SPM Chemistry

Chemical Properties of Alkohol

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Chemical Properties of Alcohol

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

C2H5OH + 3O2 → 2CO2 + 3H2O

Combustion of Propanol

C3H7OH + 9/2 O2 → 3CO2 + 4H2O
Reaction of Alcohols with Sodium

Revision:
Reaction of Water with Sodium

2Na + 2H2O → 2NaOH + H2
  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

2CH3OH + 2Na → 2CH3ONa + H2

Ethanol and Sodium

2C2H5OH + 2Na → 2C2H5ONa + H2
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 Ethanol 
C2H5OH → C2H4 + H2O

Dehydration of Propanol
C3H7OH → C3H6 + H2O

    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:

    C2H5OH + 2[O] → CH3COOH + H2O

    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.