Unit 7 · Organic Chemistry

Carboxylic Acids & Acyl Chlorides

Structure, acidity, preparation, reactions, uses — and the highly reactive acyl chloride functional group.

7.1

Structure and Nomenclature

Carboxylic Acids Organic compounds containing the carboxyl functional group (–COOH), which combines a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. General formula: R–COOH or CnH2n+1COOH. Named with the suffix –oic acid.

The Carboxyl Group

The –COOH group is planar (sp2 carbonyl carbon). The O–H bond is more acidic than in alcohols because the carboxylate anion (RCOO−) formed after deprotonation is stabilised by resonance delocalization of the negative charge over both oxygen atoms.

IUPAC Nomenclature

  1. Find the longest chain including the –COOH carbon. The –COOH carbon is always C1.
  2. Replace the final –e of the parent alkane with –oic acid.
  3. Number substituents from the –COOH end (C1).
  4. Diacids (two –COOH groups): use suffix –dioic acid.
IUPAC NameFormulaStructureCommon NameSource
Methanoic acidHCOOHH–COOHFormic acidAnt venom
Ethanoic acidCH3COOHCH3–COOHAcetic acidVinegar
Propanoic acidC2H5COOHCH3CH2–COOHPropionic acid—
Butanoic acidC3H7COOHCH3(CH2)2COOHButyric acidRancid butter
Pentanoic acidC4H9COOH—Valeric acid—
Hexadecanoic acidC15H31COOH—Palmitic acidPalm oil (saturated fat)
Octadecanoic acidC17H35COOH—Stearic acidAnimal fat
Ethanedioic acid(COOH)2HOOC–COOHOxalic acidRhubarb, spinach
Benzene-1,2-dicarboxylic acid——Phthalic acidIndustrial
Example 1

Naming Carboxylic Acids

Name: (a) CH3CH(CH3)COOH   (b) HOOCCH2CH2COOH   (c) CH3CH2CH(Br)COOH

a
Longest chain including COOH = 3 C → propanoic acid. Methyl branch at C2 → 2-methylpropanoic acid (isobutyric acid).
b
Two COOH groups; chain between them = 2 C + 2 COOH carbons = 4 C total → butanedioic acid (succinic acid).
c
Chain = 4 C (butanoic acid). Br at C2 (next to COOH) → 2-bromobutanoic acid.
7.2

Physical Properties

Boiling Points — Very High

Carboxylic acids have exceptionally high boiling points compared to alcohols, aldehydes, and ketones of similar molecular mass. This is because they form hydrogen-bonded dimers in the liquid state — two molecules pair up via two O–H···O=C hydrogen bonds, effectively doubling the apparent molecular mass.

Example: ethanoic acid (Mr=60, B.P.=118°C) vs propan-1-ol (Mr=60, B.P.=97°C).

Solubility

Short-chain acids (C1–C4) are completely miscible with water. The –COOH group forms strong H-bonds with water. Solubility decreases as chain length increases; long-chain acids (fats) are practically insoluble in water.

AcidMrB.P. (°C)M.P. (°C)Smell
Methanoic acid461018Pungent (ant sting)
Ethanoic acid6011817Vinegar
Propanoic acid74141−21Rancid, sweaty
Butanoic acid88164−6Rancid butter, vomit
Pentanoic acid102186−34Cheese, sweat
7.3

Acidity of Carboxylic Acids

Acidity Carboxylic acids are weak acids: they partially dissociate in water to give H+ (or H3O+) and the carboxylate anion (RCOO−).

RCOOH ⇌ RCOO− + H+    (Ka ~ 10−5 for ethanoic acid)

Why Carboxylic Acids are More Acidic than Alcohols

Carboxylic acids (pKa ~4–5) are far more acidic than alcohols (pKa ~16). After an alcohol loses H+, the negative charge on the alkoxide (RO−) is localised on one oxygen atom. In a carboxylate (RCOO−), the negative charge is delocalised by resonance over both oxygen atoms (both C–O bonds become equivalent, bond order 1.5), greatly stabilising the anion and favouring deprotonation.

