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

Acyclic hydrocarbons include straight chain and branched chain compounds.

Straight Chain Hydrocarbons

  • The names of these hydrocarbons consist of two parts.
  • The first one is word root and second one is suffix.
  • The root word defines the number of carbon atoms in the chain.

Some Root Words and Corresponding Number of Carbon Atoms

Some Root Words and Corresponding Number

A suffix is added to the word root to indicate saturation or unsaturation in the hydrocarbons.

Types of Hydrocarbons and Suffixes in Their Names

Types of Hydrocarbons

Some examples are below:

Types of Hydrocarbons

Branched Chain Hydrocarbons

  • In branched chain hydrocarbons, one or more alkyl groups are present as side chain attached to the main straight chain of carbon atom.
  • The carbon atom in the side chain constitute alkyl groups.
  • These alkyl groups are written as prefixes in the IUPAC names.
  • An alkyl group is obtained from an alkane by removing one hydrogen atom.

Some Alkyl Groups

Some Alkyl Groups

Branched chain hydrocarbons are named using the following rules:

Rule 1: Longest Chain Rule

  • According to this rule, the longest possible chain of carbon atoms is considered, and the compound is named as the derivative of the corresponding alkanes.
  • If some multiple bond is present, the selected chain must contain the carbon atoms of the multiple bond.

For example,

Longest Chain Rule

It has a main chain of six carbon atoms; hence, it will be named as a derivative of hexane.

Longest Chain Rule
  • The main chain of carbon atoms containing double bond consists of four carbon atom. Therefore, the compound will be a derivative of butene.
  • If two equally long chains are possible, the chain with maximum number of side chain is selected as a main chain.

Rule 2: Lowest Number or Lowest Sum Rule

The longest carbon chain is numbered from one end to another and the positions of the side chain are indicated by the number of carbon atoms to which these are attached.

The numbering is done such that:

The substituted carbon atoms have the lowest possible numbers.

Lowest Number or Lowest Sum Rule

The sum of numbers used to indicate the positions of various alkyl groups must be the lowest.

Lowest Number or Lowest Sum Rule

Developed by: