Esterification: Preparing Esters and Identifying Them by Smell

Esterification is the reaction between an alcohol and a carboxylic acid that produces an ester and water. Esters are what give many fruits and flavourings their smell, and the practical use of that is simple: change the alcohol and you change the ester, so you change the smell. In this Grade 12 experiment you make four different esters and identify each one by its smell. It is a prescribed formal assessment for CAPS Physical Sciences, Term 1, and it is marked out of 60.

Where this fits in the curriculum

Subject Physical Sciences
Grade 12
Term 1
Topic Organic molecules, matter and materials
Status Prescribed experiment for formal assessment (SBA)
Marks 60
Time needed 50 minutes

The aim

To produce different esters using a range of carboxylic acids and alcohols, and to identify each ester by its smell.

The chemistry behind it

An alcohol and a carboxylic acid join at their functional groups to form an ester, releasing a molecule of water. Because two molecules combine and a small one is eliminated, this is a condensation reaction.

Concentrated sulphuric acid does two separate jobs, and both are worth marks:

  • It catalyses the reaction, speeding it up
  • It is hygroscopic, so it absorbs the water produced. That shifts the equilibrium towards the ester and improves the yield

The second reason is the better answer. A question asking which property of sulphuric acid makes it suitable is asking about the dehydrating one, and most learners answer 'it is a catalyst', which restates the question.

The four reactions

# Alcohol Acid Ester formed Smells like
1 Ethanol Ethanoic acid Ethyl ethanoate Nail polish remover
2 Methanol Ethanoic acid Methyl ethanoate Paint
3 Propan-1-ol Ethanoic acid Propyl ethanoate Pear
4 Methanol Salicylic acid Methyl salicylate Wintergreen

The first three change the alcohol and keep the acid. The fourth changes the acid instead, which is the point of it. Reaction 4 is also the one that lands with a class, because methyl salicylate is the smell of muscle rub and learners recognise it instantly.

What you need

Apparatus

Item Qty
Test tubes, borosilicate glass 6
Test tube rack 1
Test tube holder 1
Disposable dropping pipettes 1 per chemical
Spatula 1
Beaker, 250 ml, for the water bath 1
Beaker, 100 ml 1
Glass stirring rod 1

Chemicals

  • Ethanoic acid, glacial
  • Methanol
  • Ethanol
  • Propan-1-ol
  • Salicylic acid
  • Concentrated sulphuric acid, as the catalyst
  • Sodium carbonate, for the 0,5 mol·dm⁻³ solution

You supply

A heat source with a tripod and gauze, a heat resistant mat, and water.

Method

Set the water bath up first, then run the same nine steps four times, changing one thing each time.

  1. Pour about 100 cm³ of water into the 250 cm³ beaker. Heat it until it begins to boil, then switch the heat off. The tubes stand in hot water, never over a flame.
  2. Put 1 drop of concentrated sulphuric acid into a test tube.
  3. Add 10 drops of the carboxylic acid to the same tube.
  4. Add 10 drops of the alcohol. Swirl gently to mix.
  5. Stand the tube upright in the hot water for one minute. If the mixture boils, lift it out with the holder until the boiling stops, then lower it back.
  6. Take the tube out and let it cool.
  7. Put 10 cm³ of 0,5 mol·dm⁻³ sodium carbonate solution into a second test tube. Pour the cooled mixture into it. It will fizz. Mix well.
  8. A layer of ester separates and floats on top. Waft the air above the tube towards you and identify the smell.
  9. Repeat for the remaining three combinations in the table above. Reaction 4 uses a solid acid, so use a spatula tip of salicylic acid, about 0,5 g, in place of the 10 drops.

Why sodium carbonate and not water

The fizzing at step 7 is carbon dioxide. The sodium carbonate is neutralising the leftover carboxylic acid, and that is exactly why it is used instead of plain water. It takes the sharp acid smell away so the ester underneath can actually be identified. Pour the mixture into water instead and the acid smell tends to mask the ester.

The solution is made up from solid sodium carbonate: 53 g dissolved in 1 litre of water gives 0,5 mol·dm⁻³. Make it up before the lesson.

