What is Phenol’s Nomenclature?
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).
Another common position where the functional group can be attached is meta (attached on the second carbon from the hydroxyl group). You should always remember the position of OH as it is given a high priority.
FAQs on Phenols Nomenclcature
1. What is the IUPAC name and structure of the simplest phenol?
The simplest phenol consists of a hydroxyl group (-OH) directly attached to a benzene ring. Its official IUPAC name is benzenol, but the common name phenol is universally accepted by IUPAC and is used as the parent name for its derivatives. The chemical formula is C₆H₅OH.
2. How are the positions of substituents on the phenol ring indicated?
The positions of substituents on the benzene ring are indicated by numbers. The carbon atom attached to the -OH group is always numbered as C-1. For disubstituted phenols, the prefixes ortho- (o-), meta- (m-), and para- (p-) are also commonly used for substituents at positions 2, 3, and 4, respectively. For example, 2-nitrophenol is also called o-nitrophenol.
3. What are the basic rules for the IUPAC nomenclature of substituted phenols?
The IUPAC nomenclature for substituted phenols follows these primary rules:
- The parent name is 'phenol'.
- The carbon atom bearing the -OH group is given the number 1.
- Numbering of the ring proceeds in a direction that gives the lowest possible numbers to the other substituents.
- Substituents are listed in alphabetical order before the parent name 'phenol'.
4. What are cresols, and how are they named?
Cresols are the common names for methylphenols, which are phenols with an additional methyl (-CH₃) group on the benzene ring. There are three isomers:
- Ortho-cresol (o-cresol): The methyl group is at position 2. Its IUPAC name is 2-methylphenol.
- Meta-cresol (m-cresol): The methyl group is at position 3. Its IUPAC name is 3-methylphenol.
- Para-cresol (p-cresol): The methyl group is at position 4. Its IUPAC name is 4-methylphenol.
5. What is the difference between a phenyl group and a phenol?
A phenol is a specific type of compound where a hydroxyl (-OH) group is directly bonded to a benzene ring (C₆H₅OH). In contrast, a phenyl group (C₆H₅-) is a functional group or substituent, formed when a hydrogen atom is removed from a benzene molecule. It is the benzene ring itself when it acts as a part of a larger molecule.
6. Why is 'phenol' often used as the parent name instead of 'benzenol'?
While benzenol is the systematic IUPAC name, the name phenol was the original common name for this compound. Due to its widespread and historical use, IUPAC has officially accepted 'phenol' as a retained name. It is now the preferred name and is used as the parent compound for naming its many derivatives, which helps simplify the nomenclature system for this important class of compounds.
7. When naming a compound, how do you decide between using 'phenol' and 'hydroxy'?
The choice depends on the priority of functional groups as per IUPAC rules.
- Use 'phenol' as the parent name when the hydroxyl (-OH) group is the principal (highest priority) functional group on the benzene ring.
- Use 'hydroxy' as a prefix when a functional group with higher priority, such as a carboxylic acid (-COOH) or aldehyde (-CHO), is also present. For example, in 4-hydroxybenzoic acid, the carboxylic acid group has higher priority, so the compound is named as a derivative of benzoic acid, not phenol.
8. Is 'phenyl alcohol' a correct name for phenol? Explain why.
No, 'phenyl alcohol' is an incorrect name for phenol. The term 'alcohol' refers to compounds where the -OH group is attached to a saturated (sp³-hybridised) carbon atom. In phenol, the -OH group is attached directly to an unsaturated (sp²-hybridised) carbon atom of the benzene ring. A compound like benzyl alcohol (C₆H₅CH₂OH), where the -OH group is on a carbon outside the ring, is correctly classified as an alcohol, not a phenol.
9. How would you name a compound with two hydroxyl groups on a benzene ring?
Compounds with two hydroxyl groups on a benzene ring are named as benzenediols. The positions of the two -OH groups are indicated by numbers. For example:
- Benzene-1,2-diol (Common name: Catechol)
- Benzene-1,3-diol (Common name: Resorcinol)
- Benzene-1,4-diol (Common name: Hydroquinone or Quinol)
