Alcohols

Outline the mechanism that produces ethanol

Describe fermentation process to produce ethanol

C6H12O6 → 2C2H5OH + 2CO2

Anaerobic conditions /37/ yeast

Oxygen is kept out to prevent oxidation of ethanol to ethanoic acid

Why is fermentation carried out at 37 rather than 25

• Advantage: ethanol is produced at a faster rate

• Disadvantage: more energy is used

Pure ethanol is obtained from industrial fermentation via fractional distillation to separate water and ethanol as they have different boiling points

Compare the advantages and disadvantages of producing ethanol via fermentation and hydration

Hydration of ethene:

• ethene is from crude oil-finite resource so non-renewable

• Low cost/ high set up but low labour costs

• Quality of ethanol produced is pure

• Fast rate of reaction and continuous process

  Fermentation:

• Uses sugar-renewable source

• High cost/low wet up but high labour costs

• Quality of ethanol is impure-requires fd

• Slow rate of reaction and produced in batches

Is ethanol produces from fermentation a carbon neutral biofuel?

• carbon neutral-no overall co2 emission into the atmosphere

  1. Photosynthesis 6CO2+ 6H20→ C6H12O6+ 6O2

  2. fermentation C6H12O6 → 2C2H5OH + 2CO2

  3. Combustion 6CO2 + 2C2H5OH → 4CO2+6H20

     • 6 moles of carbon dioxide taken in during photosynthesis

     • 6 moles of carbon dioxide taken out due to 2 moles of of carbon dioxide from fermentation and 4 moles of carbon dioxide from combustion

     • Carbon dioxide is released during planting,harvesting and transport as it burns fossil fuels that releases CO2

Describe how primary alcohol can be oxidised to an aldehyde

Primary alcohols can be oxidised to aldehydes:

General equation: RCH2OH + [O] → RCHO+ H2

Reagents: acidified potassium dichromate (VI) solution (K2Cr2O7/H2SO4)

Conditions: Acidified and distillation

Describe how primary alcohol can be oxidised to carboxylic acid

General equation: RCH2OH + 2[O] → RCOOH + H2O

Reagents: excess acidified potassium dichromate (VI) solution (K2Cr2O7/H2SO4)

Conditions: acidified, heat under reflux and excess reagent

Describe how secondary alcohols can be oxidised to a ketone

General equation: R2CHOH +2[O] → RCOR+ H2O

Reagents: acidified potassium dichromate (VI) solution (K2Cr2O7/H2SO4)

Conditions: acidified, heat under reflux

What things should we do when heating under reflux

• don’t seal the top of the condenser → prevent pressure build up

• Condenser must have an outer tube for water that is sealed at the top and bottom

• Condenser must have two openings water in at the bottom and out at the top→ more efficient cooling and prevents solvent being lost

• Anti-bumping granules added to distillation and reflux → produces small even bubbles so water boils smoother

Describe the tests to distinguish between aldehyde and ketone and describe the test for carboxylic acids

Tollen’s reagent

• colourless solution forms a silver mirror if aldehyde is present

• No visible change if ketone is present

Fehlings solution

• Blue solution forms a brick red precipitate if aldehyde is present

• No visible change if ketone is present

  Test for carboxylic acid:

• Reagent: solid Na2CO3

• Effervescence/gas is evolved if present → test with limewater(cloudy)

Describe the mechanism that produces alkenes from alcohols

Elimination reaction

• reagent: concentrated sulfuric acid or concentrated phosphoric acid

• Conditions: heat/warm + concentrated

  Acid→ catalyst

Outline the mechanism that forms ethene from ethanol