# Difference required for compound to be a phenol vs benzene compound

When naming organic compounds, how should one decide whether a compound should be named as a benzene compound, or as a phenol?

For example, the compound 3,4-dihydroxybenzoic acid has an -OH group, so why is it not named as a phenol derivative? I feel like I'm missing something here, but none of the numerous sources I've looked at have provided a clear-cut rule to differentiate with.

In organic nomenclature, when you have a compound with multiple functional groups, you follow the priority rules that have been nicely summed up on Wikipedia:

When compounds contain more than one functional group, the order of precedence determines which groups are named with prefix or suffix forms. The highest-precedence group takes the suffix, with all others taking the prefix form. However, double and triple bonds only take suffix form (-en and -yn) and are used with other suffixes.

These rules apply to benzene derivatives as well as aliphatic and other cyclic compounds.

Continuing,

Prefixed substituents are ordered alphabetically (excluding any modifiers such as di-, tri-, etc.), e.g. chlorofluoromethane, not fluorochloromethane. If there are multiple functional groups of the same type, either prefixed or suffixed, the position numbers are ordered numerically (thus ethane-1,2-diol, not ethane-2,1-diol.) The N position indicator for amines and amides comes before "1", e.g. $\ce{CH3CH(CH3)CH2NH(CH3)}$ is N,2-dimethylpropanamine

Here's a shorter (and quite handy) version of the Wikipedia table (shamelessly copied from masterorganicchemistry.com, I've taken a liking to their color-scheme :-))

So in the example you've posted:

You give the carbon atom that has $\ce{-COOH}$ attached to it the first locant (because $\ce{-COOH}$ takes precedence over $\ce{-OH}$, according to the priority rules), and you continue numbering clockwise so the the two $\ce{-OH}$ groups take locants 3 and 4 respectively (Lowest set of locants rule).

That's how you end up with 3,4-dihydroxybenzoic acid :-)

• The so-called “Table of Functional Group Priorities For Nomenclature” shown on masterorganicchemistry.com tries to combine the seniority order of suffixes with prefixes and endings. This can be misleading and result in wrong names. In particular, there is no suffix ‘-ane’, ‘-ene’ or ‘-yne’ (these are endings), and multiple bonds (and all groups that follow in that table) are actually not considered in the seniority order. – Loong Dec 6 '17 at 8:43
• @Loong Hmm, but I can't find a clear enough table (from Google Images) apart from this :/ Every other table's an eyesore. If you know where I can find a better (neat and brief) table online, I'd appreciate it if you could give me a link. Thanks! O:) – paracetamol Dec 6 '17 at 10:23