Chap 6: Alcohols and Carboxylic acids

# Chap 6: Alcohols and Carboxylic acids ## Objectifs et prérequis By the end of this chapter, students will be able to: - Name and draw structures of alcohols and carboxylic acids (C1–C4) using IUPAC nomenclature. - Describe laboratory and industrial methods for preparing alcohols and carboxylic acids. - Explain physical and chemical properties of alcohols and carboxylic acids, linking them to molecular structure. - Discuss common uses of alcohols, carboxylic acids, esters, soaps, and detergents. - Understand esterification and saponification reactions, including mechanisms and conditions. - Compare soaps and detergents in terms of structure, action in hard water, and environmental impact. Prerequisites: Knowledge of organic nomenclature (alkanes, alkenes), functional groups, bonding (covalent, polar), and basic reaction types (substitution, addition, oxidation). Familiarity with writing and balancing chemical equations is essential. --- ## Introduction Alcohols and carboxylic acids are two of the most important classes of organic compounds due to their widespread occurrence in nature and industrial applications. Alcohols contain a hydroxyl group (–OH) attached to a saturated carbon atom, while carboxylic acids contain a carboxyl group (–COOH). Both functional groups are polar and capable of hydrogen bonding, significantly influencing their physical properties such as boiling points and solubility. This chapter covers the structure, nomenclature, preparation, properties, and uses of alcohols and carboxylic acids with up to four carbon atoms. It further explores ester formation, saponification, and the chemistry of soaps and detergents, linking molecular structure to real-world utility in cleaning, pharmaceuticals, and materials science. --- ## I. Structure and nomenclature of alcohols (C1–C4) ### Definition and general structure An alcohol is an organic compound in which a hydroxyl group (–OH) is bonded to a saturated carbon atom (sp³ hybridised). The general formula for acyclic alcohols is $C_nH_{2n+2}O$. > **To retain**: The hydroxyl group (–OH) is the defining functional group of alcohols. ### Nomenclature (IUPAC) 1. Identify the longest carbon chain containing the –OH group. 2. Number the chain to give the carbon bearing the –OH group the lowest possible number. 3. Use the suffix "-ol" instead of "-e" for the parent alkane. 4. Number and name substituents. **Examples**: - $CH_3OH$: Methanol (not "hydroxymethane") - $CH_3CH_2OH$: Ethanol - $CH_3CH(OH)CH_3$: Propan-2-ol (isopropyl alcohol) - $CH_3CH_2CH_2CH_2OH$: Butan-1-ol - $CH_3CH_2CH(OH)CH_3$: Butan-2-ol ### Step-by-step example: Name $CH_3CH(CH_3)CH_2OH$ 1. Longest chain: 3 carbons (propane). 2. –OH on carbon 1? Number from end near –OH: $HOCH_2–CH(CH_3)–CH_3$ → –OH on C1. 3. Substituent: methyl on C2. 4. Name: 2-methylpropan-1-ol (isobutyl alcohol). ### Physical properties - **Boiling point**: Increases with molecular weight due to stronger London forces; anomalously high for low MW alcohols due to hydrogen bonding. - Methanol: 65°C, Ethanol: 78°C, Propan-1-ol: 97°C, Butan-1-ol: 118°C - **Solubility**: Decreases with chain length. Methanol, ethanol, and propan-1-ol are miscible with water; butan-1-ol has limited solubility (~7.9 g/100 mL). - **Polarity**: The –OH group makes alcohols polar, enabling H-bonding with water and themselves. ### Chemical properties 1. **Acidity**: Alcohols are very weak acids (pKa ~15–18), weaker than water. - $ROH \rightleftharpoons RO^- + H^+$ 2. **Reactivity of –OH**: - **Substitution**: With HX (X = Cl, Br, I) → alkyl halide. $CH_3CH_2OH + HBr \rightarrow CH_3CH_2Br + H_2O$ (requires catalyst like ZnBr₂ or heat) - **Oxidation**: - Primary alcohol → aldehyde → carboxylic acid $CH_3CH_2OH \xrightarrow{[O]} CH_3CHO \xrightarrow{[O]} CH_3COOH$ - Secondary alcohol → ketone $CH_3CH(OH)CH_3 \xrightarrow{[O]} CH_3COCH_3$ - Tertiary alcohol: resistant to oxidation (no H on α-carbon) - **Dehydration**: With conc. $H_2SO_4$ and heat → alkene. $CH_3CH_2OH \xrightarrow{conc. H_2SO_4, 170^\circ C} CH_2=CH_2 + H_2O$ ### Step-by-step example: Oxidation of ethanol 1. Primary alcohol: oxidisable. 