IUPAC Nomenclature of Benzene | Organic Chemistry Class 11

IUPAC Nomenclature of Benzene

Aromatic Compounds & Derivatives | Organic Chemistry

1. Benzene Structure

Benzene ($C_6H_6$) is the simplest aromatic hydrocarbon. In IUPAC nomenclature, it acts as the parent hydrocarbon for many aromatic compounds.

                    Note: IUPAC accepts many common names of benzene derivatives as their IUPAC names (e.g., Phenol, Aniline, Toluene).
                

2. Monosubstituted Benzene

Name the substituent as a prefix to the word "benzene". No numbering is needed as all positions are equivalent.

Chlorobenzene: $C_6H_5Cl$
Nitrobenzene: $C_6H_5NO_2$

Accepted IUPAC/Common Names:

Structure	Substituent	Special Name
$C_6H_5-CH_3$	Methyl	Toluene
$C_6H_5-OH$	Hydroxyl	Phenol
$C_6H_5-NH_2$	Amino	Aniline
$C_6H_5-OCH_3$	Methoxy	Anisole
$C_6H_5-CHO$	Aldehyde	Benzaldehyde
$C_6H_5-COOH$	Carboxylic Acid	Benzoic Acid

3. Disubstituted Benzene

When two substituents are present, their relative positions are indicated by numbers or prefixes:

1, 2- (Ortho, $o-$): Adjacent carbons.
1, 3- (Meta, $m-$): Alternate carbons.
1, 4- (Para, $p-$): Opposite carbons.

Example: 1,2-Dichlorobenzene is also $o$-Dichlorobenzene.

Naming with Base Compound:

If one substituent confers a special name (e.g., $-OH$ makes Phenol), that carbon gets position 1.

Example: 2-Methylphenol (o-Cresol), 4-Nitroaniline.

4. Polysubstituted Benzene

For three or more substituents:

Lowest Locant Rule: Number the ring to give the lowest possible set of locants.
Alphabetical Order: If lowest sets are tied, give lower numbers based on alphabetical order of substituents.
Priority: If a principal functional group is present, the carbon attached to it is C1.

Example: 1-Chloro-2,4-dinitrobenzene

5. Benzene as a Substituent

When benzene is attached to a carbon chain that contains a functional group or is longer than the benzene ring itself, the benzene ring is treated as a substituent called Phenyl ($C_6H_5-$).

                    Phenyl vs Benzyl:

                    Phenyl ($Ph-$): $-C_6H_5$ (H removed from Benzene).

                    Benzyl: $-CH_2-C_6H_5$ (H removed from Toluene methyl group).

Example:

$\text{CH}_3-\text{CH}(Ph)-\text{CH}_2-\text{CH}_3$: 2-Phenylbutane.
$C_6H_5-\text{CH}_2-\text{Cl}$: Benzyl chloride (Chlorophenylmethane).

Practice Quiz

Test your naming skills for Aromatic Compounds.

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