23.1 Alcohols – Structure, Naming and Classification

Learning Objectives

By the end of this section, you will be able to:

  • Identify the general structure for an alcohol.
  • Identify the structural feature that classifies alcohols as primary, secondary, or tertiary.
  • Name alcohols with both common names and IUPAC names

Incorporation of an oxygen atom into carbon- and hydrogen-containing molecules leads to new functional groups and new families of compounds. When the oxygen atom is attached by single bonds, the molecule is either an alcohol or ether.

Alcohols are derivatives of hydrocarbons in which an –OH group has replaced a hydrogen atom. Although all alcohols have one or more hydroxyl (–OH) functional groups, they do not behave like bases such as NaOH and KOH. NaOH and KOH are ionic compounds that contain OH– ions. Alcohols are covalent molecules; the –OH group in an alcohol molecule is attached to a carbon atom by a covalent bond.

We often represent alcohols by the general formula ROH, where R is an alkyl group. Alcohols are common in nature. Most people are familiar with ethyl alcohol (ethanol) (Figure 23.1a.), the active ingredient in alcoholic beverages, but this compound is only one of a family of organic compounds known as alcohols. The family also includes such familiar substances as cholesterol and the carbohydrates. Methanol (CH3OH) and ethanol (CH3CH2OH) are the first two members of the homologous series of alcohols.

 

Structure of ethanol from left to right a) structural formula, b) ball and stick model, c) space-filling model.
Figure 23.1a. Structure of ethanol a) structural formula, b) ball and stick model, c) space-filling model. (credit a: Image by Ju, PDM; b: Image by Benjah-bmm27, PDM; c: Image by Benjah-bmm27, PDM).
Alcohols have a minimum of one -OH group in the molecule.  Alcohols containing two or more hydroxyl groups can be made. Many alcohols have more than one -OH group in the structure (e.g. glucose) but rarely does any one carbon have more than one -OH group (Figure 23.1b.).

Two structures: D-glucose (on the left) and L-glucose (on the right). D-Glucose has one OH group on the left with three OH groups on the right. L-Glucose has three OH groups on the left and one OH group on the right.
Figure 23.1b. Structure of D-glucose and L-glucose showing multiple -OH groups per molecule (credit: Image by NEUROtiker, PDM).

Alcohols with two -OH groups on adjacent carbon atoms are commonly known as glycols. Additional examples include 1,2-ethanediol (ethylene glycol, used in antifreeze, sweet colourless and somewhat viscous liquid) and 1,2,3-propanetriol (glycerin or glycerol, used as a solvent for cosmetics and medicines, sweet syrupy liquid) (Figure 23.1c.). Ethylene glycol is quite toxic. Because it is sweet, pets often lap up spills of leaked antifreeze from a garage floor or driveway. Sometimes people, especially children, drink it. The oxidation of ethylene glycol by the liver results in calcium oxalate crystals forming in the kidneys which causes renal damage and possibly death. This is not true of propylene glycol which is essentially nontoxic, and it can be used as a solvent for drugs and as a moisturizing agent for foods.

Structural formulas for 1 comma 2 dash ethanediol and 1 comma 2 comma 3 dash propanetriol are shown. The first structure has a two C atom hydrocarbon chain with an O H group attached to each carbon. The O H groups are shown in red an each O atom has two sets of electron dots. Each C atom also has two H atoms bonded to it. The second structure shows a three C atom hydrocarbon chain with an O H group bonded to each carbon. The O H groups are shown in red, and each O atom has two sets of electron dots. The first C atom has two H atoms bonded to it. The second C atom has one H atom bonded to it. The third C atom has two H atoms bonded to it.
Figure 23.1c. Structure of 1,2-ethanediol and 1,2,3-propanetriol (credit: General Chemistry 1 & 2, CC BY 4.0).

Nomenclature of Alcohols

According to the International Union of Pure and Applied Chemistry (IUPAC), alcohols are named by changing the ending of the parent alkane name to –ol. Here are some basic IUPAC rules for naming alcohols:

  1. The longest continuous chain (LCC) of carbon atoms containing the OH group is taken as the parent compound—an alkane with the same number of carbon atoms. The chain is numbered from the end nearest the OH group.
  2. The number that indicates the position of the OH group is prefixed to the name of the parent hydrocarbon, and the –e ending of the parent alkane is replaced by the suffix –ol.
    • In 2013, IUPAC adopted new nomenclature guidelines that require the position number to be placed as an “infix” rather than a prefix. For example, the new name for 2-propanol would be propan-2-ol. Widespread adoption of this new nomenclature will take some time.
    • In cyclic alcohols, the carbon atom bearing the OH group is designated C1, but the 1 is not used in the name.) Substituents are named and numbered as in alkanes.
  3. If more than one OH group appears in the same molecule (polyhydroxy alcohols), suffixes such as –diol and –triol are used. In these cases, the –e ending of the parent alkane is retained.

