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How do you know when to name a benzene ring substituted by a simple haloalkane as a derivative of benzene or as a derivative of the alkyl halide.

For example, is it 1-chloro-4-isopropylbenzene or 2-(4-chlorophenyl)propane?

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In the usual substitutive nomenclature, simple halogen compounds are always expressed by prefixes (‘bromo’, ‘chloro’, etc.). Hence, the senior parent structure of such compounds is not determined by a principal characteristic group (a characteristic group chosen for citation at the end of a name by means of a suffix or a class name, or implied by a trivial name).

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the ring or the chain can be the senior parent structure; however, for the preferred IUPAC name, the ring is always selected as the senior parent structure.

P-44.1.2.2 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-44.1.2.2).

Therefore, the benzene ring is selected as the senior parent stucture for the preferred IUPAC name of the compound given in the question, which leads to ‘1-chloro-4-isopropylbenzene’. However, according to current recommendations, the preferred prefix for the isopropyl substituent is ‘propan-2-yl’; the prefix ‘isopropyl’ is retained for use in general nomenclature. Thus, the complete preferred IUPAC name is ‘1-chloro-4-(propan-2-yl)benzene’.

1-chloro-4-(propan-2-yl)benzene

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