The alcohol family
Alcohols are carbon compounds that contain the hydroxyl group, –OH. The best-known member is ethanol, formula C₂H₅OH, the alcohol found in hand sanitiser and in alcoholic drinks. Ethanol is a colourless liquid that mixes completely with water, is volatile (evaporates easily) and is flammable.
Making ethanol by fermentation
Fermentation uses the enzymes in yeast to change sugar into ethanol and carbon dioxide, with no oxygen:
Key idea
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
(glucose → ethanol + carbon dioxide, catalysed by yeast at about 37°C)
Fermentation works best in warm conditions around 30–40°C. It stops when the alcohol becomes too concentrated, because the yeast is killed, so distillation is used afterwards to separate and concentrate the ethanol using the difference in boiling points.
Properties and uses
- Burns in air with a clean blue flame to give carbon dioxide and water, releasing heat – so it is used as a fuel (gasohol).
- A good solvent for perfumes, inks and medicines.
- An antiseptic that kills microorganisms on the skin.
- Can be oxidised to ethanoic acid (the acid in vinegar) by air or oxidising agents.
Ethanol dissolves in water in any proportion because its –OH group forms hydrogen bonds with water molecules. Industrially, large amounts of ethanol are also made by adding steam to ethene using a catalyst, a faster method than fermentation. Because it is renewable when made from crops, ethanol is blended with petrol to make gasohol, a cleaner motor fuel used in several countries.
Example
When tapai is made from fermented rice or tapioca, yeast turns the sugars into a little ethanol, giving the sweet-alcoholic taste.
Remember
All alcohols carry the –OH group. Drinking too much ethanol harms the liver and slows the nervous system.