Alcohols - Structure and Nomenclature

In this article we will learn about Structure and Nomenclature of Alcohols. The Alcohols are organic compounds that consist of hydroxyl (-OH) group.

Key words: Alcohols, Structure and Nomenclature, IUPAC Names of Alcohols, Polyols, Phenols.

The word alcohol is derived from Arabic word al-kuhl means "the essence". They are one of the most common compounds used in nature and industries. 

The alcohols have general formula of C_nH_2n+1OH. The alcohols are considered as “alkyl derivative of water” because they resembles to the structure of water.  

The alcohols are categorized in to primary, secondary, tertiary alcohol and phenol based on the hydroxyl group bonded carbons. (Table 1)

Type of Alcohol	Hydroxyl group bonded Carbon	General formula	Example
Primary alcohol	Primary
Secondary alcohol	Secondary
Tertiary alcohol	Tertiary
Phenol	Aromatic  ring (benzene)

Tabel 1

Common Names of Alcohol

The common name of an alcohol is derived from alkyl group to which hydroxyl group is attached followed by the word “alcohol”. The common naming system is used for simple molecules. (Table 2)

Structure of alcohol	Alkyl group	Common name
	Methyl	Methyl alcohol
	Ethyl	Ethyl alcohol
	n-propyl	n-propyl alcohol
	iso-propyl	Iso-propyl alcohol
	Allyl	Allyl alcohol
	n-butyl	n-butyl alcohol
	Iso-butyl	Iso-butyl alcohol
	Sec-butyl	Sec-butyl alcohol
	Tert-butyl	Tert-butyl alcohol

Table 2

IUPAC Names of Alcohols

The IUPAC system delivers specific name to an alcohol based on few rules which are listed below.

1. Select the longest carbon chain which consists of hydroxyl group.
2. Derive the root name from the alkane which consists of same number of carbons. Remove "–e" from the alkane and replace it with "–ol" that gives root name.
3. Give numbers to each carbon.
4. Name all the substituents and their numbers before the root name.
5. In addition to hydroxyl group If any alkene / alkyne present in the molecule; hydroxyl groups must be given preference.
6. Cyclic alcohols are named by using suffix “cyclo” and numbers are given starting from the hydroxyl group attached carbon.

Consider the following example; it has longest chain of five carbons. So the alkane is “pentane”. Now remove suffix “–e” from alkane and replace with “1–ol” because OH present at C1. Hence the root name is “pentan-1-ol”. The “bromo” substituent is present at C2. And two methyl substituents are bonded to C4. Therefore the complete IUPAC name of the compound is “2-bromo-4,4-dimethylpentan-1-ol”.

Consider next example, here the hydroxyl group is attached to five carbon alkane at C2. So the root name is “pentan-2-ol”. There is a methyl group at C2 and “chloro” substituent at C4. The substituents should be written in alphabetical order. Therefore complete name of the given compound is “4-chloro-2-methylpentan-2-ol”.

Consider another example, Here hydroxyl group attached to cyclohexane. So the root name is “cyclohexan-1-ol”. There is ethyl group at C3 and methyl group at C5. The substituents should be written in alphabetical order. Therefore complete name of the compound is “3-ethyl-5-methylcyclohexan-1-ol”. 

In the following example, the hydroxyl group present at C3 of five member carbon chain. So the root name is “pentan-3-ol”. Also there is a double bond present at C1. In this situation the suffix “en” is used for alkene functionality. Therefore drop “an” from the root name and add “1-en” (because double bond present at C1). Hence the complete name of the compound is “pent-1-en-3-ol”.

If the internal alkene present then geometry of alkene must be given in the name. As well as if any chiral carbon present then its configuration must be included in the name. In the following example geometry of double bond is “E”. And chiral carbon has “R” configuration. Therefore complete name of the compound is “(R,E)-hex-4-en-3-ol”.

Similarly, if there is a triple bond present in the molecule, then suffix “yn” is used. In the next example, alkyne and hydroxyl group both are present. The preference must be given to hydroxyl group so the numbering starts from the end which is near to hydroxyl group. Here hydroxyl group is bonded to C2 of six member carbon chain. So the root name is “hexan-2-ol”. Since there is alkyne present at C4; drop “an” from the root name and add the suffix “4-yn”. Hence complete name of the compound is “hex-4-yn-2-ol”. 

IUPAC Names Polyols

The alcohols with two or hydroxyl groups are referred as “Polyols. They are named in the same way as we have seen for alcohols except the suffix “diol”, “triol” etc depending upon the number of hydroxyl groups present in the molecule (Table 3). Also carbon numbers are included in the name where the hydroxyl groups are located the molecule.

Table 3

Some of the examples are shown below.

IUPAC Names of Phenols

The hydroxybenzene is known as “phenol” and it is also root name of aromatic alcohols. For assigning IUPAC names; the hydroxyl bonded carbon is considered as C1. Then other substituents are written in alphabetical order.

In the following examples; based on the position of methyl groups on aromatic ring; IUPAC names are assigned. In common names the term “ortho” is used for 1,2-disubstituted; “meta” is used for 1,3-disubstituted and “para” is used for 1,4-disubstituted aromatic ring.

In the following example; two methyl groups are present at C3 and C5. So the prefix “di” is used in the name. Hence name of the compound is “3,5-dimethylphenol”.

Consider another example, here nitro-group present at C3 and methyl group at C4. Therefore the name of the compound is “4-methyl-3-nitrophenol”.

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