Factors Affecting Acid Strength

Electron-withdrawing groups (e.g. halogens, –NO2) near the –COOH increase acidity by further stabilising the carboxylate anion (withdrawing electron density, reducing the negative charge).

Electron-donating groups (e.g. alkyl groups) decrease acidity by destabilising the carboxylate anion.

AcidStructurepKaRelative Strength
Trichloroacetic acidCCl3COOH0.66Stronger (3 Cl, very electron-withdrawing)
Dichloroacetic acidCHCl2COOH1.48Stronger
Chloroacetic acidClCH2COOH2.86Stronger than ethanoic
Methanoic acidHCOOH3.75Stronger than ethanoic (H not alkyl)
Ethanoic acidCH3COOH4.76Reference
Propanoic acidC2H5COOH4.87Slightly weaker (longer alkyl)
EthanolC2H5OH~16Far weaker (no resonance stabilisation)
Example 2

Comparing Acid Strengths

Arrange in order of increasing acid strength: ethanoic acid, chloroacetic acid (ClCH2COOH), propanoic acid. Explain.

1
Propanoic acid has a longer alkyl chain (electron-donating) → weakest (pKa 4.87).
2
Ethanoic acid: alkyl group is CH3 → middle (pKa 4.76).
3
Chloroacetic acid: Cl is highly electron-withdrawing, stabilising COO− → strongest (pKa 2.86).
✓
Order: propanoic < ethanoic < chloroacetic (increasing acid strength).
7.4

Preparation of Carboxylic Acids

Method 1: Oxidation of Primary Alcohols or Aldehydes

RCH2OH --excess K2Cr2O7/H2SO4, reflux--> RCOOH RCHO --excess K2Cr2O7/H2SO4, reflux--> RCOOH e.g. CH3CH2OH --[O]--> CH3CHO --[O]--> CH3COOH (ethanol) (ethanal) (ethanoic acid)

Method 2: Hydrolysis of Nitriles

A nitrile (R–CN) is hydrolysed by heating with dilute acid (or alkali) to give a carboxylic acid. This is useful for synthesising acids with one more carbon than the starting halogenoalkane.

R-CN + H2O --dil. HCl or NaOH, reflux--> RCOOH + NH3 e.g. CH3CN + H2O --H+, heat--> CH3COOH + NH3 (ethanenitrile) (ethanoic acid)

Method 3: Hydrolysis of Esters

RCOOR' + H2O --H+ or OH-, heat--> RCOOH + R'OH e.g. CH3COOC2H5 + H2O --H+, heat--> CH3COOH + C2H5OH (ethyl ethanoate) (ethanoic acid) (ethanol)

Method 4: Reaction of Grignard Reagents with CO2

RMgX + CO2 --dry ether--> RCOOMgX --H2O/H+--> RCOOH + Mg(OH)X e.g. CH3MgBr + CO2 --> CH3COOMgBr --H+/H2O--> CH3COOH (methylmagnesium bromide) (ethanoic acid)

This method introduces a –COOH group using CO2 as the electrophile.

Example 3

Two-Step Synthesis of Propanoic Acid

Show how ethanol can be converted to propanoic acid using two steps (chain extension).

