Topic 14

Question 1

How do alcohols compare to alkanes?

Answer 1

alcohols are less volatile, have higher melting points and greater water solubility than alkanes

Question 2

Are alcohols soluble in water?

Answer 2

• Alcohols have a polar O-H bond due to the large difference in electronegativity
• Their intermolecular forces would be weak London forces but their will be stronger hydrogen bonds between the O-H groups
• alcohols have lower votality than alkanes with the same number of carbon atoms
• Alkanes are non polar therefore are insoluble in water, but alcohols are soluble in water since they can form hydrogen bonds 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 smaller

Question 3

How can we classify alcohols?

Answer 3

• methanol and ethanol are primary alcohols

- this means the -OH group is attached to a carbon atom that is attached to 2 hydrogen atoms and one alkyl group

Question 4

How can we oxidise primary alcohols?

Answer 4

• can be oxidised using oxidising agent
• Oxidising mixture contains a solution of potassium, dichromate (VI) K2Cr2O7 acidified with dilute sulfuric acid H2SO4
• If the alcohol is oxidised, the orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (III) ions

Question 5

How can we oxidise a primary alcohol to form an aldhyde?

Answer 5

• primary alcohols heated gently with acidified potassium dichromate to produce an aldhyde which is then distilled out
• dichromate ions change colour from orange to green

Question 6

How can a primary alcohol be oxidised to produce a carboxylic acid?

Answer 6

• heated strongly under reflux with an excess of potassium dichromate

Question 7

What do secondary alcohols oxidise to form?

Answer 7

• ketones to further oxidise ketones using acidified dichromate VI ions

Question 8

What do tertiary alcohols oxidise to form?

Answer 8

• remain orange, cannot oxidise

Question 9

What is the dehydration reaction of an alcohol?

Answer 9

• water molecule is removed
• alcohol heated under reflux in the presence of concentrated phosphoric acid catalyst
• the production is an alkene