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What is the IUPAC name of the following molecule? More importantly, what is the rule for cycloalkanes having a substituent with the same number of carbons.

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The undergraduate organic chemistry texts that I use when teaching tend to hide from complex nomenclature issues that cannot be addressed by the more common rules. Most texts clearly state that the parent chain in cases like these is the one with the larger number of carbon atoms.

For example, the following is butylcyclopentane.

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The following is 1-cyclopentylhexane.

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The addition of another substituent also makes it clearer, since we choose parent chains to maximize the number of substituents. The following is 1-cyclopentyl-3-methylpentane.

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Many compounds can have two or more systematic names in accordance with several methods recommended by IUPAC. Concerning systems composed of rings and chains, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

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).

The corresponding names for the examples given in the question are:

pentylcyclopentane
Method (1): “pentylcyclopentane” (preferred IUPAC name; ring preferred to chain)
Method (2): “1-cyclopentylpentane”

butylcyclopentane
Method (1): “butylcyclopentane” (preferred IUPAC name; ring preferred to chain)
Method (2): “1-cyclopentylbutane”

hexylcyclopentane
Method (1): “hexylcyclopentane” (preferred IUPAC name; ring preferred to chain)
Method (2): “1-cyclopentylhexane”

(3-methylpentyl)cyclopentane
Method (1): “(3-methylpentyl)cyclopentane” (preferred IUPAC name; ring preferred to chain)
Method (2): “1-cyclopentyl-3-methylpentane”

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