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Doc Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

10. Carboxylic acids and esters

Carboxylic acids are a group of organic compounds which have weakly acidic properties. The react with bases/alkalis to for salts and release carbon dioxide from carbonates.  What are esters? What are esters used for? What connects the chemists laboratory and the perfume industry! Carboxylic acids react with alcohols to form organic compounds called esters which are used as solvents and components in perfumes and food flavourings.

Index of KS4 Science GCSE/IGCSE Chemistry Oil & Organic Chemistry Pages: 1. Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3. ALKANES – saturated hydrocarbons and combustion : 4. Pollution, carbon monoxide, nitrogen oxides, what makes a good fuel?, climate change–global warming : 5. Alkenes – unsaturated hydrocarbons : 6. Cracking – a problem of supply and demand, other products : 7. Polymers, plastics, uses and problems : 8. Introduction to Organic Chemistry – Why so many series of organic compounds? : 9. Alcohols – Ethanol – properties, reactions, biofuels : 10. Carboxylic acids and esters : 11. Condensation polymers, Nylon & Terylene, comparing thermoplastics, fibres and thermosets : 12. Natural Molecules – carbohydrates – sugars – starch : 13. Amino acids, proteins, enzymes & chromatography : 14. Oils, fats, margarine and soaps : 15. Vitamins, drugs–analgesic medicines & food additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free radicals : 17. Extra notes, ideas and links on Global Warming and Climate Change : Multiple Choice and Gap–Fill Quizzes: m/c QUIZ on Oil Products (GCSE/IGCSE easier–foundation–level) : m/c QUIZ on Oil Products (GCSE/IGCSE harder–higher–level) : IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry : and 3 Easy linked GCSE/IGCSE Oil Products word–fill worksheets

10. CARBOXYLIC ACIDS and ESTERS – other families of organic compounds

We find members of the homologous series called carboxylic acids in fruits and in vinegar and many in fragrances and food additives as carboxylic acid derivatives called esters.

