Key Terminology
Term Definition
Biofuel A fuel derived immediately from living matter
Carbon Neutrality Net zero CO2 emissions by balancing carbon emissions with carbon removal
Carbonyl C=O group
Primary Alcohol Alpha carbon has one R group; -OH group is at the end of the chain
Secondary Alcohol Alpha carbon has two R groups; -OH group is in the body of the chain
Tertiary Alcohol Alpha carbon has three R groups; -OH group is at a branch in the chain
Producing Ethanol
In industry, ethanol is produced by hydration of ethene, using a concentrated phosphoric acid catalyst:
CH2=CH2 + H2O → CH3CH2OH. This occurs at 7MPa and 300°C, producing a pure product.
Ethanol can also be produced by fermentation of glucose. This is a slow reaction but uses renewable
sugars from sugar cane as a source. This reaction occurs at 35°C – any higher would denature the
enzymes used, at 1atm, with low yield, as yeast catalyses the reaction in anaerobic conditions. Ethanol
is then separated by fractional distillation and can then be used as a biofuel.
Biofuels are carbon neutral, as any carbon dioxide released upon combustion was absorbed as the plan
photosynthesised:
• 6H2O + 6CO2 → C6H12O6 + 6O2 : 6 moles of CO2 used
• C6H12O6 → 2C2H5OH + 2CO2 : 2 moles of CO2 released
• 2C2H5OH + 6O2 → 4CO2 + 6H2O: 4 moles of CO2 released
therefore no net CO2 is produced. However, since biofueld production uses fertiliser, distillation and
transportation methods, all of which may involve fossil fuel use, the overall fuel is not carbon neutral.
Since the fuel is renewable and releases far less CO2 than fossil fuels, the benefits do outweigh the
drawbacks of this system.
Reactions of Alcohols
Elimination
Ethanol can be dehydrated to give ethane by heating under reflux with an excess of concentrated
sulphuric acid at 170°C, over an aluminium oxide catalyst.
Alkenes produced by this method can be used to produce addition polymers without using monomers
derived from crude oil, and are therefore more environmentally friendly.
Oxidation
Alcohols can be oxidised using oxidising agents such as acidified potassium dichromate
(VI), which contains Cr2O7
2-
ions. A colour change of orange to green is seen when the
oxidising agent is reduced.
Primary alcohols are oxidised in two stages. Oxidation removes to H atoms to produce
an aldehyde, which can then be oxidised further to a carboxylic acid.
1. RCH2OH + [O] → RCHO + H2O
To favour aldehyde production, carry out the reaction at rtp. Heat then distil off
the product to prevent further oxidation. Heat below 78°C and cool the
collection vessel. Apparatus setup is shown under CP5
2. RCHO + [O] → RCOOH
To favour carboxylic acid production, heat under reflux – apparatus setup is
shown on the right
Secondary alcohols are oxidised to ketones: RCH(OH)R’ + [O] → RCOR’ + H2O
Tertiary alcohols do not undergo oxidation as 2 hydrogen atoms cannot be removed from the alpha
carbon.
