10b. ESTERS

• What are ESTERS?
  • Carboxylic acids are used to manufacture esters by reacting them with alcohols
  • Carboxylic acids react with alcohols to form members of another homologous series called esters.
  • Concentrated sulphuric acid acts as a catalyst in this reaction.
  • General word equation for esterification:
  • carboxylic acid + alcohol == acid catalyst ==> ester + water
  • e.g.
  • ethanoic acid + ethanol ethyl ethanoate + water
  • + + H₂O
    • sometimes more simply written as
    • CH₃COOH + CH₃CH₂OH CH₃COOCH₂CH₃ + H₂O
    • The reaction is reversible and the mixture reaches equilibrium, about ²/₃rds of the carboxylic acid and alcohol have been converted to the ester.
    • Without a strong acid catalyst e.g. conc. sulfuric acid, the reaction is very slow and the mixture is heated to further increase the rate of reaction (see details of the method below).
  • Structures of other esters made from ethanoic acid:
    • methyl ethanoate using methanol, and ethanoic acid
    • propyl ethanoate from using propanol (propan–1–ol, n–propyl alcohol) and ethanoic acid.
    • Note the arrangement of the atoms at the ester linkage, the functional group -COOC-.
    • The first part of an ester's name is derived from the alcohol e.g. methyl from methanol, ethyl from ethanol and propyl from propanol etc.
      • The second part of the name comes from the carboxylic acid and ends in ...anoate e.g. methanoate from methanoic acid, ethanoate from ethanoic acid and propanoate from propanoic acid etc.
• The procedure for preparing an ester
  • This is illustrated in the diagrams below and a detailed description of the method for preparing ethyl ethanoate is described.
  • Ethyl ethanoate and water are both colourless, but to help in following the procedure via the diagrams, I've coloured the ester yellow and the reaction mixture and aqueous solutions a pale grey.
  • This is a detailed description that will do for Advanced A level chemistry too!
  • Stage 1 in making an ester
  • STAGE 1 Making the ester: In the round-bottomed flask the alcohol (ethanol) is mixed with the carboxylic acid (ethanoic acid) and a small amount of concentrated sulfuric acid (catalyst) is added too. Anti-bumping granules are added to ensure a smooth boiling action. The mixture is carefully heated to get the mixture gently boiling and refluxing.
  • Stage 1 is a technique called 'heating under reflux', and ensures the reaction occurs the fastest at highest possible reaction temperature, the boiling point of the mixture. However, to prevent vapour loss by boiling/evaporation, particularly of the desired product - the ester, the vapourised liquids are condensed back into the reaction flask recycling everything.
    • The diagram shows a bunsen burner being used to supply the heat ('my days'), these days its more likely, and safer, to use an electrical heater that the round bottomed flask fits in snugly.
  • Stage 2 in making an ester
  • STAGE 2 Fractional distillation: The colourless ester liquid is separated from the reaction mixture by fractional distillation which is fully explained on the Elements, Compounds, Mixtures Notes (the example described is separating an ethanol/water mixture, but the same principal applies in separating the ester from some of the water, unreacted alcohol and acid and the sulphuric acid catalyst. Again the mixture gently heated and boiled, but this time you want the vapour of the lowest boiling component (ester) to separate out in the fractionating column and pass through into the condenser. This happens when the temperature at the top of the column reaches the boiling point of the ester. The ester and small quantities of carboxylic acid, sulfuric acid and alcohol can be collected from the condenser in a suitable glass vessel. Preferably a quick-fit one that connects to the condenser, BUT it must not be a completely sealed system otherwise pressure would build up, hence the vent to the sink. You should realise at this point in the preparation that the ester (ethyl ethanoate) is very impure.
