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4.7Unit 4. Rates, Equilibria and Further Organic Chemistry

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

5 min read 12 parts Lesson 7 of 56 1 quiz

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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

4.7 Organic Chemistry: Carboxylic Acids — properties, preparation, reactions, esters and polyesters — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Explain the properties of carboxylic acids, describe their preparation and reactions, compare acyl chlorides and esters, and describe ester …

  • Acid anhydride
  • Acid hydrolysis
  • Alkaline hydrolysis
  • Condensation polymerisation
  • Esterification
  • Saponification

Part 1 of 10Structure and physical properties

Structure and physical properties
Carboxylic acids contain -COOH, general formula CnH2n+1COOH, named alkan + oic acid; the carboxyl carbon is always carbon 1.
Two polarised groups, C=O and O-H, give strong intermolecular forces.
  • Hydrogen bonding gives high melting and boiling points;
    • ethanoic acid (Mr 60) boils at 118 °C and propan-1-ol (Mr 60) at 97 °C.
  • In the pure liquid and in non-polar solvents they form hydrogen-bonded dimers.
Short-chain acids are miscible with water through hydrogen bonds.
  • Solubility falls as the hydrocarbon chain lengthens, because the chain breaks water's hydrogen bonds.
  • Acids with more than eight carbons are solids, very slightly soluble in cold water but more soluble in hot.

Part 2 of 10Preparation of carboxylic acids

Preparation of carboxylic acids
Oxidation of primary alcohols or aldehydes under reflux with excess acidified K2Cr2O7 or KMnO4.
  • Dichromate changes from orange to green Cr3+.
  • Permanganate changes from purple to colourless Mn2+.
  • CH3CH2OH + 2[O] → CH3COOH + H2O.
Hydrolysis of nitriles, converting -CN to -COOH.
  • Dilute acidRCN + 2H2O + H+ → RCOOH + NH4+.
  • Dilute alkali gives the carboxylate salt and ammonia; acidify afterwards.
  • A nitrile made from a halogenoalkane and KCN adds one carbon.
Confirm the product with NaHCO3: effervescence of CO2.

Part 3 of 10Reactions as weak acids

1
Carboxylic acids are weak acids because they are only slightly ionised: RCOOH ⇌ RCOO− + H+, with the equilibrium to the left.
2
The carboxylate ion is stabilised by delocalisation of charge over both oxygen atoms.
3
With metals, oxides and alkalis they form salts: 2CH3COOH + MgO → (CH3COO)2Mg + H2O; CH3COOH + KOH → CH3COOK + H2O.
4
With carbonates and hydrogencarbonates they give CO2.
  • 2RCOOH + Na2CO3 → 2RCOONa + CO2 + H2O.
  • RCOOH + NaHCO3 → RCOONa + CO2 + H2O.
  • The effervescence is a test for a carboxylic acid.

Part 4 of 10Other reactions of carboxylic acids

Other reactions of carboxylic acids
Reduction with LiAlH4 in dry ether gives a primary alcohol: CH3CH2COOH + 4[H] → CH3CH2CH2OH + H2O.
PCl5 gives an acyl chloride with steamy fumes of HCl: CH3CH2COOH + PCl5 → CH3CH2COCl + POCl3 + HCl.
Esterification with an alcohol and concentrated H2SO4 catalyst forms an ester and water.
  • It is a reversible condensation reaction that needs heat.
  • The ester name comes from the alcohol first, then the acid: propanol and ethanoic acid give propyl ethanoate.
  • Esters have sweet, fruity smells and are used in flavourings, perfumes and as solvents.
  • They cannot hydrogen bond to each other, so they are more volatile than the parent acid.

Part 5 of 10Acyl chlorides and acid anhydrides

Acyl chloride
  • -COCl, named alkan + oyl chloride, e.g. ethanoyl chloride CH3COCl.
Acid anhydride
  • Acid anhydride the -OH of the acid replaced by an alkanoate, named alkan + oic anhydride.
The carbonyl carbon of an acyl chloride is very δ+ because it is bonded to two electronegative atoms, so nucleophiles attack readily.
They react by nucleophilic addition–elimination: the nucleophile adds across C=O, then HCl is eliminated.
Each reaction is vigorous, irreversible and gives misty fumes of HCl.

