RI Carbonyl Compounds Tutorial - Answers to Discussion Questions

-1- RAFFLES INSTITUTION YEAR 6 H2 CHEMISTRY 2025 Tutorial 18 Carbonyl Compounds Reaction Mindmaps  Aldehydes R C H O aldehyde RC OH H H R C OH O R C O O + Ag R C O O + Cu 2O RC OH H CN H C O O + CH I3 Na N O2N NO2N H C R H K2Cr2O7(aq) / H2SO4(aq), heat or KMnO4(aq) / H2SO4(aq), heat oxidation silver mirror Tollens' reagent, heat oxidation reddish- brown ppt Fehling's solution, heat oxidation yellow ppt I2(aq), NaOH(aq), heat oxidation orange ppt 2,4-DNPH condensation LiAlH4 in dry ether or NaBH4 in methanol or H2/Ni, heat K2Cr2O7(aq), H2SO4(aq) heat with immediate distillation oxidation reduction *Ethanal is the only aldehyde that has positive test (R=CH 3) HCN,trace amt of KOH(aq) or HCN,trace amt of KCN(aq) or KCN(aq), H2SO4(aq) RC OH H CH2NH2 nucleophilic addition LiAlH4 in dry ether or H2/Ni, heat reduction RC OH H COOH H2SO4(aq), heat acidic hydrolysis *Positive test for ALL aldehydes *Positive test only for aliphatic aldehydes 1o alcohol 2,4-dinitrophenylhydrazone tri-iodomethane cyanohydrin

-2-  Ketones

-3- Answers to self-check questions Q1 Q2(a) LiAlH4 in dry ether, room temperature OR H2(g), Ni(s), heat OR NaBH4, room temperature Q2(b) Heat propan-1-ol with acidified K 2Cr2O7 carefully with immediate distillation of the product formed to collect mainly propanal as distillate. Q2(c) Observations: Orange ppt is formed. Answers to MCQ: 3) B 4) C 5) B 6) D 7) B

-4- Suggested Solutions to Practice Questions: 1 (a) (i) Mechanism: Nucleophilic addition NaCN(aq)  Na+(aq) + CN(aq) Step 1 Step 2 (ii) NaCN provides the initial CN – ions for the nucleophilic attack on the carbonyl carbon. HCN is used in the second step as a (Bronsted) acid to protonate the anionic intermediate. Note: Do NOT merely say that NaCN helps to increase the rate of reaction and HCN is the reagent, as the question is about the roles of NaCN and HCN in the mechanism. (iii) The carbonyl carbon in CH3CHO is bonded to one alkyl group while that in (CH 3)2CO is bonded to two alkyl groups. Hence, the carbonyl carbon in (CH 3)2CO is less electron deficient (less +) due to the additional electron d onating alkyl group. There is also greater steric hindrance about the carbonyl carbon in (CH 3)2CO which hinder the approach of the attacking nucleophile. Therefore, propanone, (CH 3)2CO, reacts at a slower rate than ethanal, CH3CHO. (b) Acidic hydrolysis reaction OH O OH (c) (i) Lactic acid has a chiral carbon and no pl ane of symmetry through the molecule. Lactic acid in milk rotates the plane of polarised light because only 1 enantiomer is present (i.e. all the molecules present are of the same chirality). (ii) In the slow step, the CN  nucleophile can attack the planar carbonyl carbon of ethanal from either side with equal probability, result ing in the formation of a racemic mixture of two enanti