  • Stage 3 in making an ester
  • STAGE 3 Removing acidic impurities: The rest of the procedure is all about purifying the initial ester distillate from the fractional distillation. The condensate (liquid distillate) from the fractional distillation apparatus is transferred to a separating funnel (tap funnel). Sodium carbonate solution is added to neutralise any acids and the stopper replaced. The separating funnel is shaken to ensure complete removal of the acid, but carbon dioxide is formed, so every so often you invert the funnel, open the tap and allow the gas to escape. When there doesn't seem to be any more effervescence or gas pressure, the mixture is allowed to settle. When the two layers have fully separated, the stopper is removed, and the lower aqueous layer is careful run off, don't lose any of the ester in the process! When doing the run-off the stopper must be removed. The acidic impurities and any salts formed have now been removed in the aqueous sodium carbonate solution, therefore there should be no carboxylic acid or sulfuric acid catalyst left in the ester layer.
  • Stage 4 in making an ester
  • STAGE 4 Removing the ethanol impurity: However, despite removing some impurities there will still be some traces of the alcohol left in the ester layer. Concentrated calcium chloride solution is added to the still impure ester in the separating funnel and the mixture shaken again. The aqueous calcium chloride will remove any remaining unreacted alcohol (ethanol). Again, the lower aqueous layer is tapped off to leave only the ester layer which will still contain some water.
  • Stage 5 in making an ester
  • STAGE 5 Drying the product: By now the only impurity left is water. So, to dry the ester, it is run off (tapped off) from the separating funnel into a small conical flask and some granules of anhydrous calcium chloride added. The conical flask is stoppered and the mixture shaken, and the calcium chloride absorbs any remaining moisture in the ester. The pure ester can than be filtered off.
  • You can make butyl ethanoate and other esters by the same reaction and procedure.
    • ethanoic acid + butan–1–ol ===> butyl ethanoate + water
    • At GCSE level butan-1-ol might be just written as 'butanol'.
• The yield of ester
  • Its an equilibrium, and starting with the pure acid plus pure alcohol, you heat the mixture in and you get about ²/₃rds conversion* to the ester, and the preparation reaction is catalysed by a few drops of concentrated sulphuric acid.
    • * This means a theoretical maximum reaction yield of about 67%. - in reality, a lot less due to losses in the various steps.
    • For more on % yields and 'atom economy' see Calculations section 14.
• A very simple method of making an ester
  • This is a nice class experiment for GCSE/IGCSE or advanced level students.
  • You can mix equal volumes of small quantities of a carboxylic acid and an alcohol with an even smaller volume of concentrated sulfuric acid.
  • The mixture is gently warmed in beaker of warm water for 5-10 minutes.
  • The mixture is then poured into a beaker of sodium hydrogencarbonate solution.
  • The sodium hydrogencarbonate neutralises the acid catalyst and any unreacted carboxylic acid.
  • You should get some drops of ester left on the surface which can be carefully smelled to appreciate the aroma of the ester.
  • You can do this is as a nice class experiment with ethanoic acid and a variety of alcohols and noting what they think the esters smells like (likely to be 'fruity') alongside appreciating its molecular structure too!
• HYDROLYSIS of esters
  • If an ester is warmed with water or any dilute acid/alkali (faster), it changes back into the original carboxylic acid and alcohol.
  • This reaction is called hydrolysis or saponification i.e.
  • ethyl ethanoate + water ==> ethanoic acid + ethanol
  • whereas esterification is the opposite
  • ethanoic acid + ethanol ==> ethyl ethanoate + water
    • (more on reversible reactions and chemical equilibrium)
  • Soaps are made by hydrolysing esters from naturally occurring plant oils and animal fat
  • Esters are usually sweet/pleasant smelling colourless liquids and occur widely–naturally in plants.
• USES of ESTERS:
  • Esters occur widely in nature
  • Esters are usually sweet/pleasant smelling liquids and widely used as fragrances (components in perfumes) and food flavourings.
  • Perfumes can natural, obtained from plant sources, or artificial, since esters are readily synthesised in the laboratory.
  • Natural substances are used in many cosmetics but many mixtures contain synthetic organic compounds.
    • Many esters have pleasant sweet or fruity smells and the colourless liquids are quite volatile, that's why fruits have strong pleasing odours or aromas.
    • The pleasure of most flavours and fragrances from fruits is due to esters, the vapours from esters definitely entice the receptors in your nose to feel good!
      • How and why do we smell perfumes? (or any other substance)
      • Anything that we smell must have come from substances evaporating, no matter how little of it evaporates, the nose is quite sensitive to low concentrations of many chemicals in air.
