Alphabetical order in nomenclature

Shouldn't 4-isopropyl-2-cyclohexyl-3,5-dimethylheptane be written as 2-cyclohexyl-3,5-dimethyl-4-isopropylheptane, by following alphabetical order?

Or maybe you don't look at the cyclo or the di prefix when you look for alphabetical order.

Answer

Both names that are proposed in the question, ‘4-isopropyl-2-cyclohexyl-3,5-dimethylheptane’ and ‘2-cyclohexyl-3,5-dimethyl-4-isopropylheptane’, are unambiguous and describe the correct structure. However, they are not in accordance with IUPAC recommendations.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), simple prefixes (simple substituent groups consisting of just one part that describes an atom, or group of atoms as a unit, for example methyl, cyclohexyl, and isopropyl) are arranged alphabetically disregarding any multiplicative prefixes. For example, ‘3,5-dimethyl’ is considered to begin with ‘m’. Any multiplicative prefixes are inserted later and do not alter the alphabetical order.

P-14.5 ALPHANUMERICAL ORDER

Alphanumerical order has been commonly called ‘alphabetical order’. As these ordering principles do involve ordering both letters and numbers, in a strict sense, it is best called ‘alphanumerical order’ in order to convey the message that both letters and numbers are involved

Alphanumerical order is used to establish the order of citation of detachable substituent prefixes (not the detachable saturation prefixes, hydro and dehydro), and the numbering of a chain, ring, or ring system when a choice is possible.

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P-14.5.1 Simple prefixes (i.e., those describing atoms and unsubstituted substituents) are arranged alphabetically; multiplicative prefixes, if necessary, are then inserted and do not alter the alphabetical order already established.

Therefore, the correct alphanumerical order for the example given in the question corresponds to the name 2-cyclohexyl-4-isopropyl-3,5-dimethylheptane since ‘cyclohexyl’ begins with ‘c’, isopropyl begins with ‘i’, and methyl begins with ‘m’.

However, note that the prefix ‘isopropyl’ is retained for use in general nomenclature but the preferred prefix is ‘propan-2-yl’. When using this prefix, the correct alphanumerical order corresponds to the name 2-cyclohexyl-3,5-dimethyl-4-(propan-2-yl)heptane.

Anyway, to construct a preferred IUPAC name (PIN), the ring is selected as the parent hydride. Therefore, the PIN for the example given in the question is [3,5-dimethyl-4-(propan-2-yl)heptan-2-yl]cyclohexane. (In general nomenclature, the context may favour 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.)