbaloxy
– COCl Chloroformyl
– OH Hydroxy
– S03H Sulpho
– COX Halo formyl
– CONH2 Carbamoyl
– CHO Formyl
– CO Oxo or Keto
– CN Cyano
– SH Mercapto
– O – Epoxy
– NH2 Amino
= NH Imino
The complete IUPAC name of an organic compound may be represented as:
PREFIX + ROOT WORD + PRIMARY SUFFIX + SECONDARY SUFFIX
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