- Alcohols contain an -OH functional group which makes them polar and soluble in water. They can be classified as primary, secondary, or tertiary based on the number of carbon atoms bonded to the carbon attached to the OH group. - Primary alcohols can be oxidized to aldehydes then carboxylic acids using oxidizing agents like acidified dichromate or manganate. Secondary alcohols oxidize to ketones. Tertiary alcohols do not oxidize. - Burning alcohols releases energy with the amount increasing with the alcohol chain length due to more carbon dioxide produced. In limited oxygen, carbon monoxide is formed instead of carbon

1. Alcohols

2. • General formula is CnH2n+1OH.
• Alcohols have an –OH functional group.
• The –OH group is polar and so decreases the
volatility and increases the solubility, in water,
of alcohols, relative to alkanes of comparable
molecular mass.
• The most common alcohol, ethanol C2H5OH,
is readily soluble in water, as we know from its
presence in alcoholic drinks.

3. Combustion of alcohols
• Alcohols are important source of fuel and are
used in alcohol burners and similar heaters.
• The amount of energy released per mole of
alcohol increases as we go up the homologous
series, chiefly due to the increasing number of
carbon dioxide molecules produced.

4. Ex : burning of methanol
• 1:1 ratio of CO2 : alcohol
• ∆H𝑐 𝜃
= -726.1 kJ mol-1

5. Ex : burning of pentanol
• 5:1 ratio of CO2 : alcohol
• ∆H𝑐 𝜃
= -3330.9 kJ mol-1
In the presence of a
limited supply of
oxygen, alcohol will
produce carbon
monoxide instead of
carbon dioxide.

6. Oxidation of
alcohols• Alcohols can be classified as primary,
secondary and tertiary according to the
number of carbon atoms joined to the carbon
attached to the OH group.

7. • Primary and secondary alcohols can be
oxidised using an oxidising agent such as
acidified potassium dichromate (IV)
(K2Cr2O7/H+) or acidified potassium
manganate (VII) (KMnO4/H+).
EXAM TIP :
Don’t forget the
word ‘acidified’
when describing
the oxidising agent.

8. Primary alcohols
• Primary alcohols are oxidised, first to an
aldehyde (partial oxidation).
• Then the aldehyde is oxidised further to a
carboxylic acid (complete oxidation).
Primary alcohol ---------- aldehyde ---------- carboxylic acid
Cr2O7
2-/H+ Cr2O7
2-/H+
Heat Heat

9. Cr2O7
2-/H+ Cr2O7
2-/H+
Heat Heat
Each reaction can be shown in a balanced equation
using [O] to represent oxygen from the oxidising agent:
CH3CH2OH + [O]  CH3CHO + H2O
CH3CHO + [O]  CH3COOH

10. • However, if a fully balanced equation for the
oxidation of an alcohol using acidified
dichromate (IV), is required we must consider
the half-equations involves.
• Cr2O7
2- (orange) is the oxidising agent and is
reduced during the reaction to the Cr3+
(aq) ion
(green).

11. The half-equation of the reduction :
• Oxidation half-equation(in the first reaction) :
Orange Green
CH3CH2OH  CH3CHO + 2H+ + 2e-

12. • The two half equation can be combined by
multiplying the oxidation half-equation by
three to generate 6e- and then adding them
together.
• When H+ ions are cancelled from both sides,
the overall equation obtained is :
Cr2O7
2- + 8H+ +3CH3CH2OH  3CH3CHO + 2Cr3+ + 7H2O

13. The half-equation for the reduction of the
manganate (VII) ions (purple) to colourless
Mn2+
(aq) ions is:
• The half-equation for the oxidation of ethanal
to ethanoic acid is :
MnO4
- + 8H+ + 5e-  Mn2+ + 4H2O
CH3CHO + H2O  CH3COOH + 2H+ + 2e-

14. To get overall redox equation,
• Balance the electrons.
• So the oxidation half-equation is multiplied by
five and the reduction half-equation by two :
2MnO4
- + 5CH3CHO + 6H+  5CH3COOH + 2Mn2+ + 3H2O

15. • If a reaction mixture containing ethanol and
acidified potassium dichromate (VI) is heated
under reflux, ethanoic acid is obtained as the
main product and the aldehyde is not usually
isolated.

16. • However, it is possible to set up the apparatus
so that the aldehyde is distilled of as soon as it
is formed and before it can be oxidised
further.

17. • This technique works because aldehydes have
lower boiling points than the equivalent
alcohols (and all other components of the
reaction mixture) because they do not have
the hydrogen atom attached directly to an
oxygen atom, and therefore there is no
hydrogen bonding between molecules.

18. Secondary
alcohols• Secondary alcohols are also oxidised by
heating with acidified potassium dichromate
(VI) (or acidified manganate (VII)).
• They are oxidised to ketones, which cannot be
oxidised any further.

19. • For example :
Secondary alcohol ketone .
Cr2O7
2-/H+
Heat Heat
Cr2O7
2-/H+
Cr2O7
2-/H+
Heat

20. • The balanced equation for the reaction is :
CH3CH(OH)CH3 + [O]  (CH3)2CO + H2O
Or
Cr2O7
2- + 8H+ + 3CH3CH(OH)CH3  3(CH3)2CO + 2Cr3+ + 7H2O

21. • In terms of changes to the functional group,
the reaction can be represented as :
Cr2O7
2-/H+
Heat

22. • Looking at the reaction of another secondary
alcohol : The only change is to
the group highlighted in
red – the rest of the
molecule is unchanged.

23. Tertiary alcohol
• Tertiary alcohols are resistant to oxidation.
Cr2O7
2-/H+
Heat