1
Step 1: Convert ethanol → bromoethane: C2H5OH + HBr → C2H5Br + H2O
2
Step 2a: C2H5Br + KCN(alc) → C2H5CN + KBr (propanenitrile)
3
Step 2b: C2H5CN + H2O →H+, heat C2H5COOH + NH3 (propanoic acid)
✓
Overall: ethanol (2C) → propanoic acid (3C). Chain extended by one carbon via nitrile.
7.5

Reactions of Carboxylic Acids

Reaction 1: Acid–Base Reactions

With metals: 2 RCOOH + Mg --> (RCOO)2Mg + H2(g) 2 CH3COOH + Zn --> (CH3COO)2Zn + H2 With bases (NaOH, KOH): RCOOH + NaOH --> RCOONa + H2O CH3COOH + NaOH --> CH3COONa + H2O (sodium ethanoate) With carbonates / bicarbonates: 2 RCOOH + Na2CO3 --> 2 RCOONa + H2O + CO2(g) RCOOH + NaHCO3 --> RCOONa + H2O + CO2(g)
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Test for Carboxylic Acids Carboxylic acids effervesce with NaHCO3 solution (CO2 produced). Phenols and alcohols do NOT react with NaHCO3. This is the key test to distinguish carboxylic acids from other acidic compounds.

Reaction 2: Esterification

RCOOH + R'OH <==> RCOOR' + H2O conc. H2SO4 catalyst, heat e.g. CH3COOH + C2H5OH <==> CH3COOC2H5 + H2O (ethanoic acid) (ethanol) (ethyl ethanoate)

Reversible equilibrium. Yield improved by: excess alcohol, removing water, or removing ester by distillation.

Reaction 3: Reduction to Primary Alcohol

RCOOH + 4[H] --LiAlH4, dry ether--> RCH2OH + H2O e.g. CH3COOH + 4[H] --> CH3CH2OH (ethanol) Note: NaBH4 does NOT reduce carboxylic acids (too mild). LiAlH4 only.

Reaction 4: Conversion to Acyl Chloride

RCOOH + PCl5 --> RCOCl + POCl3 + HCl RCOOH + SOCl2 --> RCOCl + SO2 + HCl (preferred method) e.g. CH3COOH + SOCl2 --> CH3COCl + SO2 + HCl (ethanoic acid) (ethanoyl chloride)

SOCl2 is preferred because both by-products (SO2 and HCl) are gases and leave the reaction mixture, giving a pure product.

Reaction 5: Decarboxylation

RCOONa + NaOH --CaO, heat--> R-H + Na2CO3 e.g. CH3COONa + NaOH --CaO, heat--> CH4 + Na2CO3 (sodium ethanoate) (methane)

The sodium salt of the acid loses –COO− to give an alkane with one fewer carbon (Kolbe’s method — see Unit 2).

Example 4

Reactions of Ethanoic Acid

Write equations for the reactions of ethanoic acid with: (a) NaHCO3; (b) Mg; (c) ethanol (conc. H2SO4); (d) LiAlH4.

a
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2↑ (effervescence)
b
2 CH3COOH + Mg → (CH3COO)2Mg + H2↑ (magnesium ethanoate + hydrogen)
c
CH3COOH + C2H5OH ⇌conc. H2SO4 CH3COOC2H5 + H2O (ethyl ethanoate)
d
CH3COOH + 4[H] →LiAlH4 CH3CH2OH + H2O (ethanol)
7.6

Acyl Chlorides

Acyl Chlorides (Acid Chlorides) Acyl chlorides contain the acyl chloride functional group (–COCl). General formula: RCOCl. Named by replacing –ic acid of the parent carboxylic acid with –oyl chloride. Example: ethanoic acid → ethanoyl chloride (CH3COCl).

Why Acyl Chlorides are More Reactive than Carboxylic Acids

The C–Cl bond in acyl chlorides is more polarised than C–OH in acids (Cl is a better leaving group than OH). The carbonyl carbon is more electrophilic. Acyl chlorides undergo nucleophilic acyl substitution much faster than carboxylic acids, reacting vigorously with water, alcohols, and amines at room temperature.

