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These two molecules are both aromatic:

enter image description here

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:

enter image description here

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

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  • $\begingroup$ It would be phenyl bromide, but it's not used. $\endgroup$
    – Mithoron
    Commented Apr 14, 2015 at 16:55
  • $\begingroup$ why would it not be used? $\endgroup$
    – Nerdingout
    Commented Apr 14, 2015 at 16:57
  • $\begingroup$ how would you know when to use either one? $\endgroup$
    – Nerdingout
    Commented Apr 14, 2015 at 16:58
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    $\begingroup$ What does benzyl have to do with it? $\endgroup$
    – bon
    Commented Apr 14, 2015 at 16:59
  • $\begingroup$ ^the picture was mostly to show the phenyl structure, i just don't understand why they used benzene as opposed to phenyl @bon $\endgroup$
    – Nerdingout
    Commented Apr 14, 2015 at 17:10

1 Answer 1

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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.

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  • $\begingroup$ if you had a carbon chain though, with a C6H5 substituent, would you name it phenyl or benzyl? $\endgroup$
    – Nerdingout
    Commented Apr 17, 2015 at 14:02
  • $\begingroup$ like how would you know if the carbon chain was part of the benzene ring to distinguish between a "benzyl" or "phenyl" substituent? $\endgroup$
    – Nerdingout
    Commented Apr 17, 2015 at 14:05
  • $\begingroup$ Compare, for example, 2-phenyl-2-butanol to 2-benzyl-2-butanol. $\endgroup$
    – Curt F.
    Commented Apr 17, 2015 at 16:22
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    $\begingroup$ 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. $\endgroup$
    – Curt F.
    Commented Apr 17, 2015 at 16:25
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    $\begingroup$ @CurtF. if you want to take the IUPAC-approved naming perspective, n-propylbenzene. But I think using n in front of propyl is not IUPAC approved. $\endgroup$ Commented Dec 1, 2023 at 17:27

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