2025 VJC Carbonyl Compounds Tutorial

15 Victoria Junior College JC2 H2 Chemistry Tutorial: Carbonyl Compounds Complete the following tables. 1. Preparation of aldehydes and ketones starting compound reagents and conditions product (CH3)2CHCH2CHO (CH3)2CHCH(OH)CH3 K2Cr2O7(aq), H2SO4(aq), heat 2. Reactions of aldehydes and ketones reaction type reagents & conditions organic product(s) [if any] for (CH3)2CHCH2CHO (CH3)2CHCOCH3 HCN(aq), NaCN reduction @condensation K2Cr2O7(aq), H2SO4(aq), heat or KMnO4(aq), H2SO4(aq) heat

16 #Tollens’ reagebt or Fehling’s solution, heat *I2, NaOH(aq), heat Remarks: @ 2,4-DNPH is used to identify the carbonyl C=O functional group in aldehyde and ketone # Tollens’ reagent and Fehling’s solution is used to identify aldehyde and aliphatic aldehyde respectively * Alkaline aqueous I2 is used to identify CH 3CH(OH)– or CH 3CO– structural unit in organic compounds Reduction of Aldehydes and Ketones 1 A fresh salad may contain the following compounds. X CH3CH2CH2CH=CHCHO, an aroma of tomatoes Y CH3CH2CH2CH2CH=CHCOCH3, a flavour of mushrooms Z CH3CH2CH2CH2CH=CHCH=CHCHO, an aroma of cucumbers If these compounds are all present in the ratio X : Y : Z = 2 : 2 : 1, and all are reduced by a solution of an excess of NaBH 4, how many hydrogen atoms would be incorporated on average per molecule? A 2.0 B 2.4 C 3.6 D 4.4 N10/I/2

17 Organic Synthesis 2 Suggest how the following transformation can be achieved from the following starting compound: Indicate reagents and conditions you would use for each step. [7] (a) (b) (c)

18 (d) (e) (f)

19 (g) Nucleophilic Addition Mechanism 3 The simplest amide, methanamide, HCONH2, can be dehydrated to hydrogen cyanide, HCN. HCONH2 → HCN + H2O (a) Draw the mechanism of the reaction between hydrogen cyanide and ethanal, CH 3CHO, to form 2-hydroxypropanenitrile, CH3CH(OH)CN. [2]

20 (b) In some circumstances, double bonds will undergo a nucleophilic addition reaction. By applying your understanding of the following concepts: (i) electronegativity; (ii) electronic and steric effects; (iii) delocalisation of electrons, suggest reasons to explain why the following reactions occur or do not occur. [3] N18/III/1(e) [modified] ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………….

21 Distinguishing tests Use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect carbonyl functional group 4 Two female sex hormones are oestrone and oestradiol. HO OCH3 oestrone HO OHCH3 oestradiol Which of the following reagents could be used to distinguish between the two hormones? A Br2(aq) at room temperature B 2,4-dinitrophenylhydrazine C Fehling’s or Tollen’s reagent D I2 in NaOH(aq) J00/III/24 Use of Tollens’ and Fehling’s reagents to detect aldehydes 5 Which statement is correct for both 2,3-dihydroxypropanal and 1,3-dihydroxypropanone? A They are chiral. B They give a silver mirror with Tollens’ reagent. C They give a yellow precipitate with alkaline aqueous iodine. D They have the same empirical formula. N14/I/23 6(a) Three compounds E, F and G all have the molecular formula C 3H6O. E is an alcohol, F is a ketone, and G is an aldehyde. (i) Draw possible structural formulae for E, F and G. [3] E F G

22 (ii) Describe tests (reagents, conditions and observations with each compound) that would allow you to show that: 1. E is an alcohol, whereas F and G are not. 2. F and G are carbonyl compounds, whereas E is not. 3. G is an aldehyde, whereas E and F are not. Write balanced equations for all reactions that occur. [3] Use of alkaline aqueous iodine to detect presence of a CH3CO– group (b) One of the compounds responsible for the flavour of butter is butane-2,3-dione: Give the structural formula of the organic products formed when butane -2,3-dione reacts completely with J97/I/7 (i) H2, Ni catalyst, heat [1] (ii) I2, NaOH(aq) [1]

23 Structural Elucidation 7 Citral, a constituent of lemon-grass oil, gives a silver mirror with Tollens’ reagent. When 0.100 g of citral is catalytically hydrogenated at 0 C and 1 bar, 44.2 cm 3 of hydrogen react to give (CH3)2CH(CH2)3CH(CH3)CH2CH2OH. Oxidation of citral with hot acidified potassium manganate(VII) gives initially three compounds, D, C2H2O4, E, C3H6O and F, C5H8O3. Suggest structural formulae for citral, D, E and F from the information given above. Explain your reasoning. [8] [Remark: Explanation should include reaction type, functional group type and stoichiometric calculation.]

24 Victoria Junior College JC2 Chemistry Supplementary Questions: Carbonyl Compounds 1 How many σ and π bonds are there in the following molecule? O σ π A 5 4 B 7 2 C 8 1 D 15 2 2 Propanone, (CH3)2CO, may be used as the starting point for synthesi sing methacrylic acid, H2C=C(CO2H)CH3. Which sequence would be most suitable? A (CH3)2CO → (CH3)2C(OH)2 → (CH3)2C(OH)CN → H2C=C(CO2H)CH3 B (CH3)2CO → (CH3)2CH(OH) → (CH3)2CH(CN) → H2C=C(CO2H)CH3 C (CH3)2CO → (CH3)2C(OH)CN → H2C=C(CN)CH3 → H2C=C(CO2H)CH3 D (CH3)2CO → H2C=C(OH)CH3 → H2C=C(CN)CH3 → H2C=C(CO2H)CH3 3 The compound C3H7Br undergoes a sequence of reactions as follows: C3H7Br OH– (aq) X acidified K2Cr2O7 Y Tollens' reagent Z + silver mirror What could be the formulae for X, Y and Z? X Y Z A CH3CH2CH2OH CH3CH2CO2H CH3CH2CHO B CH3CH2CH2OH CH3CH(OH)CH2OH CH3CO2H C CH3CH2CH2OH CH3CH2CHO CH3CH2CO2H D CH3CH(OH)CH3 CH3COCH3 CH3CO2H