2. First step: ethanol → acetaldehyde (using PCC or distill to avoid over-oxidation). $CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$ 3. Further oxidation: acetaldehyde → acetic acid. $CH_3CHO + [O] \rightarrow CH_3COOH$ 4. With strong oxidant (KMnO₄/H⁺ or K₂Cr₂O₇/H⁺, heat): direct to acid. ### Uses of alcohols - **Ethanol**: Beverages, solvent, fuel additive (gasohol), antiseptic. - **Methanol**: Solvent, formaldehyde production, fuel (toxic). - **Propan-2-ol (isopropyl alcohol)**: Disinfectant, solvent for oils. - **Butan-1-ol**: Solvent, plasticiser, precursor to butyl acetate. ### Application exercises (corrected) **Ex 1**: Name $CH_3CH_2CH(CH_3)OH$. - Chain: 4 carbons (butane). - –OH on C2? Number: $CH_3–CH(OH)–CH(CH_3)–CH_3$ → –OH on C2, methyl on C3. - Name: 3-methylbutan-2-ol. **Ex 2**: What product is formed when butan-2-ol is heated with conc. $H_2SO_4$? - Dehydration: follows Zaitsev’s rule → more substituted alkene favoured. - Butan-2-ol: $CH_3CH(OH)CH_2CH_3$ - Loss of H from C1 or C3: - From C1: $CH_2=CHCH_2CH_3$ (but-1-ene) - From C3: $CH_3CH=CHCH_3$ (but-2-ene, more stable) - Major product: but-2-ene (mixture of cis and trans). **Ex 3**: Can tert-butanol be oxidised by acidified potassium dichromate? - Tert-butanol: $(CH_3)_3COH$ - No hydrogen on the carbon bearing –OH → cannot lose H⁺ during oxidation. - **Answer**: No, resistant to oxidation. --- ## II. Preparation and manufacture of alcohols ### Laboratory methods 1. **Hydration of alkenes** (Markovnikov addition): $CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$ - Industrially: ethene + steam, $H_3PO_4$ catalyst, 300°C, 60–70 atm. 2. **Reduction of carbonyl compounds**: - Aldehydes → primary alcohols $RCHO + 2[H] \rightarrow RCH_2OH$ (NaBH₄ or LiAlH₄) - Ketones → secondary alcohols $R_2CO + 2[H] \rightarrow R_2CHOH$ 3. **Hydrolysis of alkyl halides**: $R-X + NaOH(aq) \rightarrow R-OH + NaX$ (SN1 or SN2) - Limited by competing elimination (especially with tertiary halides or strong base/heat). ### Industrial manufacture - **Ethanol**: - Fermentation: $C_6H_{12}O_6 \xrightarrow{yeast} 2C_2H_5OH + 2CO_2$ (glucose → ethanol + CO₂) - Used for beverages, biofuel. - Requires anaerobic conditions, yeast (*Saccharomyces cerevisiae*), 25–35°C. - Petrochemical: hydration of ethene (as above). - **Methanol**: - Synthesis gas (syngas) reaction: $CO + 2H_2 \xrightarrow{ZnO/Cr_2O_3, 300^\circ C, 50–100 atm} CH_3OH$ - Modern Cu/ZnO/Al₂O₃ catalyst, lower T/P. - **Propan-2-ol**: - Hydration of propene: $CH_3CH=CH_2 + H_2O \xrightarrow{H^+} CH_3CH(OH)CH_3$ ### Step-by-step example: Fermentation of glucose 1. Glucose ($C_6H_{12}O_6$) enters yeast cell. 2. Glycolysis: glucose → 2 pyruvate + net 2 ATP + 2 NADH. 3. Pyruvate decarboxylation: pyruvate → acetaldehyde + CO₂ (thiamine pyrophosphate-dependent). 4. Acetaldehyde reduction: acetaldehyde + NADH + H⁺ → ethanol + NAD⁺ (alcohol dehydrogenase). 5. Overall: $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$ ### Application exercises (corrected) **Ex 1**: Why is acid catalysis used in alkene hydration? - The alkene is nucleophilic; protonation by acid generates a carbocation, which is electrophilic and attacked by water. - Without acid, water is not electrophilic enough to add directly. **Ex 2**: Compare fermentation and petrochemical routes for ethanol. | Feature | Fermentation | Petrochemical (hydration) | |--------|--------------|----------------------------| | Raw material | Biomass (sugar, cellulose) | Ethene (from crude oil) | | Conditions | 25–35°C, anaerobic, yeast | 300°C, 60–70 atm, $H_3PO_4$ | | By-products | CO₂ | None (high atom economy) | | Renewability | Renewable | Non-renewable | | Purity | Requires distillation | Higher purity initially | **Ex 3**: Can NaBH₄ reduce carboxylic acids to alcohols? - NaBH₄ is a mild reducing agent; reduces aldehydes and ketones but **not** carboxylic acids or esters under normal conditions. - LiAlH₄ is required for carboxylic acid reduction: $RCOOH + 4[H] \rightarrow RCH_2OH + H_2O$ --- ## III. Properties and uses of alcohols ### Physical properties (detailed) - **Boiling point trend**: Increases with molecular weight due to increased surface area and London forces. Hydrogen bonding causes a significant jump from alkanes. - Ethane: –89°C; Ethanol: 78°C (difference due to H-bonding). - **Solubility in water**: - Methanol, ethanol, propan-1-ol: miscible in all proportions. - Butan-1-ol: 7.9 g/100 mL water (limited due to hydrophobic butyl chain). - Trend: solubility decreases as alkyl chain length increases. - **Density**: All common alcohols < 1 g/mL (except fluorinated); methanol: 0.79, ethanol: 0.79, propan-1-ol: 0.80. ### Chemical properties (expanded) 1. **Esterification**: Reaction with carboxylic acids (acid-catalysed). $R-OH + R'COOH \xrightleftharpoons{H^+} R'COOR + H_2O$ - Reversible; equilibrium constant ~4 for simple esters. - Driven to completion by excess reactant or removal of water/eester. 2. **Formation of alkoxides**: Reaction with active metals. $2ROH + 2Na \rightarrow 2RONa + H_2$ - Sodium ethoxide: $CH_3CH_2ONa$, used in Claisen condensation. 3. **Tosylation**: Conversion to better leaving group. $ROH + TsCl \xrightarrow{pyridine} ROSO_2C_6H_4CH_3 + HCl$ - Tosylate (OTs) is excellent leaving group for SN2. ### Uses (expanded) - **Ethanol**: - Disinfectant (70% in water): denatures proteins, disrupts membranes. - Fuel: blended with petrol (E10, E85); burns cleanly: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$ - Solvent: for perfumes, dyes, pharmaceuticals. - **Methanol**: - Feed for formaldehyde: $2CH_3OH + O_2 \rightarrow 2HCHO + 2H_2O$ - Antifreeze in windshield washer fluid. - **Toxicity**: metabolised to formaldehyde and formic acid → acidosis, blindness. - **Isopropyl alcohol**: - Rubbing alcohol (70%): disinfectant for skin, surfaces. - Acetone production via oxidation: $(CH_3)_2CHOH \xrightarrow{[O]} (CH_3)_2CO$ - **Butanol**: - Precursor to butyl acrylate (paints, adhesives). - Potential biofuel (higher energy density than ethanol, less hygroscopic). ### Application exercises (corrected) **Ex 1**: Explain why ethanol is miscible with water but hexanol is not. - Ethanol: short ethyl group; hydroxyl group dominates → strong H-bonding with water. - Hexanol: long hydrophobic hexyl chain disrupts water H-bond network; entropy decrease outweighs H-bond gain. **Ex 2**: A student adds sodium metal to ethanol and observes bubbling. What is the gas? Write the equation. - Gas: hydrogen ($H_2$). - $2CH_3CH_2OH + 2Na \rightarrow 2CH_3CH_2ONa + H_2$ **Ex 3**: Why is concentrated sulfuric acid used in esterification rather than hydrochloric acid? - $H_2SO_4$ is a strong acid and dehydrating agent; it protonates the carbonyl (increasing electrophilicity) and absorbs water, shifting equilibrium. - HCl is volatile and less effective at water removal; can cause side reactions (e.g., substitution if alcohol is primary). --- ## IV. Structure and nomenclature of carboxylic acids (C1–C4) ### Definition and general structure A carboxylic acid contains the carboxyl group: $–C(=O)OH$ or $–COOH$. The carbon is sp² hybridised, bonded to one hydroxyl and one carbonyl oxygen. General formula for saturated acyclic carboxylic acids: $C_nH_{2n}O_2$. > **To retain**: The carboxyl group (–COOH) consists of a carbonyl group and a hydroxyl group attached to the same carbon atom. ### Nomenclature (IUPAC) 1. Longest chain containing the carboxyl carbon. 