Example 23.1a

Give the IUPAC name for each compound.

  1. From left to right, there are ten carbon on the alkane straight chain with methyl groups emerging from carbon 3 and 5 and a hydroxyl group on carbon 8
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).
  2. a 5 carbon chain with an OH group at each end of the chain
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Solutions:

  1. Ten carbon atoms in the LCC makes the compound a derivative of decane (rule 1), and the OH on the third carbon atom makes it a 3-decanol (rule 2).
    A 10 carbon chain with an OH group on the 3rd carbon, and a methyl group on the 6th and 8th carbon. This is known as 6,8-dimethyl-3-decanol
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

    The carbon atoms are numbered from the end closest to the OH group. That fixes the two methyl (CH3) groups at the sixth and eighth positions. The name is 6,8-dimethyl-3-decanol (not 3,5-dimethyl-8-decanol).

  2. Five carbon atoms in the LCC make the compound a derivative of pentane. Two OH groups on the first and fifth carbon atoms make the compound a diol and give the name 1,5-pentanediol (rule 3).
    A 5 carbon chain with an OH group at both ends.
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

     

Exercise 23.1a

Name the following molecule:
The structure shown has a C H subscript 3 group bonded up and to the right to a C atom. The C atom is bonded down and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded up and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded down and to the right to a C H subscript 3 group. The second C atom (from left to right) is bonded to a C H subscript 3 group and an O H group.

Check Your Answer:[1]

Exercise source: General Chemistry 1 & 2, CC BY 4.0.

Exercise 23.1b

Give the IUPAC name for each compound.

Two molecules from left to right are a) a 3 carbon chain with an OH group at the 1st and 2nd carbon; b) a 5 carbon ring structure with an OH group and a methyl group the 1st carbon.

Check Your Answers:[2]

Exercise source: Introduction to Chemistry: GOB(V. 1.0)., CC BY-NC-SA 3.0.

Example 23.1b

Draw the structure for each compound.

  1. hexan-2-ol
  2. 3-methyl-2-pentanol

Solution:

a) The ending –ol indicates an alcohol (the OH functional group), and the hex– stem tells us that there are six carbon atoms in the LCC. We start by drawing a chain of six carbon atoms: –C–C–C–C–C–C–.

The 2 indicates that the OH group is attached to the second carbon atom.

a 6 carbon chain with an OH group at the 2nd carbon
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Finally, we add enough hydrogen atoms to give each carbon atom four bonds.

a 6 cabon chain with an OH group at the 2nd carbon.
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

 

b) Pent- means 5 carbon chain. The numbers indicate that there is a methyl (CH3) group on the third carbon atom and an OH group on the second carbon atom.

a 6 carbon chain with an OH group at the 2nd carbon and a methyl group at the 3rd carbon.
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Exercise 23.1c

Draw the structure for each compound.

  1. heptan-3-ol
  2. 2-methyl-3-hexanol

Check Your Answers:[3]

Source: Exercise 23.1c is adapted from Basics of GOB Chemistry (Ball et al.) , CC BY-NC-SA 4.0, with images drawn by Samantha Sullivan Sauer using Biovia Draw, CC BY-NC 4.0.

Alcohols with one to four carbon atoms are frequently called by common names, in which the name of the alkyl group is followed by the word alcohol (Figure 23.1d.). Structural formula of methyl alcohol, ethyl alcohol, propyl alcohol, and isopropyl alcohol with the methyl, ethyl propyl, and isopropyl groups are highlighted in green in Figure 23.1c.

The molecular structures from left to right of methyl alcohol (methanol), ethyl alcohol (ethanol), propyl alcohol (propan-1-ol), and Isopropyl alcohol (propan-2-ol). The methyl, ethyl, propyl and isopropyl are identified using the colour green.
Figure 23.1d. Structure of methyl alcohol (methanol), ethyl alcohol (ethanol), Propyl alcohol (propan-1-ol), and Isopropyl alcohol (propan-2-ol) (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Classification of Alcohols

Some of the properties of alcohols depend on the number of carbon atoms attached to the specific carbon atom that is attached to the OH group. Alcohols can be grouped into three classes on this basis.

