4.2.1 Alcohols

Question 1

Why are alcohols polar?

Answer 1

The -OH- functional group has a polar bond due to difference in electronegativity of oxygen and hydrogen atoms.
Therefore alcohol molecules are polar.

Question 2

Why are alcohols less volatile and have higher boiling points than their corresponding alkanes?

Answer 2

Hydrogen bonds between alcohol molecules must be broken which takes more energy than overcoming weaker London forces in alkanes meaning lower volatility and higher boiling point.

Question 3

Why do alcohols have a higher water solubility than corresponding alkanes.

Answer 3

Alkanes are non-polar molecules and can’t form H bonds with water, alcohols can form H bonds between polar OH group and water molecule.
(Solubility of alcohols decreases as chain length increases)

Question 4

Definition of a primary alcohol.

Answer 4

OH groups attached to a carbon atom that’s attached to one other carbon atom.

Question 5

Definition of a secondary alcohol.

Answer 5

OH group is attached to carbon atom that’s attached to two other carbon atoms.

Question 6

Definition of a tertiary alcohol.

Answer 6

OH group attached to carbon atom that’s attached to 3 other carbon atoms.

Question 7

Definition of a tertiary alcohol.

Answer 7

OH group attached to carbon atom that’s attached to 3 other carbons.

Question 8

Describe the combustion of alcohols.

Answer 8

Alcohols burn completely in plentiful supply of oxygen to produce CO2 and H2O.
Exothermic reaction, heat energy released (amount increases with chain length).

Question 9

Oxidising agent for oxidation of primary and secondary alcohols.

Answer 9

Potassium dichromate (VI) acidified with dilute sulphuric acid.
K2Cr2O7 and H2SO4

Question 10

Colour change that occurs if alcohol is oxidised and ions involved.

Answer 10

Orange solution reduced to green solution.
Dichromate (VI) ions (Cr2O72-) to chromium (III) ions (Cr3+).

Question 11

How is an aldehyde formed from a primary alcohol?

Answer 11

Gentle heating with acidified potassium dichromate and distillation.

Question 12

How is a carboxylic acid formed from a primary alcohol?

Answer 12

Heated strongly under reflux with excess acidified potassium dichromate.

Question 13

What are secondary alcohols oxidised to?

Answer 13

Ketones.

Question 14

How are secondary alcohols oxidised?

Answer 14

Heated under reflux with acidified potassium dichromate to form a ketone.

Question 15

Describe oxidation with tertiary alcohols.

Answer 15

Tertiary alcohols don’t undergo oxidation reactions.

Question 16

What is a dehydration reaction?

Answer 16

Any reaction in which a water molecule is removed from the starting material.

Question 17

Conditions of dehydration of an alcohol.

Answer 17

Heated under reflux in presence of an acid catalyst (conc. sulphuric acid/ conc. phosphoric acid).

Question 18

What is produced during dehydration of an alcohol.

Answer 18

An alkene and water.

Question 19

Conditions of a substitution reaction with alcohols.

Answer 19

Heated under reflux with sulfuric acid and a sodium halide.

Question 20

Products of a substitution reaction with alcohols.

Answer 20

Haloalkane and water.