The following structure represents the compound cyclohexylbenzene:
Why can't the benzene ring be treated as a substituent and the compound be called benzylcyclohexane?
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The relevant rule for the seniority order of parent structures in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows.
P-44.4 SENIORITY CRITERIA APPLICABLE TO RINGS, RING SYSTEMS, OR ACYCLIC CHAINS
P-44.4.1 If the criteria of P-44.1 through P-44.3, where applicable, do not effect a choice of a senior parent structure, the following criteria are applied successively until there are no alternatives remaining. These criteria are illustrated in P-184.108.40.206 through P-220.127.116.11.
The senior ring, ring system, or principal chain:
(a) has the greater number of multiple bonds (P-18.104.22.168);
P-22.214.171.124 The senior ring, ring system or principal chain has the greater number of multiple bonds; for the purpose of this criterion, mancude rings or ring systems are considered as consisting of noncumulative double bonds [criterion (a) in P-44.4.1].
Therefore, the name of the compound that is given in the question is cyclohexylbenzene rather than “phenylcyclohexane” since benzene has more multiple bonds than cyclohexane.
Note that this rule is not to be confused with the rules for systems composed of rings and chains, which are less strict. For systems composed of rings and chains, the context may favor the ring or the chain; nevertheless, the ring is selected as the parent structure for the preferred IUPAC name. In case of cyclohexylbenzene, however, benzene is always the parent structure – not only for the preferred IUPAC name.