4.2.2Organic Chemistry
Ethanol Oxidation — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 38 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 6Oxidation of Primary Alcohols
Primary alcohols are oxidised in two stages by acidified potassium dichromate(VI).
Oxidising agent
- Oxidising agent Acidified K₂Cr₂O₇ in dilute sulfuric acid provides orange dichromate ions that act as the oxidant.
First stage
- First stage CH₃CH₂OH + [O] → CH₃CHO + H₂O, giving ethanal, an aldehyde.
Second stage
- Second stage CH₃CHO + [O] → CH₃COOH, giving ethanoic acid, a carboxylic acid.
Colour change
- Colour change Orange Cr₂O₇²⁻ is reduced to green Cr³⁺, showing that oxidation has occurred.
Part 2 of 6Distillation for Ethanal
Ethanal is removed as it forms so that it cannot be oxidised further.
1
Add acidified potassium dichromate(VI) solution to a pear-shaped flask and warm it gently.
2
Add ethanol drop by drop and allow the mixture to heat so that vapour passes into the distillation apparatus.
3
Ethanal has the lowest boiling point, about 21 °C, so it evaporates immediately and condenses into a cooled receiver.
4
Collect the distillate in a flask held in ice, since ethanal is volatile and easily lost.
5
The colour of the mixture in the flask changes from orange to green as the dichromate is used up.
Part 3 of 6Reflux for Ethanoic Acid
Reflux for Ethanoic Acid
Reflux
- Reflux The mixture is heated with a condenser vertical above the flask so vapours condense and fall back into it.
Complete oxidation
- Complete oxidation Returning the ethanal to the flask lets it oxidise fully to ethanoic acid, boiling at about 118 °C.
Excess oxidant
- Excess oxidant An excess of acidified dichromate(VI) ensures the alcohol is not left as the intermediate aldehyde.
Product isolation
- Product isolation After reflux, distil the mixture and collect the fraction boiling near the acid's boiling point.
Part 4 of 6Yield and Safety
Percentage yield
- Percentage yield Actual mass obtained ÷ theoretical mass × 100, always less than 100% in practice.
Heating method
- Heating method Use an electric heater or hot-water bath because ethanol vapour is flammable.
Hazards
- Hazards Dichromate(VI) is toxic and a carcinogen, so wear gloves and eye protection and avoid skin contact.
Anti-bumping granules
- Anti-bumping granules Granules in the flask let the liquid boil smoothly and avoid sudden violent boiling.
Part 5 of 6Worked Example
Question
- 10.0 cm³ of ethanol (density 0.789 g cm⁻³, M = 46.07 g mol⁻¹) is refluxed with excess oxidant and 6.20 g of ethanoic acid (M = 60.05 g mol⁻¹) is obtained.
- Calculate the percentage yield.
- Mass of ethanol = 10.0 × 0.789 = 7.89 g.
- Moles of ethanol = 7.89 ÷ 46.07 = 0.1713 mol.
- The ratio is 1 : 1, so theoretical moles of ethanoic acid = 0.1713 mol.
- Theoretical mass = 0.1713 × 60.05 = 10.28 g.
- Percentage yield = 6.20 ÷ 10.28 × 100 = 60.3%.
Answer Percentage yield = 60.3%
Part 6 of 6Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Oxidation
- Loss of electrons;
- in organic chemistry usually gain of oxygen or loss of hydrogen.
- Loss of electrons;
- Reflux Heating a mixture with a condenser fitted so vapours return to the flask.
- Distillation Separating liquids by boiling and condensing the vapour of the most volatile one.
- Percentage yield Actual yield divided by theoretical yield, multiplied by 100.
Common Mistakes
- Using reflux when the aim is to collect the aldehyde, which then oxidises to the acid.
- Describing the colour change as orange to blue rather than orange to green.
- Closing the top of the condenser, which creates a sealed system that can explode.
- Writing the product of distillation as ethanoic acid instead of ethanal.
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