IUPAC naming of 1,2-disubstituted cyclohexane derivative

Question

I'm facing the challenge of naming this molecule:

We have to name it as an alkyl-substituted cycloalkane, since the ring has more carbons than the longest substituent (isopropyl). My questions:

1. When numbering carbons on the ring, we can start counting from either the methyl group (1-methyl...), or the isopropyl group (1-isopropyl...). Which one should I start with? Or do I need to start numbering with the substituent which name comes first, alphabetically?

2. I set the carbon number 1 on the methyl substituent, giving me the following name: 1-methyl-2-isopropylcyclohexane. However, my book says the correct name is: 1-isopropyl-2-methylcyclohexane. So the solution managed to sort the name both numerically and alphabetically. Is this what I need to do? Is the name I gave incorrect?

3. Which prefixes count towards alphabetically-naming a compound? Which prefixes don't? All I know is that the numeric prefixes like di- or tri- don't count. Are there any others which don't count either?

Answer 1

Simple prefixes (simple substituent groups such as methyl and isopropyl) are arranged alphabetically disregarding any multiplicative prefixes. Any multiplicative prefixes are inserted later and do not alter the alphabetical order.
For example, ‘methyl’ is considered to begin with ‘m’; ‘isopropyl’ is considered to begin with ‘i’.

On this matter, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

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.

(…)

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.

Thus, the correct alphanumerical order in your example corresponds to the name x-isopropyl-y-methylcyclohexane.

Furthermore,

P-14.4 NUMBERING

When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:

(…)

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(g) lowest locants for the substituent cited first as a prefix in the name;

(…)

Note that Rule f takes precedence over Rule g; however, Rule f does not permit a decision to be reached between ‘1-isopropyl-2-methylcyclohexane’ and ‘2-isopropyl-1-methylcyclohexane’ since both names correspond to the locant set ‘1,2’.

According to Rule g, your example is named as 1-isopropyl-2-methylcyclohexane rather than 2-isopropyl-1-methylcyclohexane since isopropyl is cited first as a prefix in the name.

Note, however, that the prefix ‘isopropyl’ is retained for use in general nomenclature but the preferred IUPAC name is ‘propan-2-yl’.

Therefore, the preferred IUPAC name for your example is actually 1-methyl-2-(propan-2-yl)cyclohexane because ‘methyl’ is cited first in the name (since alphabetical order is used to establish the order of citation of substituent prefixes in the name).

Answer 2

Let me first quote some rules given by IUPAC (1) and then come to your question.

ref.1

http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm#a_2__25

Acyclic Hydrocarbons

Rule A-2. Saturated Branched-chain Compounds and Univalent Radicals

2.25 - Univalent branched radicals derived from alkanes are named by prefixing the designation of the side chains to the name of the unbranched alkyl radical possessing the longest possible chain starting from the carbon atom with the free valence, the said atom being numbered as 1. Examples to Rule A-2.25

The following names may be used for the unsubstituted radicals only:

2.3 - If two or more side chains of different nature are present, they are cited in alphabetical order .

The alphabetical order is decided as follows:

(i) The names of simple radicals are first alphabetized and the multiplying prefixes are then inserted.

(ii) The name of a complex radical is considered to begin with the first letter of its complete name.

2.4 - If two or more side chains are in equivalent positions, the one to be assigned the lower number is that cited first in the name.

For arranging substituents , which are treated as prefixes the following rule is followed.

R-0.1.8 Order of prefixes

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

Now comming to your question , it has two substituents methyl and isopropyl at equivalent positions. Using rule 2.4 and R-0.1.8 , numbering is started from isopropyl end and named.

References:

1.http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm#a_2__25

2.http://www.acdlabs.com/iupac/nomenclature/93/r93_93.htm