enter image description here Should the benzene group be taken as a substituent, so as to select the longest C atom chain? In what cases is it taken as as a substituent?


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The correct answer is

(d) none of these

In the usual substitutive IUPAC 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- 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-

Therefore, the benzene ring is selected as the senior parent structure for the preferred IUPAC name of the compound given in the question.

According to P-29.1.2, The chloromethyl ($\ce{-CH2Cl}$) substituent is a compound substituent group, i.e. a substituent group consisting of two parts, a simple substituent group (the parent substituent group, here: methyl) to which is attached one or more simple substituent groups (here: chloro).

According to P-, parentheses are used around such compound prefixes. Therefore, the preferred IUPAC name for the compound that is given in the question is (chloromethyl)benzene.

  • $\begingroup$ What if it were cyclohexane other than benzene? $\endgroup$ – user161158 Oct 28 '17 at 16:41
  • $\begingroup$ However, answer (a) does come close. $\endgroup$ – Jan Oct 29 '17 at 2:35

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