2. Number the chain to give the carboxyl carbon number 1. 3. Replace "-e" of alkane with "-oic acid". 4. Name and number substituents. **Examples**: - $HCOOH$: Methanoic acid (formic acid) - $CH_3COOH$: Ethanoic acid (acetic acid) - $CH_3CH_2COOH$: Propanoic acid - $CH_3CH_2CH_2COOH$: Butanoic acid - $CH_3CH(CH_3)COOH$: 2-methylpropanoic acid (isobutyric acid) ### Step-by-step example: Name $CH_3CH_2CH(CH_3)COOH$ 1. Carboxyl carbon is C1. 2. Chain: 4 carbons (butanoic acid). 3. Number from carboxyl end: $HOOC–CH_2–CH(CH_3)–CH_3$ 4. Substituent: methyl on C3. 5. Name: 3-methylbutanoic acid. ### Physical properties - **Boiling point**: Higher than alcohols of similar MW due to dimer formation via two H-bonds. - Methanoic acid: 101°C - Ethanoic acid: 118°C (dimers in liquid and gas phases) - Propanoic acid: 141°C - Butanoic acid: 164°C - **Solubility**: - Methanoic, ethanoic, propanoic acids: miscible with water. - Butanoic acid: soluble (~10 g/100 mL); longer chains less soluble. - **Acidity**: pKa values: - Methanoic acid: 3.75 - Ethanoic acid: 4.76 - Propanoic acid: 4.87 - Butanoic acid: 4.82 - Trend: inductive effect of alkyl groups slightly destabilises conjugate base; but effect is small for C1–C4. - **Odour**: Sharp, pungent (vinegar for ethanoic); longer chains smell rancid or cheesy. ### Chemical properties 1. **Acidity**: $RCOOH \rightleftharpoons RCOO^- + H^+$ - Resonance stabilisation of carboxylate anion: two equivalent C–O bonds (bond order 1.5). - More acidic than alcohols and phenols (except very electron-withdrawing substituted phenols). 2. **Reactions**: - **With metals**: $2RCOOH + 2Na \rightarrow 2RCOONa + H_2$ - **With bases**: $RCOOH + NaOH \rightarrow RCOONa + H_2O$ (neutralisation) - **With carbonates**: $2RCOOH + Na_2CO_3 \rightarrow 2RCOONa + H_2O + CO_2$ (effervescence) - **Esterification**: with alcohols (acid-catalysed, reversible) - **Reduction**: to primary alcohols (LiAlH₄, then $H_2O^+$) $RCOOH + 4[H] \rightarrow RCH_2OH + H_2O$ - **Decarboxylation**: rare for simple acids; requires β-keto acid or heating with soda lime (for aromatic). ### Step-by-step example: Reaction of ethanoic acid with sodium bicarbonate 1. Acid-base reaction: proton transfer from acid to bicarbonate. $CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2$ 2. Observation: effervescence due to CO₂ gas. 3. Sodium acetate formed: soluble salt, used in buffering. ### Uses of carboxylic acids - **Methanoic acid**: - Preservative in livestock feed (antibacterial). - Coagulant in rubber latex. - Used in textile dyeing and leather tanning. - **Ethanoic acid (vinegar)**: - 5–8% solution: food preservative, flavouring. - Glacial acetic acid (>99%): solvent, precursor to vinyl acetate monomer (for PVA glue, paints). - Production of cellulose acetate (fibres, films). - **Propanoic acid**: - Food preservative (E280): inhibits mould in bread, cheese. - Herbicide. - **Butanoic acid**: - Component of rancid butter, body odour. - Used in flavourings (e.g., cheese, butter flavours). - Precursor to butyl esters (solvents, plasticisers). ### Application exercises (corrected) **Ex 1**: Why is ethanoic acid more acidic than ethanol? - Ethanoic acid: conjugate base (acetate) stabilised by resonance: two equivalent O–C bonds. - Ethoxide ion: negative charge localised on oxygen; no resonance stabilisation. - Resonance energy > inductive effect of methyl group. **Ex 2**: Write the equation for the reaction of propanoic acid with sodium hydroxide. - Neutralisation: $CH_3CH_2COOH + NaOH \rightarrow CH_3CH_2COONa + H_2O$ **Ex 3**: A carboxylic acid reacts with sodium carbonate and produces CO₂. What does this indicate? - Indicates acidic proton capable of protonating carbonate: confirms carboxylic acid (or stronger acid) functionality. - Alcohols do not react with Na₂CO₃ to produce CO₂. --- ## V. Preparation, properties and uses of carboxylic acids ### Laboratory preparation 1. **Oxidation of primary alcohols**: - Requires strong oxidant and heat to go beyond aldehyde. - $RCH_2OH + 2[O] \rightarrow RCOOH + H_2O$ - Reagents: acidified $K_2Cr_2O_7$ or $KMnO_4$, reflux. - Apparatus: reflux condenser to prevent aldehyde escape. 