RI 2026 Carboxylic Acids Derivatives Tutorial Answers

1 RAFFLES INSTITUTION YEAR 6 H2 CHEMISTRY 2026 Tutorial 19 – Carboxylic Acids and Derivatives ___________________________________________________________________________________ Answers to Self-check questions 1 A. C O CH3 OH ROH, conc H2SO4, heat NH3(aq) Na(s) or NaOH(aq) or Na2CO3(aq) C O CH3 O − Na+ C O CH3 O − NH4 + C O CH3 Cl C OH CH3 H H C O CH3 OR C O CH3 NR2CH3CN or or C O CH3 OR or C O CH3 H H2SO4(aq) heat C OH CH3 H H C O CH3 Cl C C CH3 HH CH3 KMnO4(aq), H2SO4(aq), heat or K2Cr2O7(aq), H2SO4(aq), heat H2O, r.t. KMnO4(aq), H2SO4(aq), heat PCl5 LiAlH4 B. C O CH3 Cl NaOH(aq)H2O(l) ROH NR2H NH3 O − Na+ C O CH3 NH2 C O CH3 NR2 C O CH3 O − Na+C O CH3 OH C O CH3 OR C O CH3 O

2 2 (a) CO2H (b) CO2H Cl (c) O O (d) HO2C CO2H 3 (a) 2,5-dimethylhexanedioic acid (b) ethyl 2,2-dimethylpropanoate (c) 3-nitrobenzoic acid (d) 2,2-dimethylpropanenitrile (e) 3-ethylhexanoic acid (f) 4,5-dibromopentanoic acid 4a Both hydroxyethanoic acid and ethanoic acid are polar molecules held together by strong intermolecular hydrogen bonds (due to the COOH group present). Hydroxyethanoic acid has more extensive hydrogen bonding as there is an additional – OH group that can be involved in hydrogen bonding. Thus, more energy is needed to overcome the stronger and more extensive hydrogen bonds between hydroxyethanoic acid molecules and it exists as a solid whereas ethanoic acid exists as a liquid. 4b Ethanoic acid exists as polar molecules held together by stronger intermolecular hydrogen bonds while ethyl ethanoate exists as polar molecules held together by weaker permanent dipole-permanent dipole interactions. More energy is required to overcome the stronger hydrogen bonds between ethanoic acid molecules thus it has a higher boiling point. 4c Benzoic acid can form hydrogen bonds with water. However, benzoic acid has limited solubility in water as its large non-polar benzene ring interferes with the formation of hydrogen bonding between the -CO 2H group and water. The energy released in formation of hydrogen bonds between benzoic acid and water is insufficient to compensate the energy required to break the hydrogen bonds between water molecules. Sodium benzoate, an ionic compound, is soluble in water as it can form ion-dipole interactions with water. These interactions are strong enough to release sufficient energy during hydration to overcome strong ionic bonds in sodium benzoate. 5 C There are 2 ester linkages (circled in dotted rectangles) and 5 chiral carbon atoms (indicated with asterisks) as shown below. O H N O N CH3 O O O O N H O yanucamide B * * * * * 6 C Upon acidic hydrolysis of the esters, only the alcohol (ethanol) formed in option C will undergo oxidation with alkaline aqueous iodine (positive iodoform test) as it has the CH 3CH(OH)- group.

3 7 B (A) CH3COCl + 2OH− → H3COO− + Cl− + H2O Amt of Cl− = Amt of AgCl = 1 ÷ 78.5 = 0.0127 mol Mass of AgCl = 0.0127 × (35.5+108.0) = 1.83 g (C) No reaction occurs due to partial double bond character in C-Cl bond. Mass of AgCl = 0 g (B) CH2Cl CH2Cl + 2OH − + 2Cl − CH2OH CH2OH Amt of Cl− = 2(1 ÷ 153) = 0.01307 mol Mass of AgCl = 0.01307 × (35.5 + 108.0) = 1.88 g (D) + 2OH − + 2Cl − Cl Cl O O O − O − O O Amt of Cl− = 2(1 ÷ 155) = 0.0129 mol Mass of AgCl = 0.0129 × (35.5 + 108.0) = 1.85 g 8 All statements (1, 2 and 3) are correct. HO OCH3 2,4-dinitrophenylhydrazine NH NH2 NO2 NO2 + estrone HO CH3 NH N O2N NO2 + H2O (1) HO OCH3 + estrone (2) Na +Na−O OCH3 + 0.5 H2 HO OCH3 + estrone (3) CH3COCl OCH3 + HCl O O

