M4 Alcohols

Question 1

Describe the alcohol homologous series

Answer 1

• Alcohols contain the -OH function group (hydroxyl group), which is responsible for the physical and chemical properties of alcohols.
  eg. methanol, ethanol

Question 2

Describe the physical properties of alcohols

Answer 2

• Compared to alkanes of the same number of carbon atoms, alcohols are less volatile, have higher melting points and greater water solubility than the corresponding alkanes. The differences become smaller as the length of the carbon chain increases.
• These differences can be explained by considering the polarity of bonds in both alcohols and alkanes:
• The alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar.
• The alkane molecules are therefore non-polar.
• The intermolecular forces between non-polar molecules are very weak London forces.
• Alcohols have a polar O–H bond because of difference in electronegativity between the oxygen and the hydrogen atoms.
• Alcohols are therefore polar.
• The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O–H groups.

Question 3

Describe the volatility and boiling points of alcohols

Answer 3

• In a liquid state, intermolecular hydrogen bonds hold alcohols together. These bonds must be broken in order to change the liquid molecule into a gas.
• This requires more energy than overcoming weak London forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms.

Question 4

Describe the solubility of alcohols in water

Answer 4

• A compound that can form hydrogen bonds with water is far more water-soluble than a compound that cannot.
• Alkanes are non-polar molecules and cannot form hydrogen bonds with water.
• Alcohols are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules.
• As the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of the longer-chain alcohols becomes more like that of the hydrocarbons - solubility decreases.

Question 5

What are primary alcohols?

Answer 5

• Methanol and ethanol are both primary alcohols.
• In a primary alcohol, the -OH group is attached to one carbon atom that is attached to two hydrogen atoms and one alkyl group.

Question 6

What are secondary alcohols?

Answer 6

• In a secondary alcohol the -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups.

Question 7

What are tertiary alcohols?

Answer 7

• In a tertiary alcohol the -OH group is attached to no hydrogen atoms and three alkyl groups. Eg. 2-methylpropan-2-ol

Question 8

Describe the combustion of alcohols

Answer 8

• Alcohols burn in a plentiful supply of oxygen to produce carbon dioxide and water.
• The reaction of exothermic, releasing a large quantity of energy in the form of heat. As the number of carbon atoms in the alcohol chain increases, the amount of heat released per mole also increases.

Question 9

Describe the oxidation of alcohols

Answer 9

• Primary and secondary alcohols can be oxidised by an oxidising agent, usually potassium dichromate(VI), acidified with dilute sulphuric acid.
• If the alcohol is oxidised, the orange solution containing dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.

Question 10

Describe the oxidation of primary alcohols.

Answer 10

• Partial oxidation of primary alcohols produces aldehydes.
• Complete oxidation of primary alcohols produces carboxylic acids.
• On gentle heating of primary alcohols with acidified potassium dichromate an aldehyde is formed. To ensure the aldehyde is prepared rather than carboxylic acid, the aldehyde is distilled out as it forms. The dichromate(VI) ions change colour from orange to green.
• If a primary alcohol is heated strongly under reflux, with an excess of potassium dichromate(VI) (to ensure all of the alcohol is oxidised), a carboxylic acid is formed. Heating under reflux also ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid.

Preparing an aldehyde = use distillation to remove aldehyde from reaction mixture
Preparing carboxylic acid = heat alcohol under reflux

Question 11

Describe the oxidation of secondary alcohols

Answer 11

• Secondary alcohols are oxidised to ketones.
• To ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture. The dichromate(VI) ions once again change colour from orange to green.

Question 12

Describe the oxidation of tertiary alcohols

Answer 12

• Tertiary alcohols do not undergo oxidation reactions. The acidified dichromate(VI) remains orange when added to a tertiary alcohol.

Question 13

Describe the dehydration of alcohols

Answer 13

• Dehydration is any reaction in which a water molecule is removed from the starting material.
• An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulfuric or phosphoric acid.
• The product of the reaction is an alkene.
• Dehydration of an alcohol is an example of an elimination reaction.
  eg. cyclohexanol –> cyclohexene + water

Question 14

Describe substitution reactions of alcohols

Answer 14

• Alcohols react with hydrogen halides to form haloalkanes. When preparing a haloalkane, the alcohol is heated under reflux with sulphuric acid and a sodium halide, the hydrogen bromide is formed in situ (in place).
  NaBr + H2SO4 –> NaHSO4 + HBr
• The HBr formed reacts with the alcohol to produce the haloalkane.
• The overall equation for the reaction is
  CH3CHOHCH3 + NaBr + H2SO4 –> CH3CHBrCH3 + NaHSO4 + H2O