10a. The names and molecular structure of CARBOXYLIC ACIDS

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.

Carboxylic acids

The lower displayed formulae are a more accurate representation of the structure and shape of the carboxylic acid molecules

The full displayed formula for the first five members of the homologous series of CARBOXYLIC ACIDS

These diagrams show ALL the covalent bonds (C-H, C-C, C-O, C=O and O-H) in carboxylic acids

They can also be written in shorthand as:

HCOOH,  CH₃COOH,  CH₃CH₂COOH, CH₃CH₂CH₂COOH and CH₃CH₂CH₂CH₂COOH

• The molecular structure of carboxylic acids
• Carboxylic acids form another homologous series and have the functional group –COOH.
  • A homologous series is a family of compounds which have the same general formula and have a similar molecular structure and similar chemical properties because they have the same functional group of atoms (e.g. in this case the carboxylic acid -COOH group).
  • The homologous series of carboxylic acids have the general formula C_nH_2n+1COOH
    • where n = 0, 1, 2 etc. and equals the number of carbon atoms in the molecule minus 1.
    • Note when n = 0 you simply get HCOOH methanoic acid.
  • The functional group (COOH) is a group atoms common to all members of a homologous series that confer a particular set of characteristic chemical reactions on each member of the series e.g. in this series they all behave like acids with alkalis, carbonates etc., they all form compounds called esters when reacted with alcohols.
  • Members of a  homologous series have similar physical properties such as appearance, melting/boiling points, solubility etc. BUT show trends in them e.g. steady increase in melting/boiling point with increase in carbon number or molecular mass.
  • It is important to realise that members of a given homologous series like carboxylic acids have similar chemical reactions because their molecules contain the same functional group (COOH) and so you can predict the chemical reactions and products of the other members of the carboxylic series
• The structures of the first five members are shown above and the first  three members of the carboxylic acid homologous series are also shown 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.
• Carboxylic acid names end in ...oic acid (actually here they all end in ..anoic acid), and they start with meth, eth, prop etc. depending on the length of the carbon chain.
  • and check out their formula from the general formula above, the final structure is the full displayed formula showing every covalent bond in the molecule (C–H, C=O, C–O, O–H, C–C) and note the arrangement of atoms in the COOH functional group, common to all the members of the homologous series of carboxylic acids. The first three are as follows ...
  • methanoic acid (old name 'formic acid')
    • or or 1st in the series
  • ethanoic acid (old name 'acetic acid', in vinegar)
    • C₂H₅COOH or or or 2nd in the series
  • propanoic acid (old name 'propionic acid')
    • or or 3rd in the series etc.

ADVANCED A Level revision notes on the structure and naming of CARBOXYLIC ACIDS and DERIVATIVES, including nomenclature of isomers (NOT needed for GCSE/IGCSE students!)

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10b. The physical properties of carboxylic acids and some of their uses

• The properties and uses of carboxylic acids
  • This particular homologous series of carboxylic acids are colourless liquids with very strong odours (pungent smell) and tastes.
  • The obnoxious smell of rancid butter and sweaty socks are due to the formation of carboxylic acids.
  • It should be mentioned here that carboxylic acids with long carbon chains are called fatty acids, because they are combined with an alcohol called glycerol to form large ester molecules (glycerides/triesters) that make up most natural oils from plants and fat in animals.
    • Many traditional soaps are made from natural oils and fats.
    • The fatty acids may be saturated (no C=C double bonds) or unsaturated (with at least one C=C double bond) in the carbon chain.
    • For more details on glycerides and soaps see Oils, fats, margarine and soaps
• Vinegar contains ethanoic acid (old name 'acetic acid')
  • In the chemical industry 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 + H₂O 
  • + O₂ ====>  + H₂O
  • This reaction occurs if wine, beer or cider is left out open to the air, it eventually becomes vinegar and not very nice to drink!, but vinegar is very nice in salad dressings and on your fish and chips.
• 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. and contribute to the 'tarter' or 'sour' taste of fruit. The molecule contains three acidic carboxylic acid groups –COOH (molecular structure on the right).

