2023 RI H3 Chemistry Prelim Key Answer

© Raffles Institution 2023 9813/01/S/23 2023 Y6 H3 Chemistry Preliminary Exams – Suggested Solutions 1(a) It refers to the balance between high energy content with low sensitivities/good thermal stability (safety)/OWTTE. 1(b) The integration approach relies on the known properties of different types of structures/moiety, thus combining the advantages of each moiety in the design of the compound (OWTTE). Or The integration approach may accelerate the development of advanced EMs instead of developin g a molecule from scratch. 1(c)(i) AEMs need to have high energy content so that large amounts of energy can be released when detonated. 1(c)(ii) 2C(s) + N2(g) + H2(g) + ½O2(g) (g) 2 Hatomo of C BE(C–C) + 2 BE(C–H) + BE(N≡N) + BE(H–H) + 2 BE(N–O) + 2 BE(C=N) + BE(O=O) 2C(g) + 2N(g) + 2H(g) + 2O(g) Hf = [2 Hatomo of C + BE(N ≡N) + BE(H –H) + ½BE(O=O)] – [BE(C –C) + 2 BE(C –H) + 2 BE(N–O) + 2 BE(C=N)] = [(2x715) + 944 + 436 + ½(496)] – [350 + (2x410) + (2x201) + (2x610)] = +266 kJ mol–1 1(c)(iii) Standard enthalpy change of vaporisation of (liquid) 1,2,5-oxadiazole. 1(d) Tricyclic structure in compound A provides larger π-conjugated systems which increases the stability of the molecule. Hfo

© Raffles Institution 2023 9813/01/S/23 1(e) 1(f)(i) Condensation / addition-elimination 1(f)(ii) 1(f)(iii) (CH3CO)2O acts as a Lewis acid as it accepts an electron pair from HNO 3 leading to the formation of acetyl nitrate. H 2SO4 acts as a Bronsted-Lowry acid as it protonates HNO 3 leading to the formation of the NO2+ electrophile. 2(a)(i)  Amide J is less basic than amine H due to the delocalisation of the lone pair of electrons on the N atom into the C=O. H ence, the lone pair is not available to accept a proton.  Amidine K is more basic than amine H as the delocalisation of the nitrogen lone pair in the NH 2 group increases the electron density on the NH group. Accept “positive charge on conjugate acid is delocalised over both N atoms, stabilising the conjugate acid”.

© Raffles Institution 2023 9813/01/S/23 2(a)(ii) L is more basic than K as the additional NH can donate electron density onto the underlined NH, increasing electron density on the underlined N atoms. 2(a)(iii) The reaction of K is slower. The amide carbon in J is more electron deficient than the amidine carbon in K as oxygen is more electronegative than nitrogen, and is hence more susceptible towards nucleophilic attack. 2(a)(iv) The C=O bond in J absorbs at higher frequency than C=N bond in K as the C=O bond is stronger (740 kJ mol−1) than the C=N bond (610 kJ mol−1). Can accept explanation based on lower