Nomenclature and General Principles: IUPAC Nomenclature of Acyclic Hydrocarbons (For CBSE, ICSE, IAS, NET, NRA 2022)

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IUPAC Nomenclature of Acyclic Hydrocarbons

Some Alkyl Groups

Rule 3

If some multiple bond is present in the chain the carbon atoms involved in the multiple bond should get the lowest possible numbers. e. g.

Some Alkyl Groups

Rule 4: Naming of the Compounds with One Alkyl Group as the Substituent

The name of a substituted hydrocarbon consists of the following parts Position of substituent - Name of substituent, Word root, Suffix.

Naming of the Compounds
  • In this example the longest chain consists of five carbon atoms and the substituent is methyl group at position number 3.
  • The word root is Pent, and suffix is ane. Hence, the name is 3-methylpentane.

Rule 5: Name the Same Alkyl Groups at Different Positions or More Than Alkyl Groups

  • If the compound contains more than one identical alkyl groups, their positions are indicated separately and the prefixes di (for two) , tri (for three) etc. are attached to the name of the substituents.
  • The positions of the substituents are separated by commas (,) .

For example,

Name the Same Alkyl Groups
  • In this given example, two methyl groups are attached to the main chain of five carbon atoms.
  • They are attached to the positions 2 and 3 of the main chain. Hence, the name of the compound is 2,3-dimethylpentane.

Rule 6: Naming Different Alkyl Substituents

  • If there are different alkyl substituents present in the compound, their names are written in the alphabetical order.
  • The prefixes di, tri, etc. are not consider in deciding the alphabetical order.

For example,

Naming Different Alkyl Substituents
  • In this example the longest chain consists of five carbon atoms, hence, the parent hydrocarbon is pentane.
  • The main chain has two methyl groups at and and one ethyl group at as substituents.
  • Thus, the name of the compound will be 3-ethyl-2,3-dimethylpentane.

Nomenclature of Cyclic Hydrocarbons

Cyclic hydrocarbons can be divided into alicyclic and aromatic compounds.

Alicyclic Compounds

  • Their names are derived by putting prefix “cyclo” before the root word.
  • The suffix ane, ene or yne are written according to saturation or unsaturation in the ring structure.

Examples,

Alicyclic Compound

If an alkyl substituent is present, it is indicated by the appropriate prefix and its position is indicated by numbering the carbon atom of the ring in such a way so as to assign the least possible number to the substituent.

For example,

Alicyclic Compound

Aromatic Compound

  • The most important members of this class are benzene and its derivatives.
  • For naming an alkyl substituted benzene, the carbon atoms of benzene are numbered from 1 to 6 giving the lowest possible number to the position of the side chain or substituents. e. g.
Aromatic Compound

Benzene forms only one monosubstituted derivatives like methylbenzene or ethylbenzene.

It can form three disubstituted compounds namely 1,2 , 1,3 and 1,4 derivatives. These are also known as ortho- , meta- and para- substituted compounds. e. g.

Aromatic Compound

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