  • 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, originally derived from plant oils.
  • Below are some diagrams of the organic molecules or ions involved, typically with 16 to 20 carbon atoms in the chain.
  • 
  • 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 C₁₇H₃₅COO^–Na⁺, formed when stearic acid is neutralised with sodium hydroxide is a typical soap molecule.
    • For more details on fatty acids, oils and fats see Oils, fats, margarine and soaps

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10c. The 'acidic' reactions of carboxylic acids

Reactions with metals, oxides, hydroxides, carbonates and ammonia

• Carboxylic acids are weak acids
• The solubility of carboxylic acids in water decreases as the size of the molecule increases.
• They give all the normal reactions you expect of a carboxylic acid.
• Methanoic, ethanoic and propanoic acids are soluble in water, as are other carboxylic acids found in nature e.g. citric acid and ascorbic acid (Vitamin C) are both citrus in fruits.
• Typically weak acid solutions have a pH of around 2 to 6 (yellow–orange–pink with universal indicator), which is somewhat higher than strong acid solutions with a pH of 0 to 2.
• They 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) around 98% remains unionised i.e. as the original neutral molecule and only ~2% form ethanoate ions and hydrogen ions..
  • CH₃COOH(aq) CH₃COO^–(aq) + H⁺(aq)
    • Propanoic acid and butanoic acid are equally weak carboxylic acids.
    • CH₃CH₂COOH(aq) CH₃CH₂COO^–(aq) + H⁺(aq)
    • CH₃CH₂CH₂COOH(aq) CH₃CH₂CH₂COO^–(aq) + H⁺(aq)
  • This is a reversible reaction with only 2% of the weak acid ionised on the right–hand side of the equilibrium.
    • At similar concentrations, strong acids have a low pH of 0 or 1, because they are more fully ionised.
    • Ethanoic acid will turn blue litmus pink and universal indicator gives an orangey–red colour.
    • Being a weak acid, the pH is higher than in strong mineral acids like hydrochloric acid and the rate of reaction is slowed down too.
    • For a more detailed discussion of these points see The theory of acids and bases
• Despite being a weak acid, blue litmus turns red and carboxylic acids like ethanoic acid behave like any other acid and react with metals, alkalis and carbonate to form salts and fizzing here and there!