Reactions of Acyl Chlorides

1. With Water → Carboxylic Acid (Hydrolysis)

RCOCl + H2O --> RCOOH + HCl (vigorous, steamy fumes of HCl) e.g. CH3COCl + H2O --> CH3COOH + HCl

2. With Alcohols → Ester (faster than esterification)

RCOCl + R'OH --> RCOOR' + HCl e.g. CH3COCl + C2H5OH --> CH3COOC2H5 + HCl (ethanoyl chloride) (ethanol) (ethyl ethanoate)

This reaction is fast and irreversible (unlike acid + alcohol esterification). HCl fumes are produced.

3. With Ammonia → Primary Amide

RCOCl + 2NH3 --> RCONH2 + NH4Cl e.g. CH3COCl + 2NH3 --> CH3CONH2 + NH4Cl (ethanoyl chloride) (ethanamide)

Two moles of NH3 are used: one reacts, one neutralises the HCl produced.

4. With Primary Amines → Secondary Amide (N-substituted)

RCOCl + R'NH2 --> RCONHR' + HCl e.g. CH3COCl + C2H5NH2 --> CH3CONHC2H5 + HCl (ethanoyl chloride) (ethylamine) (N-ethylethanamide)
NucleophileProductBy-productExample
H2OCarboxylic acid (RCOOH)HClCH3COCl + H2O → CH3COOH + HCl
ROH (alcohol)Ester (RCOOR')HClCH3COCl + C2H5OH → CH3COOC2H5 + HCl
NH3Primary amide (RCONH2)NH4ClCH3COCl + 2NH3 → CH3CONH2 + NH4Cl
R'NH2 (amine)Secondary amide (RCONHR')HCl (or amine salt)CH3COCl + C2H5NH2 → CH3CONHC2H5 + HCl
R'OH (phenol)Phenyl esterHClCH3COCl + C6H5OH → CH3COOC6H5 + HCl
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Acyl Chlorides vs Carboxylic Acids — Key Differences Acyl chlorides react with water instantly (fuming), with alcohols at room temperature without catalyst, and with amines without heating. Carboxylic acids require concentrated H2SO4 catalyst and heat for esterification, react slowly with amines, and do not fume in air. Acyl chlorides are used in synthesis when fast, complete reactions are needed.
Example 5

Reactions of Ethanoyl Chloride

Write equations for the reactions of ethanoyl chloride with: (a) water; (b) propan-1-ol; (c) methylamine (CH3NH2). Name all organic products.

a
CH3COCl + H2O → CH3COOH + HCl
Product: ethanoic acid
b
CH3COCl + CH3CH2CH2OH → CH3COOCH2CH2CH3 + HCl
Product: propyl ethanoate
c
CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3Cl
Product: N-methylethanamide
7.7

Uses of Carboxylic Acids and Acyl Chlorides

CompoundUseNotes
Ethanoic acid (acetic acid)Vinegar (5% solution); food preservative (E260)Inhibits microbial growth
Ethanoic acidManufacture of cellulose acetate (rayon, photographic film)Reaction with cellulose –OH groups
Ethanoic acidSynthesis of aspirin (acetylsalicylic acid) via acylationReacts with salicylic acid
Methanoic acidDescaling agent; rubber coagulation; leather tanningStronger acid than ethanoic
Long-chain fatty acidsSoap making (saponification of fats with NaOH)Palmitic, stearic, oleic acids
Benzoic acidFood preservative (sodium benzoate, E211)Inhibits yeast and fungi
Lactic acidFood industry (yoghurt, cheese); biodegradable plastic (PLA)2-hydroxypropanoic acid
Citric acidFood flavouring and preservative; cleaning agentTricarboxylic acid from citrus fruits
Ethanoyl chlorideAcetylation reactions in synthesis (aspirin, paracetamol)Introduces CH3CO– group rapidly
Acyl chlorides generallyManufacturing pharmaceuticals, dyes, agrochemicalsHighly reactive acylating agents

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Exercises

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Multiple Choice Quiz — 25 Questions

Unit 7 Quiz — Carboxylic Acids

25 Questions · Select one answer each
Q1

The IUPAC name of (CH3)2CHCOOH is:

(CH3)2CHCOOH: longest chain including COOH = 3 C &rarr; propanoic acid. CH3 branch at C2 &rarr; 2-methylpropanoic acid. &ldquo;Isobutyric acid&rdquo; is a common name only.
Q2

Which reagent is used to distinguish a carboxylic acid from a phenol?

