alcohol reactions

Chemistry only

Combustion

Alcohols are flammable, they make excellent fuels. Since alcohols contain on the elements carbon, hydrogen and oxygen, and since 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 spirit burner

methanol(l) + oxygen(g) → carbon dioxide(g) + hydrogen oxide(g)
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g)
e.g2
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 it will be more flammable.

Solubility

Alcohols are generally soluble in water though as the molecules get larger (chain length increases) the solubility drops. Alcohols dissolve to form neutral solutions, pH=7.

Reaction with sodium metal

reaction of alcohol and sodium

The reaction of sodium with alcohols is very similar to that with water. At first glance this may seem odd as alcohol and water appear to be very different. However all the parts of the alcohols circled above do not take part in any of the chemical reactions of the alcohol molecules, only the -OH group, the functional group takes part in its reactions. So if you swap all these circles groups and simply replace them with a -R label to represent them, then we have:

Remember the reaction of sodium with water:

sodium(s) + water(l) → sodium hydroxide (aq) + hydrogen(g)
2Na(s) + 2H2O(l) → 2NaOH (aq) + H2(g)
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.
R-OH + Na → RO-Na+ + H2 (It's not balanced- just to give an indication of what is happening)
The RO- ion which forms (compare 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)

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 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 -OH group 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 alcohols with sodium begin 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 palce.

sodium reacting 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 defintion of oxidati that I am sure you are familar 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 below:

oxidation of ethanol to ethanoic acid

Oxidation of alcohols

refluxing alcohols with an oxidising agentIf you study the diagrams above you will see that ethanol has oxygen added and 2 hydrogen atoms removed 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. The oxidising agent is a bright orange solid compound called potassium dichromate. Potassium dichromat 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. However this acidifie potassium dichromate is an excellent oxidising agent. The ethanol is poured slowly down the Liebig condenser and the mixture 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 drop back into the flask to be oxidised further.

Key Points

Practice questions

Check your understanding - Questions on alcohols

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