What you should see

A thin oily layer floating on top of the sodium carbonate solution, and a clear smell that is nothing like what went into the tube. None of the four smells is subtle. If a learner cannot place one, something has gone wrong rather than their nose being at fault.

Conclusion

Four different esters were prepared, three by reacting the same carboxylic acid with three different alcohols and the fourth by changing the acid, and each was identified by its characteristic smell.

The point being made is that the alcohol and the acid together determine the ester, and the ester determines the smell. Change either one and the smell changes. That is the whole finding, and a conclusion that only restates the aim without saying what was found will not earn full marks.

Safety precautions

Concentrated sulphuric acid and glacial ethanoic acid are corrosive and burn skin on contact. Lab coat and goggles on before anything is opened. If either spills on skin, rinse under a strongly running tap with plenty of water and call for help. Flood eye contact for at least 20 minutes and get medical help.

Methanol is toxic, not merely flammable. Keep the room ventilated and never let anyone smell it straight from the bottle.

  • The heat goes off before the tubes go in. All three alcohols and the ethanoic acid are flammable, so nothing organic goes near an open flame
  • Waft, never sniff. Cup a hand over the tube and draw the air towards your nose. Nobody puts their face over a tube
  • Use the test tube holder. The tubes come out of hot water
  • One pipette per chemical. Moving a pipette between bottles contaminates the bottle for every group after it
  • Spilt acid on the bench gets neutralised with sodium bicarbonate, then mopped with plenty of water

If it does not work

What you see What caused it
No smell, or only the smell of the acid Not left long enough in the hot water, or the sulphuric acid was left out. One drop is small but it is not optional
Still smells sharply of vinegar Not enough sodium carbonate, or it was not mixed. The leftover acid has to be neutralised before the ester comes through
No fizzing at step 7 The sodium carbonate solution is too dilute, or it is not sodium carbonate. Check it was made up at 53 g per litre
Nothing floating on the surface Very little ester formed. Check the catalyst actually went in
The mixture boils hard The heat was left on under the beaker. It should be off before the tubes go in
Two reactions smell the same A pipette was moved between bottles, or a tube was reused without washing
Reaction 4 gives nothing Salicylic acid is slower than the others. Give it two minutes rather than one

How the 60 marks are made up

Section Marks
Experiment 1, ethanol and ethanoic acid 16
Experiment 2, methanol and ethanoic acid 16
Experiment 3, propan-1-ol and ethanoic acid 16
Experiment 4, methanol and salicylic acid 2
Discussion of results, four questions 10
Total 60

Each 16 mark block covers the smell, the word equation, the structural formula, the chemical equation and the balanced chemical equation.

Learners lose most marks on the structural formulae, not on the practical work, and 12 of the 60 marks sit there. They draw the alcohol and the acid correctly and then get the ester linkage wrong, usually by leaving out the double bonded oxygen. Worth doing one on the board before they start.

A stronger variant, if your school has butanoic acid

Butanoic acid in place of ethanoic acid gives a much bigger effect. With methanol it gives methyl butanoate, which smells of apple, and with ethanol it gives ethyl butanoate, which smells of pineapple.

The catch is that butanoic acid itself smells revolting, like rancid butter, and the smell lingers in a room. That contrast is the point: something that smells that bad turning into something that smells of apple demonstrates what a reaction does better than any explanation.

It is a good extension and some printed versions of this practical use it as the main method. It is not what the assessment memorandum is built around, so run the four reactions above for the formal mark and keep this one for a demonstration.

Disposal

Dilute everything heavily with running water and send it down the drain. The quantities are tiny and the sodium carbonate has already neutralised the acid.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 60 marks, with the four reaction tables and the discussion questions set out for the formal assessment
  • Marking memorandum, with the mark allocation, model answers, the balanced equations and a note on the five places learners most often drop marks

Buy this experiment

We are putting together a complete Esterification Kit for Grade 12 with the glassware, the seven chemicals, the safety data sheets and a printed teacher guide in one box, plus an annual refill of the consumables. Both coming shortly.

In the meantime every item in the apparatus table above is in stock and sold separately.

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