Alcohol

General FormulaCnH2n+1OH, n = 1, 2, 3, …….

Functional Group:

Hydroxyl

First 3 Members:

Chemical Properties:

1.      Alcohols are compounds in which one or more hydrogen atoms in an alkane have been replaced by an -OH group.

Naming Alcohols

  1. All alcohol members have a suffix ‘anol’. For example, methanol, ethanol, propanol ……
  2. The nomenclature of alcohol, like alkanes, is based on the number of carbon atoms in the molecule, but the suffix ‘ane’ in the nomenclature of alkanes is replaced with the suffix ‘anol’.
  3. The positions of the alkyl and hydroxyl groups that the attached to the carbon chain are shown by numbering the carbon atoms. The hydroxyl group must always get the smallest possible number.
Example 1
(Propan-2-ol)
Longest chain: 3 carbon (propanol) Position of hydroxyl group: 2 Alkyl:Example 2
(Pentan-1-ol)
Longest chain: 5 carbon (pentanol) Position of hydroxyl group: 1 Alkyl: –
Example 3
(2-methylpropan-2-ol)
Longest chain: 3 carbon (propanol) Position of hydroxyl group: 2 Alkyl: Methyl at 2nd carbon Example 4
(3-methylbutan-1-ol)
Longest chain: 4 carbon (butanol) Position of hydroxyl group: 1 Alkyl: Methyl at 3rd carbon Example 5
(2,2-dimethylpropan-1-ol)
Longest chain: 3 carbon (propanol) Position of hydroxyl group: 1 Alkyl: 2 Methyls, both at 2nd carbon

Isomerism of Alcohol

Isomerism of alcohol is because of:
  1. the position of hydroxyl group
  2. the presence of branch (alkyl group) in the molecule

Example
Isomers of propanol



Isomers of butanol

Physical Properties Alcohol

Boiling Points

  1. The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.
  2. This is because, the presence of the hydroxyl group gives extra force between the molecules, hence more heat energy is needed to overcome the intermolecular force when boiling.
  3. The boiling points of the alcohols increase as the number of carbon atoms increases.
  4. This can be explained by the following:
    1. the number of carbon increases, the size of the molecule increases.
    2. intermolecular force increases.
    3. more heat energy is needed to overcome the intermolecular force.
The solubility of Alcohols in Water
  1. The small alcohols are entirely soluble in water.
  2. However, solubility falls as the length of the hydrocarbon chain in the alcohol increases.
  3. Explanation:
    1. Every alcohol consists of two parts, the hydrocarbon chain which is not soluble in water, and the hydroxyl functional group, which is soluble in water.
    2. In short chain alcohol, the hydroxyl group plays a significant part in forming forces (hydrogen bond) in between water and alcohol molecule, therefore they are soluble in water.
    3. For the long chain alcohol, the hydroxyl group is just a small part of the molecule. Therefore, the insoluble long hydrocarbon chain contribute more to the forces between water and alcohol molecule, causes the solubility

Preparing Alcohol

Alcohols can be prepared by using 2 methods
  1. Hydration of alkene
  2. Fermentation

Hydration of Alkene

  1. When we discuss the chemical properties of alkenes, we have learned that, when alkenes undergo addition reaction with steam, alcohol will be produced. (see hydration of alkenes).
  2. Equations below shows the hydration of ethene and propene to produce ethanol and propanol respectively.

Hydration of Ethene

    C2H4 + H2O → C2H5OH

    Hydration of Propene

    C2H4 + H2O → C2H5OH
    Fermentation
      1. This method only applies to ethanol.
      2. Fermentation is the chemical process which microorganism such as yeast act on carbohydrate to produce ethanol and carbon dioxide.
      3. When yeast is added to sugar (example: glucose), fermentation occurs. During fermentation, the yeast produce an enzyme called zymase. The zymase enzyme breaks down the glucose molecules to simpler molecule, namely ethanol and carbon dioxide. \[{C_6}{H_{12}}{O_6}\xrightarrow{{yeast}}2{C_2}{H_5}OH + 2C{O_2}\]
      4. Yeast is killed by ethanol concentrations in excess of about 15%, and that limits the purity of the ethanol that can be produced.

    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.

      Uses of Alcohols

      Alcohol can be used as
      1. beverage
      2. fuel
      3. solvent
      4. Antiseptic