Phenols are oxygenated organic compounds that must have a hydroxyl group attached to any aromatic in their constitution. See the general representation of a phenol:
Basic structural formula of a phenol
like all phenol has an OH group bonded to an aromatic, what differentiates one phenol from the other are the types of aromatics that constitute them, as well as the presence or absence of organic radicals. This difference also influences the nomenclature of these compounds.
The following images bring examples of phenols with different aromatics and also phenols with branches:
Phenol formed by the aromatic naphthalene
Phenol formed by the aromatic anthracene
Phenol with methyl and ethyl radicals in its composition
A IUPAC nomenclature rule for any and all phenols is the same, namely:
Hydroxy + name of aromatic
Thus, if the aromatic is different, the phenol will consequently have a different name. See some examples:
a) Hydroxybenzene
NOTE: The name Hydroxybenzene can be replaced by phenol according to IUPAC.
As it is a phenol, its name will obviously have the term hydroxy followed by the name benzene, as this is the aromatic in the chain.
b) α-hydroxynaphthalene
NOTE: The name hydroxynaphthalene can be changed to naphthol according to IUPAC.
Following the same reasoning as in the previous example, the name starts with hydroxy followed by naphthalene, which is the name of the aromatic in the chain. We must use the term α before the term hydroxy because the hydroxyl is on one of the vertical carbons of naphthalene, all of which are named in the α position.
c) 9-hydroxyanthracene
Following the same reasoning, the name starts with hydroxy followed by anthracene, as this is the aromatic in the chain. We must use the number 9 before the term hydroxy because the hydroxyl is on the top vertical carbon in the middle ring. The bottom carbon is numbered 10.
The numbering of an anthracene always starts with the vertical carbon above or below one of the compound’s tip nuclei. As we can see, the OH is on the top vertical, so we start numbering on the top vertical carbon in the ring on the right, following the numbering clockwise until carbon 4. Arriving at it, we jump to the ring on the other end and start over the numbering, keeping the clockwise direction, since that’s how we started.
Don’t stop now… There’s more after the publicity 😉
Model to perform the numbering of an anthracene
NOTE: The name hydroxyanthracene can be changed to anthrol according to IUPAC.
Now let’s follow examples of how to perform the nomenclature of phenols in cases of branched structures:
a) 3-ethyl-5-methylhydroxybenzene
To name a branched phenol, we must number the chain. We started the numbering by the carbon that has the hydroxyl and proceeded counterclockwise, as the ethyl radical has the lowest number in relation to the methyl radical. The radical that is written first because of alphabetical order will be the reference for numbering.
b) 1,2-methylhydroxybenzene
In this structure, we start the numbering by the carbon that presents the OH, carbon number 1, and we proceed towards the radical, which, therefore, is in position 2. As the positions are 1 and 2 in the aromatic benzene, we can use the term ortho instead of 1.2. So the name would be Ortho-cresol.
NOTE: According to IUPAC, the expression methylhydroxybenzene can be replaced by cresol.
c) 3,5-dimethyl-1-hydroxy-naphthalene
When naphthalene has more than one radical or group, it is more correct to number the carbons. It is initiated by one of the carbons in the vertical closest to the OH group. It follows towards carbon four of the same ring. Afterwards, it restarts at the vertical carbon of the other ring.
Thus, there are two methyl radicals (CH3) at positions 3 and 5, and the OH group is found at carbon 1 of the chain. So the name of the compound is 3,5-dimethyl-1-hydroxy-naphthalene.
d) 4-methyl-9-hydroxyanthracene
As we saw earlier, as the OH is at the top of the middle ring, we start the numbering with the carbon at the top of the right ring. Going counterclockwise, the methyl is on carbon 4.
By Me. Diogo Lopes Dias