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Given a simple cyclic alcohol:

cyclopentanol

We can name it cyclopentanol and using cyclopentan-1-ol wouldn't be correct. When more than one alcohol group is added:

cyclic diol

we must use numbers and name it as cyclopentane‐1,1‐diol. However, what about situation like below?

disubstituted cyclic alcohol

In my understanding such compound should be named 1‐methylcyclopentan‐1‐ol and that's how I always named them. However I got into an argument with a person claiming the name of 1‐methylcyclopentan‐1‐ol is invalid and 1‐methylcyclopentanol must be used instead.

Does anyone maybe have a better google-fu and can locate a rule which specifies how such situation should be handled according to Gold Book or Blue Book?

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  • $\begingroup$ "...1‐methylcyclopentan‐1‐ol is invalid and 1‐methylcyclopentanol..." - what's the difference? butene can be written as but-1-ene. isn't it? $\endgroup$ – Rahul Verma Apr 4 at 5:19
  • $\begingroup$ @RahulVerma I'm pretty sure he's asking which one is IUPAC official. $\endgroup$ – mpprogram6771 Apr 4 at 19:47
  • $\begingroup$ Please note that your use of hashed wedges is not correct. $\endgroup$ – user7951 Apr 4 at 20:19
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For preferred IUPAC names, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), explicitly stipulates when locants are omitted. Most of these rules apply to the omission of the locant ‘1’.

In particular:

P-14.3.4.2 The locant ‘1’ is omitted:

(…)

(c) in monosubstituted homogeneous monocyclic rings;

(…)

One example for this rule given in the Blue Book is cyclohexanethiol. Likewise, cyclopentanol is the preferred IUPAC name for the first compound that is given in the question.

cyclopentanol

This rule, however, is not applicable to the other two compounds that are given in the question since they are not monosubstituted.

Furthermore, we can find a general rule that is relevant in this case:

P-14.3.3 Citation of locants

In preferred IUPAC names, if any locants are essential for defining the structure of the parent structure or of a unit of structure as defined by its appropriate enclosing marks, then all locants must be cited for the parent structure or that structural unit. For example, the omission of the locant ‘1’ in 2-chloroethanol, while permissible in general usage, is not allowed in preferred IUPAC names, thus the name 2-chloroethan-1-ol is the PIN. (…)

Therefore, the preferred IUPAC names are cyclopentane-1,1-diol and 1-methylcyclopentan-1-ol, respectively. The omission of the locant ‘1’ is not permissible for the preferred IUPAC name.

cyclopentane-1,1-diol

1-methylcyclopentan-1-ol

Nevertheless, the name ‘1-methylcyclopentanol’ describes the correct compound and may be used in general nomenclature.

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