Module 2

Lesson 1.6 Properties of the Alcohols



Key Concepts


Alcohols display very different properties compared to similarly sized hydrocarbons. The presence of the hydroxyl (-OH) functional group determines the unique physical and chemical properties of this family.

  1. Physical Properties

    1. Boiling Point

      Alcohols boil at a much higher temperature than hydrocarbons of similar molar mass. For example, C2H6 (ethane) has a much lower boiling point than C2H5OH. Whereas the ethane molecule exhibits only London forces, the -OH group allows for hydrogen bonding to occur between neighbouring ethanol molecules.


      Study the table below. How does the boiling point change as the size of the alcohol increases?


      Fig. 1

      The increase in boiling point can be explained by the fact that larger molecules possess more electrons. The more electrons, the stronger the London forces between neighbouring alcohol molecules. In contrast, the dipole-dipole forces and hydrogen bonds stay approximately the same for all five alcohols.

    2. Solubility

      The -OH end of the alcohol promotes solubility in water whereas the hydrocarbon portion of the molecule resists solubility. The solubility of alcohols in water varies depending on the length of the carbon chain. Short-chain alcohols tend to be very soluble in water because of their size, polarity, and hydrogen bonding. However, larger alcohols, with their predominant non-polar hydrocarbon portion, are not as soluble in water. In fact, larger alcohols often are used as solvents for non-polar compounds.

  2. Chemical Properties


    The presence of the hydroxyl group determines the chemical properties of alcohols. In which reactions are alcohols involved? You will explore this question in Lesson 2.


Check Your Understanding


Complete Practice Questions 8, 9, 10, and 11 on page 430 of the textbook.

Check your work by clicking on the link below

Page 430 Practice Question 8

Methanol (CH3OH, 18 electrons) exhibits London forces, dipole-dipole forces, and hydrogen bonding. Methane (CH4, 10 electrons) has weaker London forces than methanol and exhibits no dipole-dipole forces or hydrogen bonding.


Page 430 Practice Question 9
  1. The order is ethane (C2H6, 18 electrons), fluoromethane (CH3F, 18 electrons), methanol (CH3OH, 18 electrons). Each compound has similar London forces, but fluoromethane has dipole-dipole forces, and methanol has dipole-dipole forces and hydrogen bonding.
  2. The order is pentane (C5H12, 42 electrons), 1-chlorobutane (C4H9Cl, 50 electrons), butan-1-ol (C4H9OH, 42 electrons). Pentane has only London forces. The other two have London forces and dipole-dipole forces. Although the London forces in 1-chlorobutane are slightly stronger than those of butan-1-ol, this is more than offset by the fact that butan-1-ol is the only compound exhibiting hydrogen bonding.

Page 430 Practice Question 10

With its three -OH bonds, glycerol forms strong hydrogen bonds with water molecules. This makes it more difficult for water molecules to come together in the correct arrangement to form crystals. Therefore, a lower temperature is needed to cause freezing.


Page 430 Practice Question 11
  1. Ethane (C2H6, 18 electrons) has the lowest boiling point because it exhibits only London forces. Chloroethane (C2H5Cl, 34 electrons) has a boiling point between that of ethane and ethanol because it has stronger London forces than ethane, plus it has dipole-dipole forces. Ethanol (C2H5OH, 26 electrons) has the highest boiling point because, although it has weaker London forces than chloroethane, it exhibits hydrogen bonding.
  2. Ethanol has the greatest solubility because, like the solvent water, it is a small polar hydrogen-bonded substance. Chloroethane has slight solubility because it is a small polar molecule. Ethane is the least soluble because it has no polarity or hydrogen bonding.