23.2 Physical Properties of Alcohols

Learning Objectives

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

  • Explain why the boiling points of alcohols are higher than those of ethers and alkanes of similar molar masses.
  • Explain why alcohols and ethers of four or fewer carbon atoms are soluble in water while comparable alkanes are not soluble.
Alcohols can be considered derivatives of water (H2O; also written as HOH) (Figure 23.2a.). Refer to Appendix A: Key Element Information for more details about oxygen.
Structure of water (left) showing an oxygen connected by two hydrogens and an alcohol (right) showing an oxygen connected to a hydrogen and an R group.
Figure 23.2a. Structure of water (left) and alcohol (right) (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Like the H–O–H bond in water, the R–O–H bond is bent, and alcohol molecules are polar (Figure 23.2b.).

Bent structure of alcohol showing the presence of two lone pairs of electrons on the oxygen atom. The oxygen central atom is connected to an R group on one side and a hydrogen on the other side. The carbon-oxygen-hydrogen bond angle is shown to be 109°
Figure 23.2b. Bent structure of alcohol showing the presence of two lone pairs of electrons on the oxygen atom and the carbon-oxygen-hydrogen bond angle of 109° (credit: Image by RamaKrishnaHare, CC BY-SA 4.0).

Boiling Point

The relationship between the structure of water and the structure of alcohols is particularly apparent in small molecules and reflected in the physical and chemical properties of alcohols with low molar mass. Replacing a hydrogen atom from an alkane with an OH group allows the molecules to associate through hydrogen bonding (Figure 23.2c.).

Intermolecular hydrogen bonding in methanol demonstrated by multiple methanol groups close to one another. The OH groups of alcohol molecules make hydrogen bonding possible as shown with green dotted lines between the hydrogen from one molecule and a oxygen in the other molecule
Figure 23.2c. Intermolecular hydrogen bonding in methanol. The OH groups of alcohol molecules make hydrogen bonding possible (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Recall that physical properties are determined to a large extent by the type of intermolecular forces. Table 23.2a. lists the molar masses and the boiling points of some common compounds. The table shows that substances with similar molar masses can have quite different boiling points.

Table 23.2a. Comparison of Boiling Points and Molar Masses (Image Credits: Introduction to Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0.)
Formula Name Molar Mass Boiling Point (°C)
CH4 methane 16 –164
HOH water 18 100
C2H6 ethane 30 –89
CH3OH methanol 32 65
C3H8 propane 44 –42
CH3CH2OH ethanol 46 78
C4H10 butane 58 –1
CH3CH2CH2OH 1-propanol 60 97

Alkanes are nonpolar and are thus associated only through relatively weak dispersion forces. Alkanes with one to four carbon atoms are gases at room temperature. In contrast, even methanol (with one carbon atom) is a liquid at room temperature. Hydrogen bonding greatly increases the boiling points of alcohols compared to hydrocarbons of comparable molar mass. The boiling point is a rough measure of the amount of energy necessary to separate a liquid molecule from its nearest neighbours. If the molecules interact through hydrogen bonding, a relatively large quantity of energy must be supplied to break those intermolecular attractions. Only then can the molecule escape from the liquid into the gaseous state.

Adding additional -OH groups to an alcohol molecule will increase the boiling point because there are more opportunities for hydrogen bonding.  Consider propan-1-ol (CH3CH2CHOH) which has a molar mass of 60 g/mol and a boiling point of 97oC.  Now consider 1,2-ethanediol, also known as ethylene glycol, which has a molar mass of 62 g/mol and a boiling point of 197oC.  The presence of one additional -OH group significantly increase the hydrogen bonding ability and as such the boiling point (National Center for Biotechnology Information, 2024a,b).

Ethylene glycol is the main ingredient in many antifreeze mixtures for automobile radiators. Because of its high boiling point, ethylene glycol does not boil away when it is used as an antifreeze. It is also completely miscible with water. A solution of 60% ethylene glycol in water freezes at −49°C (−56°F) and thus protects an automobile radiator down to that temperature.

Solubility

Alcohols can also engage in hydrogen bonding with water molecules (Figure 23.2d.). Thus, whereas the hydrocarbons are insoluble in water, alcohols with one to three carbon atoms are completely soluble.

Intermolecular hydrogen bonding between multiple molecules of ethanol (an alcohol) and water molecules. The OH groups of alcohol molecules allow for the hydrogen bonding to occur with the water molecules. The hydrogen from ethanol and the oxygen from water demonstrate the hydrogen bonding by dotted green lines.
Figure 23.2d. Intermolecular hydrogen bonding between ethanol (an alcohol) and water molecules. The OH groups of alcohol molecules allow for the hydrogen bonding to occur with the water molecules. (Credit: Introduction to Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

As the length of the chain increases, however, the solubility of alcohols in water decreases; the molecules become more like hydrocarbons and less like water. The alcohol 1-decanol (CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2OH) is essentially insoluble in water. We frequently find that the borderline of solubility in a family of organic compounds occurs at four or five carbon atoms.

Adding additional -OH groups to an alcohol molecule will increase the solubility because there are more opportunities for hydrogen bonding with water.  Consider hexan-1-ol (CH3CH2CH2CH2CH2CH2OH) which has a molar mass of 102 g/mol and a solubility of 5.9 g/L at 25oC.  Now consider D-glucose, also known as 2,3,4,5,6-pentahydroxyhexanal (see Figure 23.1b), which has a molar mass of 180 g/mol and a solubility of 909 g/L at 25oC.  The presence of additional -OH groups significantly increase the hydrogen bonding ability with water and as such the solubility (National Center for Biotechnology Information, 2024c; “Glucose”, 2023).

Spotlight on Everyday Chemistry: Glycerol

The physical properties of compounds greatly impact their everyday uses.  Infographic 23.2a. highlights some key uses of glycerol (1,2,3-propanetriol).

Infographic 23.2a.  Read more about “Food, Cosmetics & Explosives – The Versatility of Glycerol” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 23.6a [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

Glucose. (2023, December 19). In Wikipedia.

National Center for Biotechnology Information (2024a). PubChem Compound Summary for CID 174, Ethylene Glycol. Retrieved January 10, 2024.

National Center for Biotechnology Information (2024b). PubChem Compound Summary for CID 1031, 1-Propanol. Retrieved January 10, 2024.

National Center for Biotechnology Information (2024c). PubChem Compound Summary for CID 8103, 1-Hexanol. Retrieved January 10, 2024.

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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.

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