C14- Alcohols

Question 1

Combustion of alcohols.

Answer 1

• alcohols burn completely in a plentiful supply of oxygen.
  Alchohol + oxygen = carbon dioxide + water.
• exothermic reaction, so large quantity of heat energy is released.
  As the number of carbon atoms in alcohol chain increases, quantity of heat released per mole also increases.

Question 2

Oxidation of alcohols.

Answer 2

• primary/ secondary alcohols can be oxidised by an oxidising agent.
• usually it is a solution of potassium dichromate (vi)(K2Cr2O7) acidified with dilute sulfuric acid.
• if alcohol is oxidised, orange solution with dichromate (vi) ions is reduced to green solution with chromium (III) ions.

Question 3

Explain the oxidation of primary alcohols.

Answer 3

-can be oxidised to aldehydes or carboxylic acids.

Depends on the reaction conditions used. Aldehydes themselves are also oxidised to carboxylic acids.

Question 4

Preparation of aldehydes from primary alcohols.

Answer 4

• gentle heating of the primary alcohol with acidified potassium dichromate.
• aldehyde is distilled out of the mixture as it forms to prevent further reaction with agent and the formation of a carboxylic acid.
• dichromate ions go from orange to green.

Question 5

Preparation of carboxylic acids from primary alcohols.

Answer 5

If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (vi), carboxylic acids is formed.

• excess acidified potassium dichromate (vi) = ensures all alcohol is oxidised.
• heating under reflux= any aldehyde initially formed undergoes oxidation to create a carboxylic acid.

Question 6

Oxidation of secondary alcohols.

Answer 6

• oxidised to ketones.
• heating under reflux with the oxidising mixture.
• dichromate (vi) ions go from orange to green.

Question 7

Oxidation of tertiary alcohols.

Answer 7

Do not undergo oxidation reactions.
Acidified dichromate (vi) remains orange.

Question 8

Dehydration of alcohols.

Answer 8

Dehydration- removal of water molecule.

• alcohol is heated under reflux in the presence of an acid catalyst.
• alkene produced.
• elimination reaction.

Question 9

Substitution reactions of alcohols.

Answer 9

Alcohol + hydrogen halide = haloalkane.

• alcohol is heated under reflux with sulfuric acid and a sodium halide and hydrogen bromide are produced.
• HBr reacts with the alcohol to produce the haloalkane.

Question 10

Volatility and boiling point.

Answer 10

In the liquid state= alcohols have intermolecular hydrogen bonds holding molecules together.

More energy is needed to overcome these than weak London forces therefore alcohols have a lower volatility than the corresponding alkanes.

Question 11

Solubility in water.

Answer 11

Most alcohols are completely soluble in water as they form hydrogen bonds with water.
As the hydrocarbon chain increases = influence of -OH group gets relatively smaller therefore solubility decreases.

Question 12

Primary alcohols?

Answer 12

OH group is attached to a carbon atom that is attached to 2 hydrogen atoms and 1 alkyl group.

Question 13

Secondary alcohols?

Answer 13

OH group is attached to a carbon atom that is attached to 1 hydrogen atom and 2 alkyl groups.

Question 14

Tertiary alcohols?

Answer 14

OH group is attached to a carbon atom that is attached to 3 alkyl groups

Question 15

What is the alcohol homologous series?

Answer 15

• contain the -OH functional group (hydroxyl group).

- hydroxyl group is responsible for both the physical and chemical properties.

Question 16

How are the physical properties of alcohols different to alkanes?why?

Answer 16

-alcohols are less volatile.
-alcohols have higher mp.
- alcohols have greater water solubility.
However, these differences become smaller as carbon chain length increases.
Because alcohols have a polar O-H bond due to electronegativity difference. The IMF are weak london forces but there are strong hydrogen bonds between O-H groups.
(Alkanes= non-polar)