[Chemistry Class Notes] on Phenols Nomenclcature Pdf for Exam

IUPAC nomenclature refers to the systematic approach taken for the nomenclature of organic compounds as per the recommendation of the International Union of Pure and Applied Chemistry (often abbreviated to IUPAC). Phenol constitutes a major part of Organic Chemistry. They are considered as a class of chemical compounds which are bonded with the hydroxyl group and are attached to the carbon atom of the aromatic ring. Phenol has a hydroxyl group that is always bonded with a benzene ring and is known as an aromatic hydrocarbon group. Phenol is also referred to as Phenolic, benzenol, or carboxylic acids. 

What are Phenols?

The term Phenols refer to organic compounds that have a benzene ring that is bonded to a Hydroxyl group. A common name for these is carbolic acid. The Phenol molecule contains two parts, one aryl group part and another hydroxyl group part. Phenol is the simplest derivative of benzene.

 

Naming Phenols

Phenol consists of two parts: one aryl group and one hydroxyl group. So based on the number of aryl groups attached, Phenols can be further sub categorized as mono-, di-, tri-, or polyhydric Phenols. The IUPAC name for Phenol is Monohydroxybenzene or C6H5OH. 

Rules of Nomenclature

Before progressing further nomenclature of Phenols, let us revise some fundamental rules of IUPAC nomenclature in general.

 

According to the guidelines of IUPAC, the following rules are necessary to follow when practicing nomenclature:

• The longest chain rule: As per this rule, parent hydrocarbon should be identified and labeled accordingly

• The lowest set of locants: This rule implies the numbering of the carbon atoms that belong to the parent hydrocarbons.

• Multiple instances of the same substituent: This rule refers to the usage of prefixes such as di-, tri-, mono- or poly.

• The naming of different substituents: Alphabetical order must be followed for different substituents in one compound.

• The naming of different substituents that are present in the same position: As per this rule, ascending alphabetical order is followed to name the compounds.

• Naming complex substituents: Complex substituents must be named as alkyl groups which will be written in brackets.

Nomenclature of Phenols

IUPAC has a set of guidelines in place that must always be followed while naming Phenols. This is done to ensure uniformity for all practical purposes. 

• The IUPAC name for Phenol is benzenol. Whenever a molecule of the hydroxyl group, denoted by OH, is attached to a ring of benzene, the resultant structure is a Phenol.

• In case there are more than one of these hydroxyl groups, then the benzene ring is marked with di-, tri- or poly, whoever prefix works for it and can correctly denote the number of hydroxyl groups that are attached to these rings. Say, there are three hydroxyl groups, then the nomenclature would be benzene, 1,2,3-diol. 

• Some functional groups are attached to the Phenol molecule. These functional groups are carbonyls, carboxylic acids, amines, alcohols, and so on. These can be used to change the characteristics of the molecule. Functional groups can therefore be attached to Phenol molecules. The methyl group is a good example of this.

• The nomenclature of the Phenol structure above is 4-methyl Phenol. Moreover, depending on where this functional group is attached, the labeling of the compound will differ. 

For example, if the functional group, methyl, was attached to ortho (attached to the adjacent carbon) then the name of the compound will be different from when it is attached to para (attached to the third carbon of the hydroxyl carbon).