- The general formula: CnH(2n+1)OH
- The OH group is the hydroxyl group
- The C-O-H angle is 105 degrees.
- Classification of alcohols:
Primary- OH group at the end of a carbon chain (surrounded by1 R group and 2 H atoms)
Secondary- OH group in the body of a chain (surrounded by2 R groups and 1 H atom)
Tertiary- OH group at a branch in the chain (surrounded by 3 R groups)
- Physical properties: High melting point due to hydrogen bonding between molecules. Shorter chain alcohols are soluble in water because of H-bonds between OH groups and water.
Production of Ethanol (crude oil)
- First, crude oil is cracked, producing ethene.
- Ethene is then hydrated, which means that water is added to the double bond
Catalyst- Phosphoric acid absorbed on silica.
Pressure- 6500 kPa
- This gies a conversion of ethanol of only 5%, but the excess ethene is separated and used gain until 95% of the reaction mixture is obtained,
- The reaction is exothermic.
- Advantages: fast process, continuous flow process and produces pure ethanol
- Disadvantages: requires fairly high technology, uses lots of energy and uses non-renewable raw materials (crude oil)
Production of ethanol (fermentation)
- Sugar (glucose) is broken down into ethanol by enzymes present in yeast.
- Carbon dioxide is also another product.
- This is a process called anaerobic respiration.
Temperature: 37 degrees
Absence of air to prevent oxidation of ethanol to ethanoic acid
- Once the fermenting solution contains about 15% of ethanol the enzymes are unable to function and the fermentation stops.
- Advantages: low technology process, does not use much energy, uses renewable raw materials (sugar cane)
- Disadvantages: Batch process, relatively slow process and produces impure ethanol
Uses of ethanol
- alcoholic drinks
- motor fuel
- Carbon dioxide released when something is burnt is balanced by the carbon dioxide absorbed by the plant from which it is originally obtained.
- Ethanol made from fermentation is renewable!!
Reactions of alcohols (combustion and oxidation)
- Produces carbon dioxide and water on complete combustion.
- Primary alcohols are oxidised to aldehydes, secondary alcohols are oxidised to ketones. Aldehydes can further be oxidised to carboxylic acids.
- Tertiary alcohols are not easily oxidised. This is because a C-C bond needs to break, rather than a C-H bond.
- Solution of acidified potassium dichromate is the oxidising agent. Orange dichromate ions turn to green chromium ions.
Oxidising ethanol to ethanal
- Use dilute acid and less potassium dichromate..
- Heat the mixture under reflux