Nomenclature of Alcohols, Phenols, and Ethers: Rules & Examples

Nomenclature of Alcohols, Phenols, and Ethers: Organic molecules containing the “hydroxyl \(( – {\rm{OH}})\)” functional group are called alcohols. The carbon atom directly attached to the \( – {\rm{OH}}\) group is called the “carbinol” carbon atom. The carbinol carbon (carbon attached to OH), however, is the key to classifying alcohols into “primary”, “secondary”, and “tertiary” alcohols. Let’s learn how these compounds are named in the language of chemistry.

Alcohols

The nomenclature of alcohol is derived from their parent alkane name. The -ane suffix of the parent hydrocarbon chain is replaced by -anol and a number representing the position of the \( – {\rm{OH}}\) group on the chain.

The generic IUPAC (International Union of Pure and Applied Chemistry) name for alcohol is alkanols with the general formula \({\rm{R}} – {\rm{OH}}\).

Alcohols are common in nature, and the most familiar one is ethyl alcohol (ethanol), the active ingredient in alcoholic beverages. Methanol \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{OH}}} \right)\) and ethanol \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}} \right)\) are the first two members of the homologous series of alcohols.

IUPAC Nomenclature for Alcohols

Alcohols with one to four carbon atoms are frequently called by common names, in which the word alcohol follows the name of the alkyl group:

According to the International Union of Pure and Applied Chemistry (IUPAC). The -ane suffix of the parent hydrocarbon chain is replaced by -anol. Some basic IUPAC rules for naming alcohols are:

1. The longest continuous chain (LCC) of carbon atoms containing the \( – {\rm{OH}}\) group is chosen as the parent compound.

2. The chain is numbered so that the \( – {\rm{OH}}\) group occupies the lowest position in the chain, i.e., from the end nearest the \({\rm{OH}}\) group (lowest locant rule).

3. The -ane suffix of the parent hydrocarbon alkane chain is replaced by -anol, and the number indicating the position of the \({\rm{OH}}\) group is prefixed to the name of the parent hydrocarbon.

4. However, in cyclic alcohols, the carbon atom bearing the \( – {\rm{OH}}\) group is designated as \({\rm{C}}1\), but the \(1\) is not used in the name. Substituents are named and numbered as in alkanes.

5. If more than one \( – {\rm{OH}}\) group appears in the same molecule (polyhydroxy alcohols), suffixes such as- diol and- triol are used. The ending ‘-e’ of the parent alkane is retained while naming polyhydroxy alcohols.

6. If the \({\rm{OH}}\) group is connected to a chiral centre, the absolute configuration is included at the beginning of the name. Also, if there is a double bond the \({\rm{E}}\) and \({\rm{Z}}\) configuration should be addressed when applicable:

7. When other groups are present on the ring, it is numbered clockwise or counterclockwise depending on which direction gives the next substituent the lower number:

Not all functional groups containing \( – {\rm{OH}}\) functional groups are alcohols. If the \( – {\rm{OH}}\) group is attached to a carbonyl \(({\rm{C}} = {\rm{O}})\) group, then a “carboxylic acid” functional group is formed. An alkene to which the \( – {\rm{OH}}\) group is directly attached is called an “enol” (ene \(+\) ol). Geminal diols in which both the \( – {\rm{OH}}\) groups are attached to the same carbon atom belong to a separate category from alcohols.

Alcohols Solved Example

What is the IUPAC name of:

The longest chain of carbon atoms consisting of the \( – {\rm{OH}}\) functional group that forms the parent chain is identified. Numbering is done so that the \( – {\rm{OH}}\) group gets the lowest locant, followed by other substituents. The parent chain has five carbon atoms with \( – {\rm{OH}}\), ethyl and methyl groups at \(1, 2\) and \(4\) positions. Hence, the IUPAC name of the given compound is \(2\)-Ethyl-\(4\)-methyl-\(1\)-pentanol.

Phenols

Phenols are organic molecules consisting of a hydroxyl \(( – {\rm{OH}})\) group attached to the carbon atom part of an aromatic ring. Phenols are designated as \({\rm{Ar}} – {\rm{OH}}\). Based on the number of hydroxyl groups attached to the aryl group, it can be classified into mono-, di-, tri- or polyhydric phenols.

IUPAC Nomenclature of Phenols

The IUPAC name of Phenol is hydroxybenzene, derived in the same manner as the IUPAC names for aliphatic alcohols. It is commonly known as carbolic acid.

1. The position of the hydroxyl group attached to the benzene ring is located and numbered.

2. If more than one hydroxyl group is present, then numerical prefixes such as di, tri, and tetra denote the number of similar hydroxyl groups attached to the benzene ring.

3. In substituted phenols, the numbering of the other functional groups is done with respect to the position where the hydroxyl group is attached.

Words like -ortho (functional group is attached to the adjacent carbon atom to which \( – {\rm{OH}}\) group is attached), -meta (functional group attached to the second carbon atom from the hydroxyl group), -para (functional group attached to the third carbon atom from the hydroxyl group) are also used for the nomenclature of phenols.

Phenol Solved Example Question

What is the IUPAC name of-

The numbering is done in either clockwise or anticlockwise direction. In both directions, \( – {\rm{N}}{{\rm{O}}_2}\) groups occupy the same lowest locant. The \( – {\rm{N}}{{\rm{O}}_2}\) groups are present at \(2,4,6\) carbon atoms relative to the \( – {\rm{OH}}\) group.

