[14.2] reactions of alcohols

Question 1

combustion of alcohols

Answer 1

• alcohols burn completely in a plentiful supply of oxygen in an exothermic reaction
• as the number of carbon atoms in the alcohol increases, the quantity of heat released per mole also increases

Question 2

oxidation of alcohols

Answer 2

• some alcohols can be oxidised by an oxidising agent such as potassium dichromate (VI), Kr₂Cr₂O₇, acidified with dilute sulfuric acid
• if the alcohol is oxidised, the orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (III) ions
  > Cr₂O₇²⁻ -> Cr³⁺

Question 3

how easily primary alcohols are oxidised?

Answer 3

easily oxidised to aldehydes and then carboxylic acids

Question 4

how easily are secondary alcohols oxidised?

Answer 4

easily oxidised to ketones

Question 5

how easily are tertiary alcohols oxidised?

Answer 5

• not oxidised under normal conditions
• do break down with very vigorous oxidation

Question 6

oxidation of primary alcohols: forming aldehyde

Answer 6

alcohol + [O] —> aldehyde + water

• Kr₂Cr₂O₇ / H₂SO₄
• distillation

Question 7

oxidation of primary alcohols: forming carboxylic acid

Answer 7

alcohol + 2[O] —> carboxylic acid + water OR

aldehyde + [O] —> carboxylic acid + water

• Kr₂Cr₂O₇ / H₂SO₄
• reflux

Question 8

oxidation of secondary alcohols

Answer 8

alcohol + [O] —> ketone + water

• Kr₂Cr₂O₇ / H₂SO₄
• reflux

Question 9

what is tollen’s reagent and fehling’s solution used for?

Answer 9

• oxidises aldehyde to carboxylic acid
• simple test used to distinguish aldehydes from ketones

Question 10

dehydration of alcohols: what type of reaction it is, conditions and what it forms

Answer 10

• water molecule is removed from starting material which is an elimination reaction
• alcohol is heated under reflux with an acid catalyst such as concentrated H₂SO₄ or H₂PO₄
• forms alkenes
• no need to learn the mechanism

Question 11

substitution reaction of alcohols: what type of reaction it is, conditions and what it forms

Answer 11

• substitution with halide ions
• with sulfuric acid or sodium halide (catalyst)
• forms haloalkane
• no need to learn the mechanism

Question 12

hydration of ethene: reagents, conditions and equation

Answer 12

• reagents - ethene (from cracking of fractions from distilled crude oil)
• conditions - phosphoric acid catalyst, high temperature and pressure
• equation - C₂H₄ + H₂O —> C₂H₅OH