2.3.7 Chemical Properties of Alkohol

Combustion of Alcohol

1. As the hydrocarbon, alcohols undergo combustion with the presence of oxygen to produce carbon dioxide and water.
2. Combustion of alcohol produces less soot compares to combustion of alkanes and alkenes due to the presence of oxygen in the –OH group.
3. Owing to the presence of the –OH group, the percentage of carbon in alcohol is relatively low when compared with the percentages of carbon of alkanes and alkenes.

Equation:
Combustion of Ethanol

Combustion of Propanol

Reaction of Alcohols with Sodium

1. We have learned the reaction of group 1 metals with cold water in Form 4 chapter 4, Periodic Table.
2. The reaction of an alcohol with sodium is similar to this reaction.
3. If a small piece of sodium is dropped into some ethanol, it reacts steadily to give off bubbles of hydrogen gas and leaves a colourless solution of sodium ethoxide, CH₃CH₂ONa.
   Sodium ethoxide is known as an alkoxide.
4. In the reaction, sodium reacts with the -OH group of alcohol produces sodium alkoxide and hydrogen gas.

Example:
Methanol and Sodium

Ethanol and Sodium

Dehydration of Alcohol

1. When alcohol is heated, it will decompose to form alkene and water. 
2. For example, heating ethanol will produce ethene, heating propanol will produce propene, and so on. 
3. This process is called dehydration of alcohol.

Dehydration of Alcohols Using Aluminium Oxide as Catalyst

Catalyst:
Aluminium oxide/ Porcelain chips

Dehydration of alcohols using an acid catalyst

Catalyst:
Sulphuric acid or Phosphoric acid

Temperature: 170°C

Oxidation of Alcohol

Alcohols can be oxidised to produce carboxylic acid when reacting with oxidising agents.
Equation:

Notes:

1. The oxidising agent used:
1. Acidified potassium dichromate(VI)
2. Acidified potassium manganate (VII)
2. Observation:
1. For acidified potassium dichromate(VI), if oxidation occurs, the orange solution containing the dichromate(VI) ions are reduced to a green solution containing chromium(III) ions.
2. For acidified potassium manganate (VII), the purple colour of potassium manganate (VII) decolourised.
3. The alcohol is heated under reflux with an excess oxidising agent.
4. The reflux technique used to prevent the alcohol vapour escape to the surrounding.
5. When the reaction is complete, the carboxylic acid is distilled off.