making alcohols

Chemistry only

fermentation

The word alcohol is most commonly associated with alcoholic drinks, unfortunately most alcohols are toxic and are definitely not suitable as drinks. The one alcohol which is less harmful is ethanol, although it still causes many people to do very silly things when taken in excess! Ethanol has been made for thousands of years as an alcoholic drink in a process called fermentation. As an example consider how beer is made.

To make beer, barley or sometimes oat grains are allowed to start germinating by being placed in water. After a few days natuarlly occurring enzymes break down the starch present in the grains into the sugar maltose. This process is called malting. The germinated barley grains are then dried by hot air . These dried grains are often called malt. Hops are then added to give the beer its bitter taste and aroma.

The barley grains are then lightly crushed in a process called milling and finally the grains are transferred to large tanks filled with hot water where the starch in the barley grains is converted into simple sugars by natuarlly occuring enzymes present in the barley. These simple sugars dissolve in the water to form a solution which is often called "sweet wort". This sweet wort is then transferred to copper vessels where it is boiled. The left over grains, often called brewers grains are sold as animal or cattle feed.

Once the sweet wort solution has cooled, it is transferred into large fermenatation tanks where the yeast is added. Yeast is a natural fungus which will produce an enzyme called zymase which will convert the simple sugars in the solution into carbon dioxide gas and the alcohol ethanol. This is fermentation.

The fermentation process is slow and can take days or even weeks. Fermentation is a natural process, it is a form of anaerobic respiration. An equation for fermentation is:

glucose(aq) ethanol(aq) + carbon dioxide(g)
C6H12O6(g) C2H5OH(aq) + 2CO2(g)

fermentation of glucose

Making alcohol in the lab

Making alcohol using yeast in the lab is very simple. All you need is a source of sugar, it could be for example any sweet Sfruit e.g. apples, pears, strawberries, blackberries, grapes. The fruit is squashed, added to lukewarm water and placed in a container with a lid or air lock somewhere hot, like an airing cupboard or beside a radiator. Once the yeast is added fermentation will begin and the sugars present from the squashed friut will start to change into the alcohol ethanol. After about 2 weeks filter the mixture to remove the yeast and any other solids present. That is it.... Easy, you have made ethanol!

Yeast is a fungus, a living organism that will produces enzymes that convert the sugar glucose into ethanol, this process is a form of anaerobic respiration. That is respiration without oxygen present, this anaerobic process works best around 370C - body temperature. If the fermenting solution gets too hot then the yeast will be killed and fermentation will stop. If the solution is kept too cold then obviously fermentation will be very slow. Air also needs to be excluded since bacteria and oxygen present in the air will oxidise the ethanol produced into ethanoic acid (vinegar).

One of the main uses for alcohol is as a sterlising agent to kill bacteria and fungi, for example hand sanitisers often contain a high percentage of the alcohol ethanol, which will kill any bacteria present. So how will this fact influence what we know about fermentation? Well once the concentration of alcohol in the fermenting container reaches around 10-15% fermentation stops- this is because the alcohol has killed the yeast. For alcoholic spirits such as whisky or vodka which may contain up to 40% alcohol, the alcoholic solution from fermentation needs to be distilled.

Direct hydration

Alcohol (ethanol) used in alcoholic drinks such as beer, wine and spirits is made by fermentation. However alcohol can also be made a method called direct hydration. Using this method produces alcohol very quickly and being the only substance made there is no waste and no costs are needed in separating out unwanted substance. It is also a continuous process whereas traditional fermenatation is a batch process. This makes direct hydration much more cost effective and more efficient, however is a non-renewable process since the ethene needed for it comes from crude oil.

Alcohol used for industrial purposes such as solvents and in sanitisers will be made by the direct hydration method rather than fermentation. Direct hydration involves adding steam across the C=C in an alkene molecule, usually ethene. The reaction is shown below. A catalyst (phosphoric acid) and a high temperature (3000C) and high pressure (65 atmospheres) are both needed to get this reaction to occur smoothly.

direct hydration

Equations for this reaction are:

ethene(g) + steam(g) ethanol(g)
C2H4(g) + H2O(g) C2H5OH(g)

Key Points

Practice questions

Check your understanding - Questions on fermentation.

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