Part 6 of 10Reactions of acyl chlorides

1
Water → carboxylic acid + HCl.
2
Alcohols → esters + HCl; faster, irreversible and with a higher yield than using the acid: CH3COCl + C2H5OH → CH3COOC2H5 + HCl.
3
Ammonia → a primary amide; primary amines → N-substituted (secondary) amides.
  • Two molecules of ammonia or amine are needed: one forms the amide and one neutralises the HCl as an ammonium salt.
  • CH3CH2COCl + 2NH3 → CH3CH2CONH2 + NH4Cl.
  • CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3Cl.
4
Secondary amines give tertiary amides; tertiary amines do not react.

Part 7 of 10Hydrolysis of esters

Acid hydrolysis
  • Acid hydrolysis reflux with dilute H2SO4 or HCl.
  • It is the reverse of esterification and reaches equilibrium, so it is incomplete.
  • Products are a carboxylic acid and an alcohol: CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH.
Alkaline hydrolysis
  • Alkaline hydrolysis reflux with NaOH(aq).
  • It is irreversible and goes to completion.
  • Products are a carboxylate salt and an alcohol: CH3CH2COOCH3 + NaOH → CH3CH2COONa + CH3OH.
  • Acidify afterwards to obtain the carboxylic acid.
Alkaline hydrolysis of fats gives soap and glycerol (saponification).
FeatureAcid hydrolysisAlkaline hydrolysis
ReagentDilute H2SO4 / HCl, refluxNaOH(aq), reflux
Reversible?Yes - incompleteNo - goes to completion
ProductsCarboxylic acid + alcoholCarboxylate salt + alcohol

Part 8 of 10Polyesters

Polyesters
Condensation polymerisation
  • Condensation polymerisation monomers join by repeated condensation reactions, eliminating a small molecule such as H2O or HCl.
Condensation polymers have monomers linked by ester or amide bonds.
Polyester from a dicarboxylic acid (or diacyl dichloride) and a diol.
  • Each ester link forms with loss of H2O (or HCl).
  • PET (Terylene, Dacron) is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol and is used for bottles and fibres.
A single hydroxycarboxylic acid monomer, with an -OH at one end and a -COOH at the other, can also form a polyester.
Polyesters are biodegradable by hydrolysis at their ester links, unlike poly(alkenes).

Part 9 of 10More from the specification

More from the specification
Carboxylic acid functional group
  • Carboxylic acid functional group the -COOH group;
    • acids are weak and partly dissociate in water to give H+ and a carboxylate ion
Sodium carboxylate
  • Sodium carboxylate acids react with sodium hydroxide, sodium carbonate or sodium to form salts, with carbon dioxide formed with carbonates
Esterification
  • Esterification an acid and an alcohol with a strong acid catalyst form an ester and water in a reversible reaction
Acyl chlorides
  • Acyl chlorides react by nucleophilic addition-elimination, forming esters with alcohols, amides with ammonia, and polyesters when used with diols
Reduction
  • Reduction lithium tetrahydridoaluminate reduces carboxylic acids to primary alcohols
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.

Part 10 of 10More on this topic

  • Carboxylic acids are weak acids but still fizz with carbonates.

Key terms

TermMeaning
Carboxylic acidAn organic compound containing the carboxyl group -COOH.
Acyl chlorideA compound containing -COCl, formed by replacing the OH of a carboxylic acid with Cl.
EsterA compound containing -COO-, formed from an acid (or acyl chloride) and an alcohol.
EsterificationThe reversible reaction of an acid with an alcohol to form an ester and water.
SaponificationAlkaline hydrolysis of an ester to a carboxylate salt and an alcohol.
Condensation polymerisationPolymer formation by repeated condensation reactions, eliminating small molecules.
Nucleophilic addition–eliminationAddition of a nucleophile across C=O followed by elimination of a small molecule.

Avoid these mistakes

1
Saying carboxylic acids are strong acids — they are only partly ionised.
2
Using one mole of ammonia with an acyl chloride — two are needed, one to neutralise the HCl.
3
Saying alkaline hydrolysis of an ester gives the acid directly — it gives the carboxylate salt.
4
Saying the acid hydrolysis of an ester goes to completion — it is an equilibrium.
5
Naming esters with the acid first — the alcohol part is named first.

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