  • A primary (1°) alcohol is one in which the carbon atom (in red) with the OH group is attached to one other carbon atom (in blue) (Figure 23.1e.). Its general formula is RCH2OH.
A primary alcohol showing a central carbon atom connected to one R group, two hydrogens and an OH group
Figure 23.1e. Structure of primary (1o) alcohol (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).
  • A secondary (2°) alcohol is one in which the carbon atom (in red) with the OH group is attached to two other carbon atoms (in blue) (Figure 23.1f.). Its general formula is R2CHOH.
A secondary alcohol structure showing a carbon central atom connected to two R groups, a hydrogen and an OH group
Figure 23.1f. Structure of secondary (2o) alcohol (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).
  • A tertiary (3°) alcohol is one in which the carbon atom (in red) with the OH group is attached to three other carbon atoms (in blue) (Figure 23.1g.). Its general formula is R3COH.
A tertiary alcohol with a central carbon atom connected to three R groups and an OH group
Figure 23.1g. Structure of tertiary (3o) alcohol (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Some of the common names reflect a compound’s classification as secondary (sec-) or tertiary (tert-). These designations are not used in the IUPAC nomenclature system for alcohols. There are four butyl alcohols corresponding to the four butyl groups: the butyl group (CH3CH2CH2CH2-), and three others in Figure 23.1h.

Three structure: On the left is isobutyl group showing a 3 carbon chain with a methyl group on C2 and a dash to connect to a molecule at the end carbon; in the middle it is sec-butyl group with a 4 carbon with a dash to connect to another molecule at the C2; and on the right it is tert-butyl showing a central carbon atom with 3 carbon atoms connected on each side like a "T" shape.
Figure 23.1h. Structure of isobutyl, sec-butyl and tert-butyl groups (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Table 23.1a. names and classifies some of the simpler alcohols.

Table 23.1a. Classification and Nomenclature of Some Alcohols (Image Credits: Introduction to Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0.)
Condensed Structural Formula Class of Alcohol Common Name IUPAC Name
CH3OH — methyl alcohol methanol
CH3CH2OH primary ethyl alcohol ethanol
CH3CH2CH2OH primary propyl alcohol 1-propanol
(CH3)2CHOH secondary isopropyl alcohol 2-propanol
CH3CH2CH2CH2OH primary butyl alcohol 1-butanol
CH3CH2CHOHCH3 secondary sec-butyl alcohol 2-butanol
(CH3)2CHCH2OH primary isobutyl alcohol 2-methyl-1-propanol
(CH3)3COH tertiary tert-butyl alcohol 2-methyl-2-propanol
A structure of cyclohexanol secondary cyclohexyl alcohol cyclohexanol

Indigenous Perspectives: Traditional Plant-Based Remedies

Many modern medicines have their roots in Indigenous traditional plant-based remedies.  One such example is salicin.  Salicin is a pain killing compound derived from the willow tree.  Its structure has multiple -OH groups (Figure 23.1i.). On Turtle Island, Inuit harvest dwarf willow (a tiny bush that grows in the Arctic environment and a relative of the willow tree) as a source of pain relief (Figure 23.1i.).  We now know that salicin is the active ingredient which is transformed in the human body to salicylic acid.  Salicylic acid is related to acetylsalicylic acid commercially known under the brand name of Aspirin (Anderson & Rayner-Canham, 2022).

 

Dwarf willow (Salix herbacea) on the left which is a photo of a green plant on the ground. Salicin molecule is on the right showing multiple OH groups surrounding an oxygen substituted cyclohexane group joined by an oxygen to a benzene ring.
Figure 23.1i. Dwarf willow (Salix herbacea) (left). Salicin molecule (right) (credit left: Image by El Grafo, CC BY-SA 3.0; right: Image by Fuse809; PDM).

Spotlight on Everyday Chemistry: Hand Sanitizers

Hand sanitizers use alcohol to protect against infections including from viruses and bacteria.  Infographic 23.1a. shows some of the alcohols used and how the alcohol helps prevent infection.

 

Infographic 23.1a.  Read more about “Coronavirus: How hand sanitisers protect against infections” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 23.1a [New tab].

Attribution & References

Except where otherwise noted, this page is written and adapted by David Wegman and Samantha Sullivan Sauer from:

References cited in-text

Anderson, C. C., & Rayner-Canham, G. (2022, Fall). Chemistry of the cure: Case studies of some Inuit remedies. Chem 13 News Magazine.

  1. 2-methyl-2-pentanol

    ↵

  2. a) 1,2-propanediol b) 1-methylcyclopentanol

    ↵

  3. The answers a) a 7 carbon chain with an OH group at the 3rd carbon known as 3-heptanol; and b) a 6 carbon chain with an OH group at the 3rd carbon and a methyl group at the 2nd carbon known as 2-methyl-3-hexanol

    ↵

definition

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book