      • Therefore, in order to smell a substance, that substance must be to some extent be a volatile material.
      • If a substance isn't volatile, you are highly unlikely to smell it i.e. detect it with your nose.
      • The most volatile materials are those that most easily evaporate e.g. like petrol, how easily a liquid evaporates is referred to as its volatility.
      • The intermolecular forces between molecules are relatively weak in liquids that are volatile, so the particles don't need to much kinetic energy to escape from the surface of your skin.
      • Because of random collisions, the particles in a liquid have a variety of speeds and kinetic energies.
      • Evaporation occurs all the time from volatile liquids, but it is the higher kinetic energy particles that can overcome the attractive forces between the molecules in the bulk of the liquid and escape from the surface into the surrounding air.
      • It is these higher kinetic energy escaped molecules that diffuse through the air to reach the receptor cells in the nostril to trigger the sense of smell.
      • That is why perfume molecules must be quite volatile to work, but they must be not too volatile or their effect won't last very long.
      • On heating particles gain kinetic energy and move faster and are more are able to overcome the intermolecular forces between the molecules, therefore theoretically, perfumes should smell stronger in a warmer room.
    • Because they are volatile and pleasing to the nostril, it makes esters ideal for cosmetic perfumes and cosmetic fragrances in general, but esters are also used in air fresheners e.g. flowery smells like jasmine.
    • Because fruit sources are limited, many esters are now synthesised in large quantities so the flavourings and derived taste and aromas in fruit drinks, sweets and cakes etc. may be from manufactured esters simulating strawberry, pineapple, pear, apple, grape, orange, banana when used as food and drink additives etc.
    • Esters are used in pharmaceutical and household products e.g. ointments, washing–up liquids to give the medications or cleaning products a pleasant odour.
  • Examples of plant ester sources:
    • Lavender oil essence is distilled from the lavender plant
  • Examples of flavouring esters:
    • Pear drop sweet essence is an ester. ester name?
  • Factors affecting perfume design e.g. using esters:
    • Designing a perfume – several issues to address by way of design factors.
    • You can't just use any ester, no matter how beautiful it smells.
    • The chemicals in cosmetic perfumes must have a particular set of properties including ...
      • the chemical components (they) must evaporate easily, otherwise the molecules will never reach your nose, but different evaporation rates are needed by different components to give a prolonged effect,
      • they must not react with water in your sweat forming compounds that might not smell nice, like carboxylic acids, which could be irritating to the skin too,
      • at the same time, they must not be soluble in water or they would be easily washed away,
      • they must be non-toxic and not be absorbed by the skin to cause irritation or poisoning, but you do want them to be absorbed by the skin, BUT harmlessly,
      • they must not irritate the skin, since you are applying the perfume to you skin of the neck or wrists as well as part of your clothing.
    • The perfume needs to be a mixture of compounds to give a prolonged perfumery effect.
    • The perfumer chemist has to design the mixture to give a particular fragrance which includes ...
      • the top note – the first fragrant molecule to be released,
      • and the low note, the last molecule to be vapourised.
  • Cosmetic companies are always developing new products to comply with our aesthetic desires!
    • BUT, every new product must be thoroughly tested before it is ready for the consumer.
    • Unfortunately, this sometimes involves using animals e.g. to monitor their skin response to the new cosmetic formulation.
    • Opinions can be strongly divided and divisive as to the merit and ethics (morality) of using animal testing for new cosmetic formulations (and of course using animals to test new medicines from the pharmaceutical industry).
    • One view is that animal testing is worth it to avoid possible adverse effects on consumers, so any discomfort or worse, suffered by the animals to prevent us suffering in the same way, a sort of 'health and safety' issue argument.
    • The opposing view argues that it is entirely wrong to use animals in tests. 'Animal rights' people argue its unfair to use defenceless animals who cannot speak for themselves AND the results of animal tests are not necessarily conclusive and so unnecessary animal suffering.
    • So, because of concerns about animal welfare in conducting tests of cosmetics on animals, the European Union (EU) has banned almost all of these animal test procedures.
