DHS_H3_CHEM_ANSWERS

1 © DHS 2011 9812/01 [Turn over Answer any five questions. 1 (a) 1H NMR is a common analytical technique used to determine the molecular structures of various compounds. (i) Outline the basic working principles of NMR. [2] Nucleus of proton spins and generates a magnetic moment [0.5]. When a magnetic field is applied, the magnetic moment aligns with or against the applied magnetic field. To switch from the lower energy spin state that aligns with the applied field to the higher energy state that aligns against the applied field[0.5], radio–frequency radiation is absorbed [0.5]. The exact frequency of absorption depends on the chemical environment of the proton[0.5]. An unknown compound A, C 5H10O2, was synthesised in the laboratory. It is used to manufacture various propionates which are used as anti–bacterial agents. Its MS, IR and 1H NMR spectra data given below. (i) Deduce the structure of compound A, giving your reasoning. [8]

2 © DHS 2011 9812/01 [Turn over [1] absorption bond 1740 C=O 1200 C–O  Absence of peak at 3230 to 3500 cm–1 suggests absence of O–H stretch  No alcohol [1]  The presence of a C–O (strong and broad) at 1200 cm–1 and C=O at 1740 cm–1 confirm the identity of an ester functional group. [1] chemical shift /ppm splitting pattern No. of adjacent protons relative peak area group responsible 1.15 triplet 2 3 –CH 3 (a) [1] 1.25 triplet 2 3 –CH 3 (b) [1] 2.30 quartet 3 2 –CH 2CO– (adjacent to a –CO– and –CH 3) [1] 4.15 quartet 3 2 –OCH 2– (adjacent to a –CO– and –CH3) [1] m/e species 102 (C 5H10O2)+ [1] 57 (CH 3CH2CO)+ 29 (CH 3CH2)+

3 © DHS 2011 9812/01 [Turn over (b) Opium is the air–dried milky exudate, or latex, obtained by incising the unripe capsules of the opium poppy Papaver somniferum. It contains up to 12% morphine, an alkaloid, which is frequently processed chemically to produce heroin for the illegal drug trade. Morphine is also used as a narcotic analgesic and for the treatment of dry cough and diahorrea. morphine (i) Explain the term narcotic analgesic, using morphine as an example. [1] Narcotic analgesic, like morphine, is a substance that depresses the activity of the central nervous system resulting in pain relief. (ii) Narcotic and non–narcotic analgesics act differently to prevent pain. Briefly outline the modes of action of non–narcotic analgesics. [1] Non–narcotic analgesics work by inhibiting the cox enzyme and reducing the biosynthesis of prostaglandins. c (iii) Assign the stereochemical configuration about the chiral carbon atoms 1, 2 and 3. [2]     All correct [2], 1 correct [0.5], 2 correct [1] (c) Many drugs, like morphine, contain the tetrahydrofuran ring in their structures. Shown below is the puckered conformation