4 Suggested Answers to Carboxylic Acid & Derivatives Tutorial 1(a) Decreasing acid strength: ethanoic acid > phenol > ethanol In general, HA + H2O ⇌ H3O+ + A– The acidity of a compound depends on the relative stability of its conjugate base anion (A –). The more stable the conjugate base, the more acidic the compound will be. In the ethanoate ion (CH 3CO2–), the negative charge on oxygen is dispersed over the two highly electronegative oxygen atoms resulting in two equivalent resonance structures . Hence, CH3CO2− ion is resonance-stabilised to a larger extent than C6H5O– ion. In the phenoxide ion (C6H5O–), the p-orbital containing the lone pair of electrons on the O atom overlaps with the π-electron cloud of the benzene ring so that the negative charge on O delocalises into the benzene ring. The dispersal of negative charge stabilises C6H5O– ion but this resonance stabilisation is not as great as that in the CH3CO2− ion. In the ethoxide ion (CH3CH2O–), the electron-donating alkyl (ethyl) group intensifies the negative charge on O atom , which destabilises CH3CH2O– ion. Therefore, CH3CH2O– is the least stable. (b) Decreasing acid strength: trifluoroethanoic acid > trichloroethanoic acid > chloroethanoic acid > ethanoic acid In general, HA + H2O H3O+ + A– The acidity of a compound depends on the relative stability of its conjugate base anion (A–). The more stable the conjugate base, the more acidic the compound will be. Compared to CH 3COO− ion, the CF 3COO−, CC l3COO− and C lCH2COO− ions are more stable due to the presence of electron-withdrawing F or C l group(s) to disperse the negative charge and hence stabilising the anion. CCl3COO− ion is more stable than C lCH2COO− ion as it has 2 more electron-withdrawing Cl groups to disperse the negative charge. Since F is more electronegative than Cl, the F group is more electron -withdrawing than the Cl group and the negative charge is dispersed to a greater extent in CF3COO− ion than in CCl3COO− ion. Hence CF3COO− ion is more stable than CCl3COO− ion. 2 Decreasing ease of hydrolysis: C6H5COCl > C6H5CH2Cl > C6H5Cl The carbon of the acyl group in C 6H5COCl has a higher δ+ charge (or is more electron deficient) as it is bonded to two electronegative atoms (O and C l). The carbon bonded to the chlorine atom in C6H5CH2Cl has lower δ+ charge (or is less electron deficient) as it is bonded to only one electronegative atom (Cl). Hence, C6H5COCl can attract nucleophiles more easily and is more susceptible to nucleophilic attack as compared to C6H5CH2Cl. In addition, the carbon of the acyl group in C 6H5COCl, being sp2 hybridised and trigonal planar, provides less steric hindrance during nucleophilic attack compared to the carbon bonded to the chlorine atom in C6H5CH2Cl, which is sp3 hybridised and tetrahedral. Thus, C6H5COCl undergoes hydrolysis with ease with water (a weak nucleophile) while C6H5CH2Cl requires a stronger nucleophile OH− under heating.

5 C6H5Cl is the least susceptible to hydrolysis. This is because the p orbital containing the lone pair of electrons on the C l atom overlaps with the π electron cloud of the benzene ring, resulting in a lone pair of electrons in the p orbital of Cl delocalising into the benzene ring. As a result, the C-Cl bond has partial double bond character. Since the bond is strengthened, the cleavage of this bond (which is necessary during hydrolysis) is made very difficult. 3 (a) Test: Add aq Na 2CO3 (or aq NaHCO3) to each compound in a test-tube. Observation: For C 6H5COOH: Effervescence observed, CO 2 gas evolved formed white ppt with limewater/aq Ca(OH)2. For C6H5OH: No effervescence / no gas is evolved. Other tests: Or aqueous Br 2: Phenol decolourises orange Br2 and a white ppt is formed. No decolourisation of Br2 and no white ppt for benzoic acid. (b) Test: Add aq AgNO3 to each compound in a test-tube. Observation: For CH 3CH2COCl: White ppt ( AgCl) formed and white fumes (HCl) evolved. For ClCH2CH2COOH: No white ppt and no white fumes. Note: Aq. Na 2CO3 should not be used as a distinguishing test between acid chlorides and carboxylic acids. Acid chlorides s