NaHCO3: carboxylic acids react with NaHCO3 to give CO2 (effervescence). Phenols are too weak to react with NaHCO3 (both react with NaOH). Litmus would show both as acidic.
Q3

Why is chloroacetic acid (ClCH2COOH) stronger than ethanoic acid (CH3COOH)?

Cl is highly electronegative and withdraws electron density from the carboxylate anion, dispersing the negative charge further and lowering its energy &rarr; more stable anion &rarr; stronger acid. CH3 donates electrons (inductive), slightly destabilising the anion &rarr; weaker acid. Both B and D a
Q4

The product of reducing propanoic acid with LiAlH4 is:

LiAlH4 reduces carboxylic acids all the way to primary alcohols: C2H5COOH + 4[H] &rarr; C2H5CH2OH = propan-1-ol. NaBH4 is too mild to reduce carboxylic acids.
Q5

The reaction of an acyl chloride with ammonia gives:

RCOCl + 2NH3 &rarr; RCONH2 + NH4Cl. The product is a primary amide. Two moles of NH3 are needed: one for the reaction, one to neutralise HCl.
Q6

Why does ethanoic acid have an unusually high boiling point (118&deg;C) compared to other organic compounds of similar Mr?

Carboxylic acids form hydrogen-bonded dimers in the liquid: two molecules pair via two O&ndash;H&middot;&middot;&middot;O=C hydrogen bonds. This effectively doubles the molecular mass, requiring much more energy to vaporise &rarr; very high boiling point.
Q7

Acyl chlorides react with water to produce:

RCOCl + H2O &rarr; RCOOH + HCl. Acyl chlorides are hydrolysed rapidly by water, producing the corresponding carboxylic acid and HCl (steamy fumes in moist air).
Q8

Which method is preferred for preparing an acyl chloride from a carboxylic acid?

SOCl2 is preferred because both by-products (SO2 and HCl) are gases and leave the reaction mixture, giving a pure product without difficult separation. PCl5 leaves POCl3 as a liquid by-product that must be separated.
Q9

Hydrolysis of a nitrile (R&ndash;CN) gives:

R&ndash;CN + H2O &rarr; R&ndash;COOH + NH3. The nitrile is made from R&ndash;X + CN&minus; (adds one C), then hydrolysed to RCOOH. So the acid has one more carbon than the original halogenoalkane. This is a key chain-extension strategy.
Q10

Which of the following is a use of ethanoic acid?

Ethanoic acid (acetic acid) is used as vinegar (5% aqueous solution) and in manufacturing cellulose acetate (synthetic fibre, photographic film). Oxy-acetylene uses ethyne; polyethylene monomer is ethene.
Q11

Carboxylic acids are stronger acids than alcohols because:

RCOO⁻: –C(=O)–O⁻ ↔ –C(–O⁻)=O. Resonance stabilises the anion — both C–O bonds equivalent (partial double bond character). R–O⁻ has no such stabilisation.
Q12

Ethanoic acid reacts with Na₂CO₃ solution to give:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑. Effervescence (CO₂) is the positive test. Carboxylic acids (pKa~5) are stronger than H₂CO₃ (pKa~6.3) → displace CO₂.
Q13

Reduction of a carboxylic acid with LiAlH₄ gives:

LiAlH₄ (powerful reducing agent) fully reduces –COOH → –CH₂OH. NaBH₄ cannot reduce carboxylic acids (only reduces C=O of aldehydes/ketones). LiAlH₄ used in dry ether.
Q14

