4.4 More Aryl Compounds
When two substituents exist on the same aryl ring, their positions relative to each other are called ortho, meta, or para, often abbreviated o-, m- and p-. Here is the example of the three xylenes:
o-xylene |
m-xylene |
p-xylene |
An aryl group having a methyl substitution is called cresyl, for example cresyl alcohol or cresol whose formula is CH3C6H4OH, and occurs in orth-, meta-, and para- isomers.
The benzenediols are aryl compounds having two hydroxy substituents. Ortho-benzenediol is called catechol; meta-benzenediol is called resorcinol; para-benzenediol is called hydroquinone.
Another common aryl compound is salicylic acid, C6H4(OH)CO2H, with the two substituents in the ortho position. Its ester acetyl salicylic acid, with the acetate group esterified onto the hydroxy position, is aspirin.
Phthalic acid has the structure C6H4(CO2H)CO2H, with the two carboxyls in the ortho position. When they are in the para position, the molecule formed is terephthalic acid, a precursor to PET plastic: polyethylene terephthalate.
With more than two substituents, naming reverts to the numbering system, for example the xylenols, which are dimethyl phenols. The -OH is considered to be in position 1, leaving positions 2 through 6 available for the methyl groups. Consequently, the xylenols are 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-xylenol. Other combinations of numbers could simply flip over to produce the same molecule as one of these.
2,3-xylenol |
2,4-xylenol |
2,5-xylenol |
2,6-xylenol |
3,4-xylenol |
3,5-xylenol |