A-Level Chemistry AQA Notes

3.3.5 Alcohols

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Alcohol Production

Ethanol is produced industrially by fermentation of glucose using yeast undergoing anaerobic respiration.

C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)

The conditions required are 35⁰C, anaerobic conditions and less than 15% ethanol solution, for optimum enzyme function.

Ethanol can be distilled from the reaction mixture using fractional distillation to increase its purity.
The ethanol produced can be used as a biofuel, which is a fuel produced from renewable sources.

Advantages	Disadvantages
Basic equipment needed The sugar used is renewable Carbon-neutral (in theory) Increased income for farmers	Slow rate of reaction Ethanol not pure Batch process – high production costs Land used to grow crops could be used to grow food

The carbon dioxide that is released upon combustion and fermentation is balanced by the carbon dioxide absorbed during the photosynthesis. (carbon neutral)

Photosynthesis:
6H2O (I) +6CO2 → C6H12O6 (aq) + 6O2 (g)
Fermentation:
C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)
Combustion:
2C2H5OH (aq) + 6O2 (g) → 4CO2 (g) + 6H2O (l)

However, there is CO2 emission during fuel transportation:

Ethene, derived from crude oil, can be used to produce ethanol through hydration in the presence of an acid catalyst

C2H4 (g) + H2O (g) → C2H5OH (g)

Advantages	Disadvantages
Fast rate of reaction Ethanol produced is pure Continuous process – low production costs	Advanced equipment needed Ethene used is non-renewable High pressure means high energy costs

Oxidation of Alcohols

Alcohols are a homologous series of saturated organic compounds with the general formula CnH2n+1OH.

Alcohols can form hydrogen bonds in water, making them soluble.
Primary alcohols can be oxidised to aldehydes. Conditions: gentle heating. Oxidising agent and immediate distillation

Primary alcohols can be further oxidised to carboxylic acids. Conditions: heat under reflux, no distillation, oxidising agent.

Secondary alcohols can be oxidised to ketones. Conditions: heat under reflux and oxidising agent.

Tertiary alcohols cannot be oxidised as they do not have an H atom on the carbon bonded to the -OH group
Acidified potassium dichromate(VI) is a suitable oxidising agent

Reactions of Alcohols

Alcohols can be combusted as a fuel source, forming carbon dioxide and water e.g.
- C2H5OH + 3O2 → 2CO2 + 3H2O
Alcohols can be dehydrated to form an alkene and water, using a concentrated acid catalyst and heating under reflux e.g.

The alkenes produced can be used to form addition polymers without using monomers derived from crude oil.

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