Alcohols

• 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.

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.

Alcohol + oxygen carbon dioxide + water + energy

Ex : burning of methanol

• 1:1 ratio of CO2 : alcohol• H = -726.1 kJ mol-1

2 CH3OH(l) + 3 O2 (g) 2 CO2 (g) + 4 H2O (g)

Ex : burning of pentanol

• 5:1 ratio of CO2 : alcohol• H = -3330.9 kJ mol-1

2 C5H11OH (g) + 15 O2 (g) 10 CO2 (g) + 12 H2O (g)

In the presence of a limited supply of oxygen, alcohol will produce carbon monoxide instead of carbon dioxide.

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.

• 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.

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

Cr2O72-/H+ Cr2O7

2-/H+

Heat Heat

Cr2O72-/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 + H2OCH3CHO + [O] CH3COOH

• 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.

• Cr2O72- (orange) is the oxidising agent and is

reduced during the reaction to the Cr3+ (aq) ion

(green).

The half-equation of the reduction :

• Oxidation half-equation(in the first reaction) :

Cr2O72- + 14 H+ + 6e- 2Cr3+ + 7H2O

Orange Green

CH3CH2OH CH3CHO + 2H+ + 2e-

• 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 :

Cr2O72- + 8H+ +3CH3CH2OH 3CH3CHO + 2Cr3+ + 7H2O

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-

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

• 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.

• 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.

• 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.

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.

• For example :

Secondary alcohol ketone .

Cr2O72-/H+

Heat Heat

Cr2O72-/H+

Cr2O72-/H+

Heat

• The balanced equation for the reaction is :CH3CH(OH)CH3 + [O] (CH3)2CO + H2OOr

Cr2O72- + 8H+ + 3CH3CH(OH)CH3 3(CH3)2CO + 2Cr3+ + 7H2O

• In terms of changes to the functional group, the reaction can be represented as :

Cr2O72-/H+

Heat

• 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.

Tertiary alcohol • Tertiary alcohols are resistant to oxidation.

Cr2O72-/H+

Heat

Bye