According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the ring is selected as the parent structure to construct the preferred IUPAC name.
P-18.104.22.168 Systems composed of rings and chains (exclusive of linear phanes)
Two methods are recognized to name systems composed of rings and chains (exclusive of linear phanes).
(1) Within the same class, a ring or ring system has seniority over a chain. When a ring and a chain contain the same senior element, the ring is chosen as parent. Rings and chains are chosen regardless of their degree of hydrogenation. As a consequence, this approach prefers the choice of a ring over a chain in systems composed of cyclic and acyclic hydrocarbons.
(2) The context may favor the ring or the chain, so that, for example, substituents may be treated alike or an unsaturated acyclic structure may be recognized, or the one chosen has the greater number of skeletal atoms in the ring or in the principal chain of the acyclic structure.
(…) For selection of a preferred IUPAC name, see P-52.2.8.
P-52.2.8 Selection between a ring and a chain as parent hydride
Within the same heteroatom class and for the same number of characteristic groups cited as the principal characteristic group, a ring is always selected as the parent hydride to construct a preferred IUPAC name. In general nomenclature, a ring or a chain can be the parent hydride (see P-22.214.171.124).
Thus, the compound given in the question is named as cyclohexane. The substituents are expressed as prefixes, i.e. x-(butan-2-yl)-y-methylcyclohexane.
The most important simplified criteria for the numbering of the locants x and y are:
- lower locants for the group that is expressed as suffix
- lower locants for multiple bonds
- lower locants for prefixes
- lower locants for substituents cited first as a prefix in the name
The corresponding actual wording in the current IUPAC Recommendations reads as follows:
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(c) principal characteristic groups and free valences (suffixes);
(i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;
(ii) low locants are given first to multiple bonds as a set and then to double bonds (…);
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
(g) lowest locants for the substituent cited first as a prefix in the name;
The compound that is given has no principal characteristic group that would be expressed as a suffix and no multiple bonds. Thus, Rules (c) and (e) are not relevant in this case.
Rule (f) does not lead to a decision since the locant set (all considered together in a series of increasing numerical order) is always ‘1,4’.
Finally, Rule (g) stipulates that the lowest locant is assigned to the substituent that is cited first as a prefix in the name. Therefore, the correct name for the compound that is given in the question is 1-(butan-2-yl)-4-methylcyclohexane.