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14.6 Glycols and Glycerol

Learning Objective

  1. Describe the structure and uses of some common polyhydric alcohols.

Alcohols with two OH groups on adjacent carbon atoms are commonly known as glycolsAn alcohol with two OH functional groups.. The most important of these is 1,2-ethanediol (the common name is ethylene glycol), a sweet, colorless, somewhat viscous liquid.

Another common glycol, 1,2-propanediol, is commonly called propylene glycol. Its physical properties are quite similar to those of ethylene glycol.

Commonly called glycerol or glycerin, 1,2,3-propanetriol is the most important trihydroxy alcohol. Like the two glycols, it is a sweet, syrupy liquid. Glycerol is a product of the hydrolysis of fats and oils. (For more information about fats and oils, see Chapter 17 "Lipids", Section 17.2 "Fats and Oils".)

Ethylene glycol is the main ingredient in many antifreeze mixtures for automobile radiators. The two OH groups lead to extensive intermolecular hydrogen bonding. This results in a high boiling point—198°C; thus 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. Ethylene glycol is also used in the manufacture of polyester fiber and magnetic film used in tapes for recorders and computers.

To Your Health: Glycols and Human Health

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. As with methanol, its toxicity is due to a metabolite. Liver enzymes oxidize ethylene glycol to oxalate ion.

In the kidneys, the oxalate ion combines with the calcium (Ca2+) ion, precipitating as calcium oxalate (CaC2O4).

Ca2+(aq) + C2O42−(aq) → CaC2O4(s)

These crystals cause renal damage and can lead to kidney failure and death.

Although propylene glycol has physical properties much like those of ethylene glycol, its physiological properties are quite different. Propylene glycol is essentially nontoxic, and it can be used as a solvent for drugs and as a moisturizing agent for foods. Like other alcohols, propylene glycol is oxidized by liver enzymes.

In this case, however, the product is pyruvate ion, a normal intermediate in carbohydrate metabolism. (For more information about metabolic reactions, see Chapter 20 "Energy Metabolism".)

Glycerol, a product of fat metabolism, is essentially nontoxic.

Concept Review Exercises

  1. In the oxidation of propylene glycol to pyruvic acid, what functional groups in the reactant are involved? What new functional groups appear in the product?

  2. Oxalate ion is formed by the oxidation of ethylene glycol. In what kind of reaction is the oxalate ion involved?

Answers

  1. two OH groups; a ketone group and a carboxylic acid group

  2. precipitation

Key Takeaways

  • Glycols are alcohols with two OH groups on adjacent carbon atoms.
  • Glycerol is the most important trihydroxy alcohol.

Exercises

  1. What is a glycol?

  2. Why is ethylene glycol so much more toxic to humans than propylene glycol?

  3. Draw the structure for each compound.

    1. 1,5-pentanediol
    2. propylene glycol

  4. Draw the structure for each compound.

    1. 1,3-hexandiol
    2. glycerol

Answers

  1. an alcohol with two OH groups on adjacent carbon atoms

    1. HOCH2CH2CH2CH2CH2OH