2021 YIJC Prelim P3 Answer

2021 YIJC Preliminary Examination H2 Chemistry Paper 3 with Mark Schemes Section A Answer all the questions in this section. 1 5-bromo-4-methyl-3-penten-2-ol can be made from 5 -bromo-4-methyl-3-penten-2-one in one step. The molecules are labelled as A and B respectively in Fig. 1.1. Fig. 1.1 (a) (i) State the total number of σ and π bonds in a molecule of A. [1] 16 σ and 2 π (ii) Describe the hybridisation of the orbitals and the bonds between the carbon atoms of the C=C double bond in A. [2] sp2 1 σ bond – head on overlap of sp2 hybridised orbitals of the 2 C atoms. 1 π bond – side on overlap of the unhybridised p orbitals of the 2 C atoms. (ii) State the total possible number of stereoisomers that can be exhibited by B. [1] 1 chiral carbon + 1 C=C that can exhibit cis trans isomerism = 22 = 4 (iv) Sodium borohydride is used to reduce A to form B.

Explain why only the ketone functional group in A undergoes reduction with sodium borohydride but the alkene functional group does not. [2] The carbonyl carbon in ketone is electron deficient due to the difference in electronegativity between C and O. Hence it attracts the :H– nucleophile. The C=C in alkenes are electron rich and will repel the negatively charged :H– nucleophile. (v) Describe a chemical test that can be used to distinguish A and B. [2] Test : 2,4 – DNPH, rtp A: orange ppt formed B : no orange ppt formed Test : KMnO4 , H2SO4(aq), heat A: purple KMnO4 remains B : purple KMnO4 decolourises Test : K2Cr2O7 , H2SO4(aq), heat A: orange K2Cr2O7 remains B : orange K2Cr2O7 turns green Test : Na, rtp A: no effervescence B : effervescence observed, gas extinguish lighted splint with a pop sound Test : anhydrous PCl5 , SOCl2, rtp A: no white fumes B : white fumes observed

Compound E can be made from A by a four-step synthesis. Fig. 1.2 (b) (i) Suggest the reagents and conditions for steps 2 and 3 in Fig. 1.2. [2] Step 2: ethanolic KCN, heat Step 3: H2SO4(aq), heat (ii) Compound E is neutral and does not react with 2,4 -dinitrophenylhydrazine or sodium metal. Draw the structures for compounds C, D and E. [3] C: D:

E: (iii) 5-chloro-4-methyl-3-penten-2-ol can be used as the reactant in step 2 instead of B in the reaction scheme in Fig. 1.2. Suggest how the rate of reaction will change if 5-chloro-4-methyl-3-penten-2-ol is used. [2] The chlorine atom has a smaller valence orbital / less diffused/ smaller size / smaller radius/ smaller electron cloud size than that of bromine. Hence, the effectiveness of orbital overlap is more effective between C and Cl, making the C–Cl bond stronger . Hence more ener