# Phenyl or benzene when naming compounds

These two molecules are both aromatic:

However, the first molecule has a halogen substituent and is named "bromobenzene". The second molecule has a chlorine and a hydroxyl group and is named "4-chlorophenol". Assuming there was no alcohol group, how would we know when to use benzene or phenyl?

I thought the following:

therefore, shouldnt the first molecule have been written as "bromophenyl" and not as "bromobenzene"?

• It would be phenyl bromide, but it's not used. – Mithoron Apr 14 '15 at 16:55
• why would it not be used? – Nerdingout Apr 14 '15 at 16:57
• how would you know when to use either one? – Nerdingout Apr 14 '15 at 16:58
• What does benzyl have to do with it? – bon Apr 14 '15 at 16:59
• ^the picture was mostly to show the phenyl structure, i just don't understand why they used benzene as opposed to phenyl @bon – Nerdingout Apr 14 '15 at 17:10

You are confused about several overlapping but mostly unrelated issues.

First, you are right that "benzyl" is the $\ce{C7H8\bond{~}}$ radical and that "phenyl" is the $\ce{C6H5\bond{~}}$ radical.

Second, although "phenol" and "phenyl" sound almost the same, you would probably do well to think of them as entirely separate naming roots. "Phenol" is the simplest aromatic alcohol. Simple aromatic alcohols with other substitutions (e.g. 4-chlorophenol) can thus be named as substituted phenols.

Third, "benzyl" and "benzene" similarly sound the same but should be treated as distinct concepts. Substituted aromatic rings can be named as benzene derivatives, such as chlorobenzene.

Fourth, for shorthand and convenience, "phenyl" and "benzyl" are sometimes used to name substituents. Note that IUPAC has detailed priority rules for what is a substituent and what is the base molecule, but chemists don't always follow these. "chlorobenzene" is a correct IUPAC name, and it views the molecule as a substituted benzene. That is, benzene is the base molecule and "chloro" is a substitution. Chemists sometimes find it convenient to reverse their perspective, even though the resulting names would not be IUPAC-approved. From a reverse perspective, the chlorine atom is the "base molecule", and it is substituted by a phenyl group. Thus you could call the chlorobenzene "phenyl chloride" if you wanted to take this reverse perspective.

Lastly, "bromophenyl" ends in "-yl" and so is the name of a "radical", i.e. a substituent or fragment, not of a complete molecule. Without any other context, I would think that "bromophenyl" was a $\ce{C6H4Br\bond{~}}$ substituent. In contrast, "bromobenzene" is a complete molecule, derived from benzene by substitution.

• if you had a carbon chain though, with a C6H5 substituent, would you name it phenyl or benzyl? – Nerdingout Apr 17 '15 at 14:02
• like how would you know if the carbon chain was part of the benzene ring to distinguish between a "benzyl" or "phenyl" substituent? – Nerdingout Apr 17 '15 at 14:05
• Compare, for example, 2-phenyl-2-butanol to 2-benzyl-2-butanol. – Curt F. Apr 17 '15 at 16:22
• Also, you would never call something "benzyl" unless the benzylic carbon is part of a branch. If the benzylic carbon is part of the main chain, then it makes no sense to use "benzyl". For example, "1-benzylethane" doesn't make sense and no one would use that name. That molecule should be called "1-phenylpropane" instead, or, if you want to take the IUPAC-approved naming perspective, n-propylbenzene. – Curt F. Apr 17 '15 at 16:25
• In general, benzene is used as a "noun" and "phenyl" an adjective. – user55119 Sep 13 '18 at 18:37