• Carboxylic acids react with metals and are neutralised by bases/alkalis
  • This includes insoluble or soluble metal oxides, hydroxides and carbonates, to form salts.
  • The salt names depends on the name of the acid, but the end of the name is ... oate.
  • So aqueous solutions of methanoic acid form methanoate salts, ethanoic acid gives ethanoate salts, propanoic acid gives propanoate salts and butanoic acid gives butanoate salts on neutralisation.
  • The salts can be crystallised from the solution by evaporation.
  • Examples of carboxylic acid salt formation:
  • Metals react and dissolve to form a salts and hydrogen e.g.
    • ethanoic acid + magnesium ==> magnesium ethanoate + hydrogen
    • 2CH₃COOH + Mg ====> (CH₃COO)₂Mg + H₂
    • butanoic acid + zinc ====> zinc butanoate + hydrogen
    • 2CH₃CH₂CH₂COOH + Zn ====> (CH₃CH₂CH₂COO)₂Zn + H₂
    • Not on the syllabus, but an interesting tragic story of 'old acetic acid' and lack of appreciation of chemical hazards.
    • Ethanoic acid very slowly reacts with lead to form lead(II) ethanoate (old name lead acetate), once called 'sugar of lead'
    • 2CH₃COOH + Pb ====> (CH₃COO)₂Pb + H₂
    • The salt formed was called 'sugar of lead' because it had a sweet taste!
    • Cider makers in the past had dipped rods of lead into cider to neutralise any acetic acid that had formed and sweeten the beverage.
    • Unfortunately, lead is one of many heavy metals and that are highly toxic and lead compounds affect the brain and nervous systems and can be fatal.
    • Cases of lead poisoning have occurred through millennia, including the Romans, by using lead pots in food preparation or concentrating liquids.
    • So, any cider left over that goes sour, dispose of it or let turn completely into cider vinegar for the kitchen!
  • 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
    • ethanoic acid + potassium hydroxide ===> potassium ethanoate + water
    • CH₃COOH + KOH ====> CH₃COOK + H₂O
    • propanoic acid + sodium hydroxide ===> sodium propanoate + water
    • CH₃CH₂COOH + NaOH ====> CH₃CH₂COONa + H₂O
  • Insoluble bases dissolve and react to form salt and water e.g.
    • zinc oxide + ethanoic acid ====> zinc ethanoate + water
    • 2CH₃COOH + ZnO ====> (CH₃COO)₂Zn + H₂O
    • ethanoic acid + calcium hydroxide ====> calcium ethanoate + water
    • 2CH₃COOH + Ca(OH)₂ ====> (CH₃COO)₂Ca + 2H₂O
    • ethanoic acid + magnesium hydroxide ====> magnesium ethanoate + water
    • 2CH₃COOH + Mg(OH)₂ ====> (CH₃COO)₂Mg + 2H₂O
    • butanoic acid + magnesium hydroxide ====> magnesium butanoate + water
    • 2CH₃CH₂CH₂COOH + Mg(OH)₂ ====> (CH₃CH₂CH₂COO)₂Mg + 2H₂O
  • Carbonate and hydrogencarbonate bases produce a carboxylic acid salt, water and carbon dioxide gas
    • This is a simple test for a carboxylic acid - effervescence observed, follow up by test the gas with limewater for CO₂ which gives a white milky precipitate
    •  e.g.
    • ethanoic acid + sodium hydrogen carbonate ==> sodium ethanoate + water + carbon dioxide
    • CH₃COOH + NaHCO₃ ====> CH₃COONa + H₂O + CO₂
    • ethanoic acid + sodium carbonate ====> sodium ethanoate + water + carbon dioxide
    • 2CH₃COOH + Na₂CO₃ ====> 2CH₃COONa + H₂O + CO₂
    • propanoic acid + sodium carbonate ====> sodium propanoate + water + carbon dioxide
    • 2CH₃CH₂COOH + Na₂CO₃ ====> 2CH₃CH₂COONa + H₂O + CO₂
    • ethanoic acid + magnesium carbonate ==> magnesium ethanoate + water + carbon dioxide
    • 2CH₃COOH + MgCO₃  ====> (CH₃COO)₂Mg + H₂O + CO₂
    • ethanoic acid + calcium carbonate ==> calcium ethanoate + water + carbon dioxide
    • 2CH₃COOH + CaCO₃  ====> (CH₃COO)₂Ca + 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

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10d. The reaction of carboxylic acids with alcohols to form esters

• ESTERS:
  • 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
    • A whole variety of esters can be made using an alcohol + carboxylic acid + a strong acid catalyst.
  • Detailed notes on esters are on a separate page now, and they are a very important group of compounds and more than merit their own page!

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10e. The structure of amino acids, polypeptides and proteins

• AMINO ACIDS
  • Amino acids have two functional groups.
  • The carboxylic acid group -COOH and the amino or amine group -NH₂
  • The simplest one is aminoethanoic acid (glycine) H₂N-CH₂-COOH or
  • Amino acids can undergo condensation polymerisation via the two functional groups to form peptides, and all sorts of combinations of amino acids produce the huge variety of proteins found in living systems (see diagram below).
  • For glycine the condensation polymerisation to give a glycine peptide can be shown as ..
  • n H₂N-CH₂-COOH  ====>  -(NH-CH₂-COO-)_n-  +  n H₂O
• 
  • Diagram showing the formation of peptide linkage in polypeptide proteins from amino acids by elimination of water
  • where n is a very large number, with the elimination of n water molecules.
  • R is variable, as in proteins, so you can get quite complicated sequences of amino acids in these VERY important natural polymers.
  • See Amino acids, proteins, polypeptides, enzymes & chromatography for more details

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

Multiple Choice Quizzes and Worksheets

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