alcohol reactions

Chemistry only

Combustion

Alcohols are flammable; they make excellent fuels. Since alcohols contain only the elements carbon, hydrogen and oxygen and combustion simply involves adding oxygen to the elements present in a compound then combustion of alcohols will release carbon dioxide and water vapour e.g.

Ethanol in a spirit burner.

methanol(l) + oxygen(g) carbon dioxide(g) + hydrogen oxide(g)
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g)
e.g.2
ethanol(l) + oxygen(g) carbon dioxide(g) + hydrogen oxide(g)
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)

The smaller the alcohol molecule the more volatile it will be and so the more flammable it will be.



Solubility

butanol and water do not mix so form two separate layers

Alcohols are generally soluble in water however as the alcohol molecules get larger (chain length increases) the solubility drops. While methanol and ethanol are all miscible in water, that is they can be mixed in any ratio without separating out into two separate phases or layers, as oil and water do. However as the chain length increases the solubility of the alcohol molecule in water begins to fall. Propanol will dissolve in water but butanol is barely soluble in water. If a small amount of butanol is added to water it will dissolve in the water but as more is added then the butanol will begin to form its own separate layer which floats on top of the water. This is shown in the image opposite. Alcohols which do dissolve in water will form neutral solutions with a pH of 7.



Reaction with sodium metal

The reaction of sodium metal with alcohols is very similar to that with water. At first glance this may seem odd as alcohols and water molecules appear to be very different from each other. However all the parts of the alcohols circled in the image below do not take part in any of the chemical reactions of the alcohol molecules; only the -OH group on the alcohol; the functional group takes part in its chemical reactions (other than combustion).

Comparison of the reaction of alcohols and water with metals.

So if you swap all these circles groups and simply replace them with a -R label to represent them, then we have:

Comparison of the reactions of alcohol and water

Remember the reaction of sodium with water:

students working on a chemistry experiment
sodium(s) + water(l) sodium hydroxide (aq) + hydrogen(g)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

Well the hydroxide ion (OH-) in the sodium hydroxide is simply a water molecule (H2O), or H-OH which has lost a hydrogen ion; this lost hydrogen is given off as hydrogen gas during the reaction.

When sodium reacts with alcohols the same reaction occurs. The sodium removes the hydrogen attached to the hydroxyl functional group in the alcohol. The lost hydrogen is given off as hydrogen gas.

2R-OH + 2Na 2RO-Na+ + H2
The RO- ion which forms (compared with the hydroxide ion (OH-) which forms when sodium and water react) is called the alkoxide ion e.g.
sodium(s) + methanol(l) sodium methoxide (aq) + hydrogen(g)
2Na(s) + 2CH3OH(l) 2CH3ONa (aq) + H2(g)

students working on a chemistry experiment now an almost identical equation; but time the alcohol reacting is ethanol:
sodium(s) + ethanol(l) sodium ethoxide (aq) + hydrogen(g)
2Na(s) + 2C2H5OH(l) → 2C2H5ONa(aq) + H2(g)
and finally sodium reacting with the alcohol propanol:

sodium(s) + propanol(l) sodium propoxide (aq) + hydrogen(g)
2Na(s) + 2C3H7OH(l) → 2C3H7ONa(aq) + H2(g)
At first glance these equations may look a bit complicated but if you take the time to look you see that they are actually very straight forward- the alcohol loses its hydrogen from the hydroxyl (-OH) group present in all alcohols and it is replaced by sodium- that’s all there is to it!

Trends in the reactions of alcohols with sodium metal

As mentioned above the structure of an alcohol can be considered in some ways similar to that of a water molecule. However as the chain length of the alcohol grows then the reactions of the alcohol with sodium metal begins to slow down. Methanol (CH3OH) is the "smallest" alcohol and it reacts the fastest of all alcohols with sodium. Ethanol (C2H5OH) the next alcohol reacts more slowly with sodium and propanol the next alcohol in the homologous series reacts even more slowly. This is shown in the image below where the rate of release of hydrogen gas gives an excellent indicator of the speed of the reaction taking place.

Comparing the rate of reaction of sodium  with methanol, ethanol and propanol

Oxidation of alcohols

When a substance is oxidised oxygen is added to it or hydrogen is removed. This is one definition of oxidation that I am sure you are familiar with. If wine which contains the alcohol ethanol is left exposed to air it will go off. The ethanol in the wine will react with oxygen in the air to give it a sour taste. The reason the wine tastes sour is that the ethanol has been oxidised into ethanoic acid; a carboxylic acid. The oxidation of the alcohol ethanol to ethanoic acid is shown in the image below where the oxidising agent is represented by the symbol [O]:

Model, word and symbolic equations to show the oxidation of ethanol to ethanoic acid.

Oxidation of alcohols

Refluxing alcohols with an oxidising agent, apparatus diagram and reagents used.If you study the diagrams above you will see that alcohol ethanol has an oxygen atom added and 2 hydrogen atoms removed from it when it is oxidised to ethanoic acid. The alcohol has been oxidised to a carboxylic acid. The apparatus diagram on the right gives an outline on how this oxidation reaction is carried out in the lab.

The set-up shown opposite where the Liebig condenser is attached directly on top of the pear shaped flask is called reflux. The oxidising agent is a bright orange solid compound called potassium dichromate. Potassium dichromate is a particularly unpleasant compound; it is a well known carcinogen and to make matters worse it is dissolved in dilute sulfuric acid, a corrosive solution, then a few mls of concentrated sulfuric acid is added to the mixture as well! However this acidified potassium dichromate solution is an excellent oxidising agent. The ethanol is simply poured very slowly down the Liebig condenser and into the pear shaped flask containing the oxidising mixture and then heated for around 20 minutes.

The condenser is inserted vertically into the pear shaped flask containing the mixture of chemicals because the ethanol and other intermediate compounds produced are volatile and would simply evaporate out of the flask before the acidified potassium dichromate solution has a chance to oxidise them fully. This way when the volatile substances evaporate, they are cooled and condense inside the Liebig condenser and drip back into the pear shaped flask to be oxidised further.

Colour change when an alcohol is oxidised to a carboxylic acid when dichromate is used.  Ornage to green

The dichromate ion has the formula Cr2O72-, it contains chromium ions with a +6 charge (Cr+6). It is the presence of these ions which gives the dichromate solution its distinctive orange colour. However when the acidified potassium dichromate solution oxidises ethanol then the orange chromate ions (Cr6+) gain 3 electrons and are reduced to form the green chromium(III) ion (Cr3+). This is shown in the image opposite.



Key Points

Practice questions

Check your understanding - Questions on reactions of alcohols

Next