IUPAC Nomenclature of Alkanes | Organic Chemistry Class 11

IUPAC Nomenclature of Alkanes

Systematic Naming of Saturated Hydrocarbons | Organic Chemistry

1. Basics of Alkanes

Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds ($\sigma$-bonds).

General Formula: $C_nH_{2n+2}$
Suffix: -ane (e.g., Methane, Ethane, Propane).
Alkyl Group ($R-$): Formed by removing one H from alkane ($C_nH_{2n+1}$). Suffix changes to '-yl' (Methyl, Ethyl).

2. Rule 1: Longest Chain Rule (Selection)

Select the longest continuous chain of carbon atoms as the Parent Chain.

                    Tie-Breaker: If two chains have the same length, choose the one with the maximum number of substituents (branches).
                

3. Rule 2: Lowest Locant Rule (Numbering)

Number the carbon atoms in the parent chain from the end that gives the lowest number (locant) to the first substituent.

                    Lowest Set of Locants: If there are multiple substituents, number the chain such that the set of locants $(2, 4, 7)$ is lower than $(2, 5, 6)$ at the first point of difference.
                

                    Alphabetical Tie-Breaker: If substituents get the same number from both ends, assign the lower number to the substituent that comes first alphabetically (e.g., Ethyl gets priority over Methyl).
                

4. Rule 3: Writing the Name

The name is written in a single line following this format:

$\text{Locant} - \text{Prefix} + \text{Parent Alkane}$

Prefix: Name of substituent (e.g., chloro, methyl).
Order: Substituents are listed Alphabetically (Ethyl before Methyl), regardless of their number.
Multipliers: Use di-, tri-, tetra- for identical groups (e.g., 2,2-dimethyl). Note: 'di', 'tri' are NOT considered for alphabetical ordering.

Punctuation

Use a hyphen (-) between numbers and text (e.g., 2-methyl).
Use a comma (,) between numbers (e.g., 2,2-dimethyl).

5. Complex Substituents

If a substituent itself is branched, it is named as a substituted alkyl group.

The carbon attached to the main chain is numbered 1.
The full name of the complex substituent is placed in brackets ().
Example: $-\text{CH}(\text{CH}_3)_2$ is named as $(1\text{-methylethyl})$ or commonly Isopropyl.
Example: $-\text{C}(\text{CH}_3)_3$ is named as $(1,1\text{-dimethylethyl})$ or commonly tert-Butyl.

Practice Quiz

Test your naming skills.

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