alcohol reactions

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

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 in the image below 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.

reaction of alcohol and sodium

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)

Well the hydroxide ion (OH-) in the sodium hydroxide is simply a water molecule (H2O), or H-OH which has lost a hydrogen, 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 (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

Key Points

Practice questions

Check your understanding - Questions on reactions of alcohols

Next