• More on USES OF ESTERS
  • Esters are used as solvents
    • e.g. nail varnish remover (the solvent propanone/acetone is also used), but also in paints, glues and ink formulations as a medium compatible with the other ingredients.
    • Some larger ester molecules are used in plastic formulations as plasticisers which are added to make the polymer more flexible.
    • They aren't totally free of health issues but esters have replaced more harmful aromatic hydrocarbon solvents like benzene (a carcinogen – a cancer promoting chemical) and methylbenzene (old name toluene, also carcinogenic) in paint and varnishing products.
    • BUT take care ...
      • (i) although imparting a pleasant odour, ester fumes can irritate mucous membranes in the nose and mouth,
      • (ii) because they are volatile and combustible, the fumes are highly flammable and easily ignited by a naked flame. The vapour is heavier than air and will not disperse quickly,
      • (iii) some people may be allergic to ester fumes, or indeed their use as food additives.
• Why does a substance dissolve in one liquid solvent but not another?
  • There are three particle interactions going on if you mix one substance with another e.g. a liquid solvent that may or may not dissolve a solid.
  • The three possible attractions are (i) solid ... solid, (ii) solid ... liquid and (iii) liquid ... liquid.
  • The relative strength of these attractive intermolecular forces decides whether e.g. a solid will dissolve in a particular solvent.
  • For example, nail varnish will not dissolve in water, but will dissolve in organic solvents like an ester, alcohol or acetone.
  • Nail varnish is insoluble in water because the intermolecular forces between the nail varnish molecules themselves, and between the water molecules themselves are much stronger than the attraction between water and the nail varnish molecules, so the nail varnish cannot possibly dissolve in water. Forces (i) and (iii) override force (ii)
  • However, nail varnish will dissolve in organic solvents like butyl ethanoate or ethyl ethanoate (esters, old names butyl acetate and ethyl acetate), ethanol ('alcohol') and propanone (old name acetone) solvents. Here the organic solvent intermolecular attraction to the nail varnish molecules can override the nail varnish ... nail varnish and the solvent ... solvent intermolecular forces and the nail varnish will dissolve. In this case attractive force (ii) overrides both attractive forces (i) and (iii).
  • Since different solvents are different molecular affinities for different substances, the solubility of a solute in a solvent can vary quite considerably from one solvent to another.
  • The question of which solvent you choose to use to dissolve a substance depends on two main factors ..
    • (a) How soluble is the substance in the solvent?
    • (b) How safe is to use the solvent? e.g. in terms of inhaling vapour or spillage on the skin (gloves!), is it harmful?, irritating?, even toxic?, and is it highly flammable, so more dangerous to use.
    • Chlorinated organic solvents e.g. trichloromethane ('chloroform') tend to be harmful, alcohols and esters are safer but are more flammable.
    • This section is repeated in alcohols
• Other natural esters - triglycerides
  • Esters from the 'triol' alcohol glycerol , which has three C-O-H groups, is the alcohol plants and animals use to make oils and fats - which are esters we use in food and soaps.
  • Animals and plants combine glycerol and long chain fatty acids to make triglyceride esters - fats from animals and oils from plants.
  • For more details on fatty acids, oil and fat esters see Oils, fats, margarine and soaps

• Polymers - polyesters like Terylene (diagram above)
  • The diagram above shows part of structure of Terylene, a very useful polymer used for making plastic objects and also manufactured as fibres for use in fabrics for the clothing industry.
  • You don't have to know any detailed molecular structure at GCSE/IGCSE level, but I have highlighted the -COOC- ester linkage, which is the same functional group structure as in the 'little' esters described on the page above.
  • The most common use of polyester today is called PET (for short!) and is used to make the plastic bottles for storing liquids in like soft drinks, PET is very useful because it is transparent, shatterproof and cheap!
  • Fine polyester fibres can be made into a variety of articles of clothing which are lighter and cheaper than traditional materials like wool.
  • Plastic bottles made from polyester can recycled and turned into fibres again and reused in clothing.
  • For more see section 11.  Condensation polymers including Terylene

INDEX of Advanced A Level revision notes on the chemistry of CARBOXYLIC ACIDS and DERIVATIVES

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