Alcohols

Question 1

How do you name alcohols?

Answer 1

1. The suffix -ol is added to the stem name of the longest carbon chain
2. The position of the alcohol functional group in the chain is indicated using a number

Question 2

How are alcohols and alkanes different?

Answer 2

• Although same number of carbon atoms e.g. methane and methanol
  1. Alcohols are less volatile
  2. Alcohols have higher melting points
  4. Greater water solubility than the corresponding alkanes
• The differences become much smaller as the length of the carbon chain increases

Question 3

Why are alkanes and alcohols different?

Answer 3

1. Alkanes have non-polar bonds because the electronegatvitiy of hydrogen and carbon are very similar and so alkanes molecules are therfore non-polar
2. The intermolecular forces between non-polar molecule are very weak London forces
3. Alcohols have a polar O-H bond because of the difference in electronegatvcity of the oxygen and hydrogen atoms. Alcohols are therefore polar
4. The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H bonds

Question 4

Explain the difference in volatility and boiling points

Answer 4

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

Question 5

Explain the difference in solubility in water

Answer 5

1. A compound that can form hydrogen bonds with water is far more water-soluble than a compound that cannot
2. Alkanes are non-polar molecules and cannot form hydrogen bonds with water
3. Alcohols such as methanol and ethanol 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 longer-chain alcohols becomes more like that of hydrocarbons, solubility decreases

Question 6

What does the classification of alcohols depend on?

Answer 6

• Alcohols can be classified as primary, secondary or tertiary
• This classification depends on the number of hydrogen atoms and the alkyl groups attached to the carbon atoms that contains the alcohol functional group

Question 7

Describe a primary alcohol

Answer 7

1. In a primary alcohol the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
2. Methanol with three hydrogen atoms and no carbon atoms attached, is an expectation that is still classified as a primary alcohol
3. Methanol and ethanol, the two simplest alcohols in the alcohols homologous series are both primary alcohols

Question 8

What is a secondary alcohol?

Answer 8

1. In a secondary alcohol the -OH group is attached to a carbon atom that is attached to one hydrogen atoms and two alkyl groups
2. -Propan-2-ol and pentan-3-ol are both examples of secondary alcohols

Question 9

What is a tertiary alcohol?

Answer 9

1. In a tertiary alcohol the -OH group is attached to a carbon atoms that is attached to no hydrogen atoms and three alkyl groups
2. 2-Methylpropan-2-ol and 2-methylbutan-2-ol re both examples of secondary alcohols

Question 10

Describe the combustion of alcohols

Answer 10

1. Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
2. The reaction is exothermic
3. As the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increases

Question 11

What happens in the oxidation of alcohols?

Answer 11

1. Primary and secondary alcohols can be oxidised by an oxidising agent
2. The usual oxidising mixture is a solution of potassium dichromate(VI) k2Cr2O7, acidified with dilute sulphuric acid H2SO4
3. If the alcohol is oxidised the orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (II) ions

Question 12

Describe the oxidation of primary alcohols

Answer 12

1. Primary alcohols can be oxidised to either aldehydes or carboxylic acids
2. The product of the oxidation depends on the reaction conditions used because aldehydes themselves also oxidised to carboxylic acids

Question 13

How do you prepare aldehydes?

Answer 13

1. On gentle heating of a primary alcohols with acidified potassium dichromate, an aldehyde is formed
2. To ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms
3. This prevents any further reaction with the oxidising agent
4. The dichromate (VI) ions change colour from orange to green

Question 14

How do you prepare carboxylic acids?

Answer 14

1. If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (VI), a carboxylic acid is fromed
2. Use of an excess of the acidified potassium dichromate (VI) ensures that all of the alcohol is oxidised
3. Heating under reflux ensures that any aldehyde formed initially in the reaction undergoes oxidation to the carboxylic acid

Question 15

In summary what is the difference between preparing for a carboxylic acid against an aldehyde?

Answer 15

1. When preparing an aldehyde, use distillation to remove the aldehyde form the reaction mixture
2. When preparing the carboxylic acid, heat the acid under reflux

Question 16

Describe the oxidation of secondary alcohols

Answer 16

1. Secondary alcohols are oxidised to ketones
2. It is not possible to further oxidise ketones using acidified dichromate ions
3. To ensure that the reaction goes to completion, the secondary alcohol is heated under reflux with he oxidising mixture
4. The dichromate (VI) ions once again change colour from orange to green

Question 17

What happens when you try to oxidise tertiary alcohols?

Answer 17

1. Tertiary alcohols do not undergo oxidation reaction

2. The acidified dichromate (VI) remains orange when added to a tertiary alcohol

Question 18

What happens in the dehydration of alcohols?

Answer 18

1. Dehydration is nay reaction in which a water molecule is removed form the starting material
2. An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulphuric acid H2SO4 or concentrated phosphoric acid H3PO4
3. The product of the reaction is an alkene
4. Dehydration of an alcohol is an example of an elimination reaction

Question 19

Describe the substitution of alcohols

Answer 19

1. Alcohols react with hydrogen halides to from haloalkanes
2. When preparing a haloalkanes, the alcohol is heated under reflux with sulphuric acid and a sodium halide the hydrogen bromide is formed in situ
3. The HBR reacts wit the alcohol to produce the haloalkane

Question 20

Describe the oxidation reactions of propan-1-ol when using a suitable oxidising agent?

Answer 20

• Reagents: Acid/H* and dichromate/Cr2O7 m
• Observations: Orange to Green/blue
• Distillation produces aldehyde CH3CH2CHO
• CH3CH2CHO + [O] —> CH3CH2CHO + H2O
• Reflux produces carboxylic acid