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4.7Unit 4. Rates, Equilibria and Further Organic Chemistry

Organic Chemistry: Carboxylic Acids — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

5 min read 13 parts Lesson 7 of 8 1 quiz

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4.7 Organic Chemistry: Carboxylic Acids — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Explain the properties of carboxylic acids, describe their preparation and reactions, compare acyl chlorides and esters, and explain ester hydrolysis and polyester formation.Carboxylic acids — contai…

  • Weak acids
  • Hydrogen bonding
  • Solubility
  • Oxidation of primary alcohols and aldehydes
  • Hydrolysis of nitriles
  • With carbonates and hydrogencarbonates

Section 1 of 13Structure and naming

Structure and naming
Carboxylic acids
  • Carboxylic acids contain the carboxyl group –COOH, with general formula CnH2n+1COOH or RCOOH
  • Names follow alkan + oic acid, such as propanoic acid.
  • No number is needed, because the –COOH carbon is always carbon 1.
Weak acids
  • Weak acids only slightly ionised in water: RCOOH ⇌ RCOO− + H+
  • The equilibrium lies well to the left.
  • The carboxylate ion is stabilised by delocalisation of charge over both oxygen atoms.

Section 2 of 13Physical properties

Hydrogen bonding
  • Hydrogen bonding both the C=O and O–H groups are polarised, and the O–H allows hydrogen bonds
  • Boiling points are higher than those of alcohols of similar Mr, such as ethanoic acid (118 °C) and propan-1-ol (97 °C).
  • In the pure liquid and in non-polar solvents the acids form hydrogen-bonded dimers.
Solubility
  • Solubility short-chain acids (up to about C4) mix with water by hydrogen bonding to it
  • Solubility falls as the hydrocarbon chain lengthens, because the chain disrupts hydrogen bonds between water molecules.
  • Acids with more than eight carbons are solids and only very slightly soluble in cold water.

Section 3 of 13Preparation of carboxylic acids

Preparation of carboxylic acids
Oxidation of primary alcohols and aldehydes
  • Oxidation of primary alcohols and aldehydes heat under reflux with acidified K2Cr2O7 or acidified KMnO4
  • Orange Cr2O72− is reduced to green Cr3+.
  • Purple MnO4− is reduced to colourless Mn2+.
  • CH3CH2OH + 2[O] → CH3COOH + H2O.
Hydrolysis of nitriles
  • Hydrolysis of nitriles the –CN group at the end of the chain becomes –COOH
  • Dilute acid gives the carboxylic acid and an ammonium salt: RCN + 2H2O + H+ → RCOOH + NH4+.
  • Dilute alkali gives the sodium carboxylate and ammonia, which then needs acidifying to form the acid.
Reflux
  • Reflux keeps volatile intermediates in the flask, and the acid is then distilled off and tested with sodium hydrogencarbonate

Section 4 of 13Reactions of carboxylic acids

Reactions of carboxylic acids
With carbonates and hydrogencarbonates
  • With carbonates and hydrogencarbonates give a salt, water and carbon dioxide, which fizzes
  • 2RCOOH + Na2CO3 → 2RCOONa + CO2 + H2O.
  • RCOOH + NaHCO3 → RCOONa + CO2 + H2O.
  • This is a useful test for a carboxylic acid.
With metals, oxides and alkalis
  • With metals, oxides and alkalis give a salt
  • 2CH3COOH + MgO → (CH3COO)2Mg + H2O.
  • CH3COOH + KOH → CH3COOK + H2O.
With LiAlH4 in dry ether
  • With LiAlH4 in dry ether reduced to a primary alcohol
  • CH3CH2COOH + 4[H] → CH3CH2CH2OH + H2O.
  • Water is added at the end to destroy excess LiAlH4.
With PCl5
  • With PCl5 forms an acyl chloride
  • CH3CH2COOH + PCl5 → CH3CH2COCl + POCl3 + HCl.
  • Steamy fumes of HCl are seen and the liquids are separated by fractional distillation.
With alcohols
  • With alcohols form esters by esterification, catalysed by concentrated H2SO4
  • Esters contain –COOR and have sweet, fruity smells.
  • The first part of the name comes from the alcohol, so propanol and ethanoic acid give propyl ethanoate.

Section 5 of 13Acyl chlorides and acid anhydrides

Acyl chlorides
  • Acyl chlorides carboxylic acid derivatives in which –OH is replaced by Cl
  • Named with the suffix -oyl chloride, such as propanoyl chloride.
Acid anhydrides
  • Acid anhydrides derivatives in which –OH is replaced by an alkanoate group
  • Named with the suffix -oic anhydride, such as ethanoic anhydride.
Acylating agents
  • Acylating agents acyl chlorides and acid anhydrides build acyl groups into other molecules
Reactivity
  • Reactivity the carbonyl carbon is very δ+ because it is bonded to two electronegative atoms
  • They undergo nucleophilic addition–elimination reactions, which are vigorous and irreversible.