2. **Oxidation of aldehydes**: - Mild oxidants (Tollens’ reagent) stop at acid; strong oxidants ensure completion. - $RCHO + [O] \rightarrow RCOOH$ 3. **Hydrolysis of nitriles**: - $RCN + 2H_2O \xrightarrow{H^+ \text{ or } OH^-} RCOOH + NH_3$ - Two-step: amide intermediate. - Example: $CH_3CN + 2H_2O \rightarrow CH_3COOH + NH_3$ ### Industrial manufacture - **Ethanoic acid**: - **Fermentation**: acetobacter oxidises ethanol: $CH_3CH_2OH + O_2 \rightarrow CH_3COOH + H_2O$ - Requires aerobic conditions, *Acetobacter* spp., surface culture (Orleans process) or submerged. - Used for vinegar production. - **Carbonylation of methanol** (Monsanto/Cativa process): $CH_3OH + CO \xrightarrow{[Rh/Ir] \text{ catalyst}, 150–200^\circ C, 30–60 atm} CH_3COOH$ - High atom economy, >95% yield. - Dominant industrial method (>70% of global production). - **Methanoic acid**: - Byproduct of acetate oxidation or formaldehyde production. - $CH_3OH + O_2 \rightarrow HCOOH + H_2O$ (catalytic oxidation). - Also from hydrolysis of methyl formate: $HCOOCH_3 + H_2O \rightarrow HCOOH + CH_3OH$ - **Propanoic and butanoic acids**: - Fermentation by certain bacteria (e.g., *Propionibacterium*). - Oxidation of corresponding alcohols or aldehydes. - Reppe synthesis: hydrocarboxylation of alkenes (e.g., ethene + CO + H₂O → propanoic acid). ### Step-by-step example: Oxidation of ethanol to ethanoic acid 1. Set up reflux apparatus: round-bottom flask, condenser, heating mantle. 2. Add ethanol and excess acidified potassium dichromate solution (orange). 3. Heat gently: ethanol → acetaldehyde (distills if not refluxed). 4. Under reflux: acetaldehyde retained, further oxidised to ethanoic acid. 5. Colour change: orange (Cr₂O₇²⁻) → green (Cr³⁺). 6. After reaction, distil mixture: ethanoic acid boils at 118°C; water at 100°C. 7. Test product: turns blue litmus red; reacts with NaHCO₃ to give CO₂. ### Properties (reinforced) - **Acidity**: as above; carboxylic acids are weak acids but stronger than phenols with electron-donating groups. - **Reactivity**: carbonyl carbon electrophilic; hydroxyl can be protonated or converted to better leaving group. - **Intermolecular forces**: strong dimerisation via H-bonding increases effective molecular weight. ### Uses (expanded) - **Ethanoic acid**: - Production of vinyl acetate: $CH_3COOH + C_2H_4 \rightarrow CH_2=CHOCOCH_3$ (then polymerised to PVAc). - Manufacture of acetone via ketene: $2CH_3COOH \xrightarrow{700^\circ C} CH_2CO + H_2O + CO_2$; $CH_2CO + H_2O \rightarrow CH_3COCH_3$ - Inorganic acetate: e.g., copper(II) acetate (fungicide, pigment). - **Longer chain acids**: - Soap production (see Section VI). - Plasticisers: phthalates (from phthalic acid, aromatic). - Flavours and fragrances: esters of fatty acids (e.g., ethyl butyrate = pineapple). ### Application exercises (corrected) **Ex 1**: Why is reflux necessary when oxidising a primary alcohol to a carboxylic acid? - To prevent loss of volatile aldehyde intermediate; allows further oxidation to acid. - Without reflux, aldehyde escapes and yield is low. **Ex 2**: Compare the fermentation and carbonylation methods for ethanoic acid production. | Feature | Fermentation | Carbonylation (Monsanto) | |--------|--------------|---------------------------| | Raw material | Ethanol (from fermentation or petrochemical) | Methanol, CO | | Catalyst | *Acetobacter* (enzyme) | Rh/Ir complex, iodide promoter | | Conditions | 30°C, aerobic, aerobic | 150–200°C, 30–60 atm | | By-products | Water, biomass | None (high atom economy) | | Purity | Dilute (~10%), requires distillation | Concentrated (~99%), pure | | Scale | Small (vinegar, specialty) | Large (industrial, >90% of supply) | **Ex 3**: Can ethanoic acid be prepared by oxidation of acetaldehyde? - Yes: $CH_3CHO + [O] \rightarrow CH_3COOH$ - Oxidising agents: acidified $K_2Cr_2O_7$, $KMnO_4$, or even Tollens’ reagent (in basic medium, gives acid after acidification). --- ## VI. Esters and saponification ### Ester formation (esterification) - Reaction between a carboxylic acid and an alcohol, catalysed by acid. $RCOOH + R'OH \xrightleftharpoons{H^+} RCOOR' + H_2O$ - Mechanism: nucleophilic acyl substitution. 1. Protonation of carbonyl oxygen (makes C more electrophilic). 2. Attack by alcohol on carbonyl carbon → tetrahedral intermediate. 3. Proton transfer. 4. Elimination of water (after protonation of OH). 5. Deprotonation to yield ester. - Reversible; equilibrium constant typically 1–10. - To drive forward: excess alcohol or acid, or remove water/ester (distillation, molecular sieves). > **To retain**: Esterification is a reversible acid-catalysed nucleophilic acyl substitution reaction producing an ester and water. ### Step-by-step example: Synthesis of ethyl ethanoate 1. Mix ethanoic acid and ethanol in 1:1 molar ratio (or excess ethanol). 2. Add few drops concentrated $H_2SO_4$ as catalyst. 3. Heat under reflux for 1–2 hours. 4. Distil mixture: ethyl ethanoate (bp 77°C) separates; water and unreacted acids remain. 5. Wash distillate with sodium carbonate solution to remove acid, then dry ($MgSO_4$). 6. Redistil to purify. - Overall: $CH_3COOH + CH_3CH_2OH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O$ ### Hydrolysis of esters - **Acidic hydrolysis**: reverse of esterification. $RCOOR' + H_2O \xrightleftharpoons{H^+} RCOOH + R'OH$ - **Basic hydrolysis (saponification)**: $RCOOR' + NaOH \rightarrow RCOONa + R'OH$ - Irreversible because carboxylate anion is stabilised and does not reprotonate under basic conditions. - Produces alcohol and carboxylate salt (soap if R is long alkyl chain). > **To retain**: Saponification is the base-promoted hydrolysis of an ester, yielding a carboxylate salt and an alcohol; it is irreversible under the reaction conditions. ### Step-by-step example: Saponification of ethyl ethanoate 1. Mix ethyl ethanoate with aqueous sodium hydroxide. 2. Heat gently (to increase rate; avoid boiling off ethanol). 3. Reaction: $CH_3COOCH_2CH_3 + NaOH \rightarrow CH_3COONa + CH_3CH_2OH$ 4. After reaction, acidify mixture with dilute HCl: $CH_3COONa + HCl \rightarrow CH_3COOH + NaCl$ 5. Distil to recover ethanol (bp 78°C) and ethanoic acid (bp 118°C). - Note: saponification is used to make soap from triglycerides (see next section). ### Uses of esters - **Fragrances and flavours**: - Ethyl butyrate: pineapple - Isoamyl acetate: banana oil - Methyl salicylate: wintergreen - Benzyl acetate: jasmine, pear - **Solvents**: ethyl ethanoate (nail polish remover), butyl ethanoate (lacquers). - **Plasticisers**: dioctyl phthalate (DOP) in PVC. - **Polyesters**: terephthalic acid + ethylene glycol → PET (bottles, fibres). ### Application exercises (corrected) **Ex 1**: Why is saponification irreversible? - The carboxylate anion formed is resonance-stabilised and a poor electrophile; under basic conditions, it is not protonated to reform the carboxylic acid, so the equilibrium lies far to the right. **Ex 2**: Write the equation for the saponification of methyl propanoate. - $CH_3CH_2COOCH_3 + NaOH \rightarrow CH_3CH_2COONa + CH_3OH$ **Ex 3**: An ester has a fruity smell. When boiled with NaOH, it produces a salt that turns red litmus blue and an alcohol. What does this indicate? - Indicates basic hydrolysis (saponification) occurred: ester → carboxylate salt (basic) + alcohol. - Confirms the presence of an ester functional group. ---