There are three \( – {\rm{N}}{{\rm{O}}_2}\) groups, hence, the prefix tri- is used. Putting everything together, the IUPAC name of the given compound is \(2,4,6\)-trinitrophenol.

Ethers

Ethers are organic compounds in which both the ends of the oxygen atom are connected to two hydrocarbon groups (alkyl or aryl). An ether is represented by the general formula \({\rm{R}} – {\rm{O}} – {{\rm{R}}^\prime }\) as shown below.

The \({{\rm{R}}^\prime }\) in the formula means that the hydrocarbon group can be the same as \({\rm{R}}\) or different.

IUPAC Nomenclature of Ethers

1. The longest carbon chain is chosen as the parent chain, and one of the alkoxy (alkyl with the oxygen) groups is treated as a substituent connected to a parent chain.

2. The oxygen atom forms a part of the alkyl group comprising less number of carbon atoms, and together they constitute the alkoxy group.

3. In the IUPAC system, the -yl ending of alkyl substituents is replaced by -oxy. For example \( – {\rm{C}}{{\rm{H}}_3}\) methyl as \( – {\rm{OC}}{{\rm{H}}_3}\) methoxy, \( – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}\) ethyl as \( – {\rm{OC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}\) ethoxy.

For example-

4. As alkoxy \(( – {\rm{O}} – {\rm{R}})\) groups are considered as substituents, they do not rank in the priority chart of the functional groups. Hence, they do not change the suffix of the parent chain.

For example, let’s compare the effects of the \( – {\rm{OH}}\) and \( – {\rm{OR}}\) groups on naming structurally similar compounds:

The \( – {\rm{OH}}\) group has the highest priority, and the parent chain is altered from “ane” to “ol”. On the other hand, the \( – {\rm{OR}}\) group has no priority and is added as a prefix in alphabetical order.

5. The substituents present in the ether are named alphabetically.

Ethers can also be named as follows.

The alkyl groups are identified and arranged in alphabetical order. The parent compound is given by the word ether, which appears at the end of the name. The names of each alkyl group come before the word ether. For unsymmetrical ethers, the alkyl groups are listed in alphabetical order.

The IUPAC name of the above compounds is as follows:

For symmetrical ethers, the prefix –di is used. Spaces are left between the names of the alkyl groups (if different) and before the word ether. For example-

The IUPAC name of the above compounds is as follows:

Solved Examples on Ether

What is the IUPAC name of:

The parent chain, along with the substituents, is identified. The end of the oxygen atom that has a shorter alkyl group becomes the alkoxy substituent. The parent chain is numbered in a way that the alkoxy group gets the lowest locant. The substituents are arranged in alphabetical order, and the IUPAC name is written with numbers and words separated by a hyphen(-). The IUPAC name of the given compound is as follows-

Summary

Alcohols, Phenols, and Ethers are an essential class of organic compounds with a wide range of applications. From beverages, fuels to sanitisers and perfumes. Hence, it is important to know their nomenclature. These compounds are named according to IUPAC (International Union of Pure and Applied Chemistry).

Alcohols and ethers are functional isomers, i.e. they have the same molecular formula but differ in their functional groups. Due to the difference in the functional groups, they are also named differently. Phenols that resemble alcohols differing only in the alkyl group are named based on different rules. We learnt the rules of naming alcohol, phenols, and ethers through this article and how they differ from each other.

FAQs

Q.1. What is the generic structure of alcohols?
Ans: Organic molecules containing the “hydroxyl \(( – {\rm{OH}})\)” functional group attached to an aliphatic carbon atom are called alcohols. \( – {\rm{OH}}\) is the functional group of all alcohols; hence alcohols are represented by the general formula \({\rm{ROH}}\), where \({\rm{R}}\) is an alkyl group.

Q.2. Why is Phenol more acidic than ethanol?
Ans: Phenol is more acidic than ethanol because phenol forms phenoxide ion on losing \({{\rm{H}}^ + }\) ions, and ethanol on losing \({{\rm{H}}^ + }\) ion forms ethoxide ion. Moreover, phenoxide ion is stabilised by resonance; hence Phenol is more acidic than ethanol.

Q.3. What is the IUPAC name and structure of the products formed by the hydration of propene in the presence of dilute sulphuric acid?
Ans: Acid catalysed hydration of alkenes leads to the formation of alcohols. The hydration of propene in the presence of dilute sulphuric acid forms Propan-\(2\)-ol \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CH}}({\rm{OH}}){\rm{C}}{{\rm{H}}_3}} \right)\).

Q.4. What is the IUPAC name of m-cresol?
Ans: The IUPAC name of m-cresol is \(3\)-methyl phenol. The methyl group is present at the second carbon atom relative to the \( – {\rm{OH}}\) group.

Q.5. The IUPAC name of \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{C}}{{\rm{H}}_3}\) is ethyl methyl ether or methoxy ethane?
Ans: The IUPAC name of the given compound is methoxyethane. The end of the oxygen atom that has a shorter alkyl group becomes the alkoxy substituent. The longer alkyl group becomes the alkane base name. Each alkyl group on each side of the oxygen is numbered separately. The numbering priority is given to the carbon closest to the oxygen. The alkoxy side (shorter side) has an “-oxy” ending with its corresponding alkyl group.

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