Section 6 of 13Nucleophilic addition–elimination of acyl chlorides

1Hydrolysis
  • Hydrolysis water adds across C=O and HCl is eliminated, giving the carboxylic acid
  • CH3CH2COCl + H2O → CH3CH2COOH + HCl.
2With alcohols
  • With alcohols give esters, and HCl is eliminated
  • The reaction is faster, irreversible and gives a higher yield than using the acid.
  • CH3COCl + C2H5OH → CH3COOC2H5 + HCl.
3With ammonia
  • With ammonia gives a primary amide
  • The first NH3 forms the amide and the second neutralises HCl to form NH4Cl.
  • CH3CH2COCl + 2NH3 → CH3CH2CONH2 + NH4Cl.
4With primary amines
  • With primary amines give N-substituted amides
  • CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3Cl.
5Mechanism steps
  • Mechanism steps the nucleophile attacks the C=O carbon with its lone pair, then chloride is eliminated and HCl forms, shown as misty fumes
6Other amines
  • Other amines secondary amines give N,N-disubstituted amides, and tertiary amines do not react

Section 7 of 13Esters

Properties
  • Properties sweet, fruity smells, used in flavourings, perfumes and as solvents
  • They cannot hydrogen bond to each other, so they are more volatile than the parent acid.
Naming
  • Naming alkyl alkanoate, such as ethyl ethanoate

Section 8 of 13Hydrolysis of esters

Acid hydrolysis
  • Acid hydrolysis heating under reflux with dilute acid is the reverse of esterification
  • It reaches an equilibrium mixture, so the hydrolysis is not complete.
  • CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH.
Alkaline hydrolysis
  • Alkaline hydrolysis heating under reflux with dilute NaOH is irreversible
  • It gives the sodium carboxylate and the alcohol, as in CH3CH2COOCH3 + NaOH → CH3CH2COONa + CH3OH.
  • The carboxylate does not react with the alcohol, so the reaction goes to completion.
  • Acidifying with HCl then forms the carboxylic acid.
Saponification
  • Saponification hydrolysis of fats with NaOH produces soap and glycerol

Section 9 of 13Polyesters

Polyesters
Condensation polymerisation
  • Condensation polymerisation repeated condensation reactions between monomers, each losing a small molecule
  • Condensation polymers have monomers linked by ester or amide bonds.
Polyester
  • Polyester made from a dicarboxylic acid and a diol, joined by ester links
  • A diol has two –OH groups and a dicarboxylic acid has two –COOH groups.
  • H2O is lost for each ester link formed (HCl if a diacyl dichloride is used).
PET (poly(ethylene terephthalate), Terylene or Dacron)
  • PET (poly(ethylene terephthalate), Terylene or Dacron) made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol
  • It is used for bottles and fibres.
Hydroxycarboxylic acids
  • Hydroxycarboxylic acids a single monomer with –OH at one end and –COOH at the other can form a polyester
Biodegradability
  • Biodegradability polyesters can be hydrolysed at their ester links, unlike poly(alkenes)

Section 10 of 13More from the specification

More from the specification
Nucleophilic addition-elimination
  • The mechanism type for reactions of acyl chlorides.
Functional group
  • The atom or group of atoms that determines the reactions of a series.

Section 11 of 13More on this topic

  • Carboxylic acids
    • weak acids that still fizz with carbonates
    • With an alcohol and an acid catalyst they form an ester.

Section 12 of 13Key terms

TermMeaning
Carboxyl groupThe –COOH functional group of carboxylic acids.
Acyl chlorideA carboxylic acid derivative in which the –OH is replaced by Cl.
EsterA compound containing –COO– formed from an acid and an alcohol.
Nucleophilic addition–eliminationA mechanism in which a nucleophile adds to C=O and a small molecule is then eliminated.
Condensation polymerA polymer formed with the loss of a small molecule at each link.
SaponificationAlkaline hydrolysis of a fat to give soap and glycerol.
AmideA compound containing the –CONH– group.

Section 13 of 13Exam technique

1
Saying carboxylic acids are strong acids — they are weak and only slightly ionised
2
Distilling instead of refluxing in the oxidation of a primary alcohol to an acid
3
Writing the products of alkaline ester hydrolysis as the acid — the product is the carboxylate salt until acidified
4
Forgetting that two molecules of ammonia or amine are needed with an acyl chloride
5
Saying esterification goes to completion — the acid-catalysed reaction is reversible

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