For a complex alkyl substituent, (at least the ones I have come into contact with thus far) you name it as a chain numbering from the first non-parent carbon and placing the name in square brackets: e.g. isopentyl can also be named [3-methylbutyl]. What is the convention for more complex benzene-based substituents? Specifically, can you still use the more common names like phenol and toluene as a parent for your name if say you bond C5 of 2,3-dimethylphenol to your parent and what suffix will you use?

  • $\begingroup$ “For a complex alkyl substituent, you name it as chain numbering from the first non-parent carbon and placing the name in square brackets.” This is not generally true. $\endgroup$
    – user7951
    Mar 12, 2018 at 18:44
  • $\begingroup$ Note that your example 3-hydroxy-4,5-dimethylphenyl is a compound substituent group and not a complex substituent group. $\endgroup$
    – user7951
    Mar 12, 2018 at 18:45
  • $\begingroup$ But when adding it to your parent name won't you use [3-hydroxy-4,5-dimethylphenyl]? $\endgroup$
    – Venter
    Mar 12, 2018 at 19:18

1 Answer 1



Your statement, that complex substituent names are placed in square brackets, is incorrect. These are to be placed in parentheses (round brackets). Correct preferred IUPAC name (PIN)[1] example:

2-(3-methylbutyl)phenol structure
2-(3-methylbutyl)phenol (PIN)

Square brackets are used when the substituent name already contains parentheses, e.g.:

2-[(2<i>R</i>)-2-methylbutyl]phenol structure
2-[(2R)-2-methylbutyl]phenol (PIN)


Retained aromatic compounds allowed for substitution

You can see that the names above are based on phenol, which is one of retained aromatic functional parent compounds, with allowed substitution, in preferred names.

The other ones are aniline and benzoic acid (see IUPAC rule P-34.1.2.[1]). E.g.:

2-hydroxybenzoic acid structure
2-hydroxybenzoic acid (PIN)
(not 2-carboxyphenol, as $\ce{-COOH}$ group is preferred over $\ce{-OH}$)


Retained alkyl-substituted aromatic hydrocarbons (not allowed for substitution in PINs)

There are also retained alkyl substituted aromatic parent hydrocarbons (P-22.1.3):

toluene structure
toluene (PIN)


1,2-xylene structure
1,2-xylene (PIN)

(and the two other xylenes). But none of them is allowed for substitution in preferred names, e.g.:

α,4-dichlorotoluene structure
1-chloro-4-(chloromethyl)benzene (PIN)

Xylenes are not allowed for substitution even in the general non-preferred names:

enter image description here
1,2-bis(bromomethyl)benzene (PIN)
(not α,α′-dibromo-o-xylene)

(Yet another retained name mesitylene for 1,3,5-trimethylbenzene (PIN) is allowed only for general names, not allowed for substitution.)


Complex aromatic substituents

Now if a complex substituent is made from substituted phenol, aniline or benzoic acid, the substituent name is no longer based on these names. They can be thought as suffixes, but now we have another higher-priority “suffix”, -…yl. E.g.:

3-hydroxy-4,5-dimethylphenyl acetate structure
3-hydroxy-4,5-dimethylphenyl acetate (PIN)
(not 2,3-dimethylphenol-5-yl acetate)

(In case you didn't hear ‘phenyl’ yet – it's not to be confused with ‘phenol’, it's the only, notorious, name for “benzenyl”, $\ce{-C6H5}$)



  1. Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)
  • $\begingroup$ Suppose the carboxylic acid was not attached directly attached to the benzene ring and it were an ethanoic acid derivative attached to ring, how would you name it? $\endgroup$ Oct 8, 2021 at 21:00

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