Esterification of propanoic acid with methanol gives:

CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O. The methanol's –OH replaces the –OH of the carboxylic acid. Conc. H₂SO₄ catalyses and removes water → better yield.
Q15

Formation of an acyl chloride from a carboxylic acid uses:

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl. Acyl chlorides are more reactive acylating agents than carboxylic acids. SOCl₂ preferred (gaseous byproducts easy to remove).
Q16

The carboxylate group shows two C–O bonds of equal length in the anion because:

RCOO⁻: one structure has C=O and C–O⁻; the other has C–O⁻ and C=O. Both equivalent → actual structure is hybrid with bond length between single (0.136nm) and double (0.124nm) — ~0.127nm.
Q17

Methanoic acid (HCOOH) is unusual among carboxylic acids because:

HCOOH has H directly on C=O (like an aldehyde character). It reduces Tollens' and Fehling's, and KMnO₄. Higher carboxylic acids (CH₃COOH, etc.) do not do this.
Q18

The reaction of ethanoic acid with PCl₅ gives:

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl. The fumes of HCl and the acyl chloride product confirm the reaction. POCl₃ is the other byproduct.
Q19

Dicarboxylic acids (e.g. ethanedioic acid, HOOC–COOH) differ from monocarboxylic acids in that:

Two –COOH groups can react with two –OH groups of a diol → long chain polyester (condensation polymer). Also: dicarboxylic acids are stronger acids (lower pKa₁) than monocarboxylic.
Q20

The smell of many fruit esters is due to:

Esters have distinctive fruity/floral smells. Naturally occurring: banana (3-methylbutyl ethanoate), pineapple (ethyl butanoate). Carboxylic acids often smell rancid/acidic.
Q21

Alpha-hydroxy acids (AHAs) used in skincare contain:

AHAs: lactic acid (from milk), glycolic acid (from sugar cane), citric acid (from citrus). They exfoliate by weakening bonds between dead skin cells. Bifunctional: –OH and –COOH.
Q22

The boiling points of carboxylic acids are higher than expected because:

RCOOH forms dimers: two –COOH groups link via two O–H···O hydrogen bonds. Dimer effectively doubles the Mr. Ethanoic acid boiling point (118°C) >> ethanol (78°C) despite similar Mr.
Q23

Reaction of a carboxylic acid with NaOH gives:

RCOOH + NaOH → RCOONa + H₂O. Carboxylate salts are ionic, water-soluble, and non-volatile (used in soaps — sodium/potassium salts of long-chain fatty acids).
Q24

The decarboxylation of a carboxylic acid (losing CO₂) occurs when:

Kolbe's method: heating sodium carboxylate with soda lime (NaOH + CaO) → alkane + Na₂CO₃. The carboxylate carbon is lost as CO₃²⁻/CO₂. Product alkane has one fewer C.
Q25

Benzoic acid (C₆H₅COOH) is obtained by:

Hot conc. KMnO₄ oxidises any alkyl substituent on benzene ring to –COOH. Toluene → benzoic acid. Even long chains give benzoic acid (C6 ring + COOH). Ring is resistant to KMnO₄.
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Unit Test

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Instructions Total: 50 marks  |  Time: 50 minutes  |  Attempt all questions  |  Show all working.

Section A — Short Answer

30 marks
Q1 [4 marks]

Name the following:
(a) CH3CH2CH2COOH   (b) (CH3)2CHCH2COOH   (c) HOOCCOOH   (d) BrCH2CH2COOH

(a) Butanoic acid
(b) 3-methylbutanoic acid (branch at C3 from COOH end)
(c) Ethanedioic acid (oxalic acid)
(d) 3-bromopropanoic acid (Br at C3, counting from COOH as C1)
Q2 [4 marks]

Explain why carboxylic acids have higher boiling points than alcohols of similar molecular mass, and why they are more acidic than alcohols. Use ethanoic acid vs ethanol as your example.

