# Confusion in naming a carboxylic acid with a cyclohexyl group

For the below compound .

According to me the answer should be 1-carboxymethylcyclohexane.

But the answer given as 2-cyclohexylethan-1-oic acid

Why the name given by me is wrong .

Whereas the name given to

Is cyclohexane-1-carboxylic acid

Why it is not 1-cyclohexylmethan-1-oic acid

• – Loong Jan 21 '17 at 10:43

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the senior parent structure has the maximum number of substituents corresponding to the principal characteristic group.

P-44.1 SENIORITY ORDER FOR PARENT STRUCTURES

When there is a choice, the senior parent structure is chosen by applying the following criteria, in order, until a decision is reached. These criteria must always be applied before those applicable to rings and ring systems (see P-44.2) and to chains (see P-44.3). Then criteria applicable to both chains and rings or ring systems given in P-44.4 are considered.

P-44.1.1 The senior parent structure has the maximum number of substituents corresponding to the principal characteristic group (suffix) or senior parent hydride in accord with the seniority of classes (P-41) and the seniority of suffixes (P-43).

(…)

If there is still a choice (for example, if the ring and the chain each have a substituent corresponding to the principal characteristic group), the ring or the chain can be the senior parent structure; however, for the preferred IUPAC name, the ring is always selected as the senior parent structure.

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 compound given in the question contains a characteristic group $(\ce{-COOH})$. Since there is only one characteristic group, the seniority order of classes is not relevant in this case; thus, the $\ce{-COOH}$ substituent corresponds to the principal characteristic group.

Since the principal characteristic group is a substituent of the chain, and the ring does not have any substituents corresponding to the principal characteristic group, the senior parent structure of the compound given in the question is the chain and not the ring. The rules P-44.1.2.2 and P-52.2.8 for the selection between a ring and a chain as senior parent structure are not relevant in this case.

Simple carboxylic acids are named substitutively using the suffix ‘oic acid’ as follows.

P-65.1.2.1 Carboxylic acid groups, $\ce{-COOH}$, that conceptually replace a $\ce{-CH3}$ group of methane or terminate an unbranched hydrocarbon chain are named by replacing the final ‘e’ of the name of the corresponding hydrocarbon by the suffix ‘oic acid’. No locants are necessary to denote the positions of the carboxylic acid groups in a hydrocarbon chain; locants are used when hydrocarbon chains are modified by skeletal replacement, as shown in P-15.4.3.2.3. Except for formic acid, acetic acid, oxalic acid (see P-65.1.1.1), and oxamic acid (see P-65.1.1.1), systematically formed names are preferred IUPAC names; the names given in P-65 .1.1.2 are retained names for use in general nomenclature.

Therefore, the systematic name for the unsubstituted carboxylic acid structure that is given in the question is ethanoic acid (not ‘ethan-1-oic acid’ since no locants are used to denote the positions of the carboxylic acid groups in a hydrocarbon chain). However, the preferred IUPAC name (PIN) of ethanoic acid is the retained name acetic acid according to rule P-65.1.1.1.

The acetic acid chain is substituted using the usual principles of substitutive nomenclature.

However, according to rule P-14.3.4.6, locants are omitted where there is only one kind of substitutable hydrogen.

P-14.3.4.6 All locants are omitted for parent compounds when all substitutable hydrogen atoms have the same locant.

Therefore, the preferred IUPAC name name for the complete compound given in the question is cyclohexylacetic acid since the locant ‘2’ of ‘2-cyclohexylacetic acid’ is omitted.

• So is it cyclohexylmethanoic acid or cyclohexanecarboxylic acid? – DHMO Jan 21 '17 at 11:58

A good explanation of IUPAC rules for naming organic compounds may be useful. For carboxylic acids (which it is due to the $\ce{-COOH}$ group), the webpage says:

Carboxylic acids are named by counting the number of carbons in the longest continuous chain including the carboxyl group and by replacing the suffix -ane of the corresponding alkane with -anoic acid.

The reason it should be named as a carboxylic acid (instead of using carboxyl as a mere group) is due to precedence (and this is related to the comment by Loong):

The carboxyl group takes precedence over alkyl groups and halogen substituents, as well as double bonds, in the numbering of the parent chain.

The correct name is thus 2-cyclohexylethan-1-oic acid, as you said.