Why is the correct name 5-cyclopentyl-2-methylheptane rather than 2-methyl-5-cyclopentylheptane? I am using the 2nd edition Hornback Organic Chemistry textbook. I got this answer from the solutions manual.
I have searched the textbook and the internet for the explanation and am still very confused. I thought the order of the side chain in the name was alphabetical, not including the prefixes? So by this logic, "methyl" should be before "pentyl," right?
In view of the release date of your book (2005), the used names for organic compounds are probably in accordance with the 1993 Guide to IUPAC Nomenclature of Organic Compounds. However, note that the current version of the recommendations can be found in Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book).
According to the 2013 recommendations, two methods are possible for naming systems composed of rings and chains: either the ring or the chain can have seniority. To construct the preferred IUPAC name (PIN), the ring is always selected as the parent structure. Therefore, the PIN for the compound given in the question is (6-methylheptan-3-yl)cyclopentane since the cyclopentane ring has seniority over the heptane chain.
In general nomenclature, however, the context may favour the chain, so that the parent structure has the greater number of atoms in the principal chain. The corresponding systematic name for the compound given in the question is 5-cyclopentyl-2-methylheptane.
According to the 1979 recommendations, a hydrocarbon containing a small ring attached to a long chain was generally named as a derivative of the acyclic hydrocarbon. The 1993 recommendations do not explicitly mention this rule but refer to the 1979 recommendations. Hence, it is not surprising that your book still uses such a name, although the 1998 Guide to IUPAC Recommendations already explains that the ring is always selected to be the parent structure.
Now coming to your actual question:
According to the 2013 recommendations, the prefixes ‘cyclopentyl’ and ‘methyl’ in the name 5-cyclopentyl-2-methylheptane are arranged alphabetically.
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.
P-14.5.2 The name of a prefix for a substituent is considered to begin with the first letter of its complete name.
Therefore, ‘cyclopentyl’ begins with ‘c’ and ‘methyl’ begins with ‘m’. Note that, for example, ‘2,3-dimethyl’ is considered to begin with ‘m’ since the multiplicative prefix ‘di’ does not alter the alphabetical order. Whereas, for example, the compound substituent name ‘(1,2-difluorobutyl)’ in 7-(1,2-difluorobutyl)-5-ethyltridecane is considered to begin with ‘d’.