Higher boiling point: Ethanoic acid (B.P. 118°C) forms hydrogen-bonded dimers in the liquid state via two O–H···O=C hydrogen bonds per pair of molecules. This effectively doubles the molecular mass, requiring much more energy to separate molecules. Ethanol (B.P. 78°C) forms H-bonds but only as single O–H···O links → lower B.P.

Greater acidity: On losing H+, ethanoic acid forms CH3COO− where the negative charge is delocalised by resonance over both oxygens → very stable anion. Ethanol forms C2H5O− with no resonance stabilisation → localised charge → less stable → much weaker acid (pKa ~16 vs 4.76).
Q3 [6 marks]

Write balanced equations for the following reactions:
(a) Ethanoic acid + Na2CO3   (b) Propanoic acid + NaHCO3   (c) Ethanoic acid + Mg   (d) Butanoic acid + LiAlH4   (e) Ethanoic acid + SOCl2   (f) Ethanoic acid + PCl5
Name all organic products.

(a) 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 — sodium ethanoate
(b) C2H5COOH + NaHCO3 → C2H5COONa + H2O + CO2 — sodium propanoate
(c) 2CH3COOH + Mg → (CH3COO)2Mg + H2 — magnesium ethanoate
(d) C3H7COOH + 4[H] →LiAlH4 C3H7CH2OH + H2O — butan-1-ol
(e) CH3COOH + SOCl2 → CH3COCl + SO2 + HCl — ethanoyl chloride
(f) CH3COOH + PCl5 → CH3COCl + POCl3 + HCl — ethanoyl chloride
Q4 [5 marks]

Write equations for all the reactions of ethanoyl chloride (CH3COCl) with: (a) water; (b) ethanol; (c) phenol (C6H5OH); (d) ammonia; (e) ethylamine (C2H5NH2). Name all organic products.

(a) CH3COCl + H2O → CH3COOH + HCl — ethanoic acid
(b) CH3COCl + C2H5OH → CH3COOC2H5 + HCl — ethyl ethanoate
(c) CH3COCl + C6H5OH → CH3COOC6H5 + HCl — phenyl ethanoate
(d) CH3COCl + 2NH3 → CH3CONH2 + NH4Cl — ethanamide
(e) CH3COCl + 2C2H5NH2 → CH3CONHC2H5 + C2H5NH3Cl — N-ethylethanamide
Q5 [5 marks]

Describe two different methods for preparing propanoic acid from ethanol. Write equations and state all conditions. (Hint: one method uses direct oxidation; the other extends the chain via a nitrile.)

Method 1 — Direct oxidation of propan-1-ol:
First make propan-1-ol (not directly from ethanol in one step, but showing oxidation):
CH3CH2CH2OH →excess K2Cr2O7/H2SO4, reflux CH3CH2COOH

Method 2 — Chain extension via nitrile (from ethanol):
Step 1: C2H5OH + HBr → C2H5Br + H2O
Step 2: C2H5Br + KCN(alc) → C2H5CN + KBr (propanenitrile; chain extended 2C → 3C)
Step 3: C2H5CN + H2O →dil. HCl, reflux C2H5COOH + NH3 (propanoic acid)
This route uses ethanol (2C) → bromoethane → propanenitrile → propanoic acid (3C).
Q6 [6 marks]

Arrange the following in order of increasing acid strength and explain the trend using electronic theory: methanoic acid (HCOOH), ethanoic acid (CH3COOH), trifluoroacetic acid (CF3COOH), propanoic acid (C2H5COOH).