  • The molecular structure and uses of carboxylic acids
  • Carboxylic acids form another homologous series and have the functional group –COOH.
  • The structures of the first three members are given below.
  • The last alcohol structure given below is the full displayed formula which you should definitely know, but you also need to know the various abbreviated ways of writing the molecular structure of alcohols.
  • Names end in ...oic acid (actually here they all end in ..anoic acid).
  • Vinegar contains ethanoic acid (old name 'acetic acid')
    • Ethanoic acid can be manufactured on a large scale by oxidising the alcohol ethanol.
    • See in section 9 Oxidation of the alcohol ethanol
    • It is used as a food preservative and in food flavourings.
    • It is the oxidation of ethanol to ethanoic acid that results in alcoholic drinks turning sour (e.g. cider, wine) when exposed to air.
    • The fruit material already contains the enzymes that catalyse the oxidation of ethanol ('alcohol') in the presence of air.
    • ethanol + oxygen ==> ethanoic acid + water
    • CH3CH2OH + O2 ==> CH3COOH + H2
    • + O2 ==>  + H2O
  • Ethanoic acid is used in the manufacture of the fibre, acetate rayon.
  • Citrus fruits like oranges and lemons and many soft drinks contain the tri–carboxylic acid citric acid.CITRIC ACID and contribute to the 'tarter' or 'sour' taste of fruit. The molecule contains three acidic carboxylic acid groups –COOH.
    • Citric acid is a natural preservative (E330 on food labels) and is found in the largest quantities in oranges, lemons, limes and grapefruit. It is an anti–oxidant. Metal salts from citric acid, i.e. citrates, are used in dietary supplements to deliver trace metal minerals in a biologically available/absorbable chemical form.
    • Citric acid can be used in baking powder to react with sodium bicarbonate giving the raising action from carbon dioxide gas formation. The same combination can be used to give the fizzy drink effect in medicines like ant–acid stomach powders.
    • Citric acid is made in large quantities for the food industry, including fizzy drinks - which can be quite acid - dental concerns about the health of teeth here!
    • Citric acid is strong enough to be used in some limescale removers - so think about your teeth when consuming all those fizzy drinks we might like!
  • Aspirin is a carboxylic acid. Aspirin is a drug used for pain relief and is taken regularly by those at risk from heart attacks (see also Drugs).
  • Ascorbic acid (vitamin C) is another carboxylic acid and is present in fresh fruit and vegetables and is vital for good health AND the body cannot synthesise it, so you must eat fruit and vegetables regularly!
    • A lack of vitamin C can cause the disease scurvy. The symptoms of scurvy are skin sores, spongy gums and bleeding from mucous membranes. This is one example of malnutrition diseases caused by a vitamin deficiency in a diet.
  • Long chain carboxylic acids, known as 'fatty acids', are used to make soaps and detergents.
    • Below are some diagrams of the organic molecules or ions involved
    • Diagram S1: The stearic acid molecule C17H35COOH or CH3(CH2)16COOH is a typical long chain fatty acid obtained from naturally occurring plant oils and used to make traditional soaps.
    • Diagram S2: The salt sodium stearate C17H35COONa+, formed when stearic acid is neutralised with sodium hydroxide is a typical soap molecule.
  • Carboxylic acids are weak acids, typically solutions are around pH 2 to 6 (yellow–orange–pink with universal indicator).
    • Advanced Chemistry Page Index and LinksThey are called weak acids because only a few % of the molecules in aqueous ionise to release protons (hydrogen ions, H+).
      • It is the presence of hydrogen ions that makes aqueous solutions of carboxylic acids acid.
      • e.g. for ethanoic acid (vinegar)
      • 98% CH3COOH(aq) (c) doc b CH3COO(aq) + H+(aq)
      • This is a reversible reaction with only 2% of the weak acid ionised on the right-hand side of the equilibrium.
  • Carboxylic acids react with metals and are neutralised by bases (insoluble or soluble – alkalis and carbonates) to form salts ... with examples of neutralisation ...
    • metals react to form salts and hydrogen e.g.
      • ethanoic acid + magnesium ==> magnesium ethanoate + hydrogen
      • 2CH3COOH + Mg ==> (CH3COO)2Mg + H2
    • alkalis (soluble bases) react to form a carboxylic acid salt and water  e.g.
      • ethanoic acid + sodium hydroxide ==> sodium ethanoate + water
      • CH3COOH + NaOH ==> CH3COONa + H2O
      • and
      • ethanoic acid + potassium hydroxide ==> potassium ethanoate + water
      • CH3COOH + KOH ==> CH3COOK + H2O
    • insoluble bases dissolve and react to form salt and water e.g.
      • zinc oxide + ethanoic acid ==> zinc ethanoate + water
      • ZnO + 2CH3COOH ==> (CH3COO)2Zn + H2O
    • carbonate and hydrogencarbonate bases to produce a carboxylic acid salt, water and carbon dioxide  e.g.
      • ethanoic acid + sodium hydrogen carbonate ==> sodium ethanoate + water + carbon dioxide
      • CH3COOH + NaHCO3 ==> CH3COONa + H2O + CO2
      • and
      • ethanoic acid + sodium carbonate ==> sodium ethanoate + water + carbon dioxide
      • 2CH3COOH + Na2CO3 ==> 2CH3COONa + H2O + CO2
      • and
      • propanoic acid + sodium carbonate ==> sodium propanoate + water + carbon dioxide
      • 2CH3CH2COOH + Na2CO3 ==> 2CH3CH2COONa + H2O + CO2
    • aqueous ammonia solution forms ammonium salts e.g.
      • methanoic acid + ammonia ==> ammonium methanoate
        • HCOOH + NH3 ==> HCOONH4
      • ethanoic acid + ammonia ==> ammonium ethanoate
        • CH3COOH + NH3 ==> CH3COONH4
      • Strictly speaking, ammonium hydroxide doesn't really exist, but in older texts you will find these reactions written in this way, but NOT correct e.g.
        • propanoic acid + ammonium hydroxide ==> ammonium propanoate + water
        • CH3CH2COOH + NH4OH ==> CH3CH2COONH4 + H2O
  • Advanced Chemistry Page Index and LinksESTERS: Carboxylic acids are used to manufacture esters.
    • 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: carboxylic acid + alcohol ==> ester + water
    • ethanoic acid + ethanol ethyl ethanoate + water
    • + + H2O
      • sometimes more simply written as
      • CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O
    • Structures of other esters made from ethanoic acid:
      • (c) doc bmethyl ethanoate using methanol, and
      • (c) doc b propyl ethanoate from using propanol (propan–1–ol, n–propyl alcohol).
      • 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 ...oate e.g. methanoate from methanoic acid, ethanoate from ethanoic acid and propanoate from propanoic acid etc.
    • The procedure for preparing an ester are illustrated in the diagram below.
    • This technique is 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, the vapourised liquids are condensed back into the reaction flask.
      • 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.
    • The colourless ester liquid is separated and purified from the reaction mixture by fractional distillation which is fully explained on the Elements, Compounds, Mixtures Notes (the example described is separating an ethanol/alcohol mixture, but the same principal applies in separating the ester from the water, unreacted alcohol and acid and the sulphuric acid catalyst.
    • You can make butyl ethanoate by the reaction:
      • ethanoic acid + butan–1–ol ==> butyl ethanoate + water
    • 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%.
      • For more on % yields and 'atom economy' see Calculations section 14.
    • If the ester is warmed with water or any dilute acid (faster), it changes back into the original acid and alcohol. This reverse reaction is called hydrolysis i.e.
    • Esters are usually sweet/pleasant smelling colourless liquids and occur widely–naturally in plants.
  • Esters occur widely in nature and are usually sweet/pleasant smelling liquids and widely used as fragrances (components in perfumes) and food flavourings.
    • Natural substances are used in many cosmetics but many mixtures contain synthetic organic compounds.
      • Many esters have sweet or fruity smells and the colourless liquids are quite volatile, that's why fruits have strong pleasant odours or aromas.
      • Because they are volatile makes them ideal for perfumes and fragrances in general.
      • 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.
      • 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.
    • Esters are used as solvents e.g. nail varnish remover, but also in paints, glues and ink formulations.
      • 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.

Advanced Chemistry Page Index and Links

 

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier–foundation–level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder–higher–level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

ALSO gap–fill ('word–fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap–fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes


keywords equations: CH3COOH + CH3CH2OH ==> CH3COOCH2CH3 + H2O * CH3CH2OH + O2 ==> CH3COOH + H2O * 2C2H5OH + 2Na ==> 2C2H5ONa + H2 * CH3CH2OH ===> CH2=CH2 + H2O *


Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences Chemistry revision notes for GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising AQA GCSE Science AQA GCSE Chemistry, Edexcel GCSE Science, Edexcel GCSE Chemistry, OCR 21st Century Science Chemistry, OCR Gateway Science chemistry, WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10)

Advanced Chemistry Page Index and Links
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