Order (weakest → strongest):
Propanoic acid (pKa 4.87) < Ethanoic acid (4.76) < Methanoic acid (3.75) < Trifluoroacetic acid (0.52)

Explanation:
Acid strength depends on stability of the carboxylate anion (RCOO−).
• Propanoic < Ethanoic: longer alkyl chain = more electron-donating (inductive) = more electron density on anion = less stable = weaker acid.
• Ethanoic < Methanoic: methanoic has H instead of CH3; H is less electron-donating than CH3 → slightly less destabilisation of anion → stronger.
• Methanoic < CF3COOH: three F atoms are strongly electron-withdrawing, withdrawing electron density from COO−, greatly stabilising the anion by dispersing the negative charge → much stronger acid.

Section B — Extended Response

20 marks
Q7 [10 marks]

(a) Compare acyl chlorides and carboxylic acids as acylating agents. In your answer: define acylation, explain why acyl chlorides are more reactive, and compare their reactions with water, alcohols, and amines with equations. [7 marks]

(b) Describe the industrial manufacture of aspirin (acetylsalicylic acid) from salicylic acid using ethanoyl chloride. Write the equation. [3 marks]

(a) Acylation: The introduction of an acyl group (RCO–) into a molecule, replacing an active H on O, N, or C.

Why acyl chlorides are more reactive: The C–Cl bond is more polarised than C–OH; Cl− is a better leaving group than OH−; the carbonyl carbon in RCOCl is more electrophilic → faster nucleophilic acyl substitution.

With water:
RCOOH + H2O: very slow (essentially no reaction at RT)
RCOCl + H2O → RCOOH + HCl (immediate, steamy fumes)

With alcohols:
RCOOH + R’OH ⇌ RCOOR’ + H2O (slow, reversible, needs H2SO4 catalyst, heat)
RCOCl + R’OH → RCOOR’ + HCl (fast, irreversible, RT, no catalyst)

With amines:
RCOOH + R’NH2 → RCOOH·NH2R’ (ammonium salt first; amide only on strong heating)
RCOCl + 2R’NH2 → RCONHR’ + R’NH3Cl (fast, RT)

(b) Aspirin synthesis:
Salicylic acid (2-hydroxybenzoic acid) + ethanoyl chloride:
HOC6H4COOH + CH3COCl → CH3COOC6H4COOH + HCl
(aspirin = acetylsalicylic acid)
The phenolic –OH of salicylic acid is acylated by ethanoyl chloride. The reaction is fast and gives good yield. HCl is evolved as a gas.
Q8 [10 marks]

(a) Starting from butanol, describe with equations how you would prepare: (i) butanoic acid; (ii) pentanenitrile (5C nitrile); (iii) pentanoic acid; (iv) butanoyl chloride. State reagents and conditions for each step. [8 marks]

(b) Methanoic acid (HCOOH) is unusual in that it gives a positive Tollens’ test (silver mirror). Explain why, using an equation. [2 marks]

(a)(i) Butanoic acid from butan-1-ol:
C3H7CH2OH →excess K2Cr2O7/H2SO4, reflux C3H7COOH

(ii) Pentanenitrile from butan-1-ol:
Step 1: C3H7CH2OH + HBr → C3H7CH2Br + H2O (1-bromobutane)
Step 2: C3H7CH2Br + KCN(alc) → C3H7CH2CN + KBr (pentanenitrile, 5C)

(iii) Pentanoic acid from pentanenitrile:
C3H7CH2CN + H2O →dil. HCl, reflux C3H7CH2COOH + NH3 (pentanoic acid)

(iv) Butanoyl chloride from butanoic acid:
C3H7COOH + SOCl2 → C3H7COCl + SO2 + HCl (butanoyl chloride)

(b) Methanoic acid (HCOOH) has the structure H–COOH: the carbon of the –COOH group is bonded directly to H. This gives it the structural feature of an aldehyde group (H–C=O) as well as an acid group. It can therefore be oxidised by Tollens’ reagent:
HCOOH + 2Ag(NH3)2+ + 2OH− → CO2 + 2Ag(s) + 4NH3 + H2O
(Silver mirror forms; HCOOH is oxidised to CO2.)

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