Question

The NMR spectrum of the aromatic section of a compound is presented in figure 1. 3. 78 7.7 74 7.5 74 73 7.2 7.1 70 7 64 3 62 ppm Figure 1: NMR spectrum of unknown. 6: 6.25 (d, J-3Hz ) , 6.38 (dd, J-8Hz , J-3Hz), 7.75 (d, J- 8Hz). acquire the NMR spectrum Determine which compound (A, B or C) was used to a. presented in figure 1 (1 mark). R Re H H. H. Ra HT HT H H R3 H H Compound C Compound B Compound A Justify your answer by numbering the protons according to their chemical environment and drawing the multiplicity of the aromatic protons from the correct compound b. (4 marks). In the compound below, number the aromatic protons only according to their chemical equivalence. Equivalent protons may be given the same number (1 mark). C. OH H H OCH3 With reasoning, determine which protons will be the most shielded. Illustrate your d. answer by drawing the resonance forms involved (4 marks). Gas chromatography has a narrower application range to compounds compared to that 4. of TLC and HPLC but is frequently used in the field of biological and clinical analysis Suggest the reasons for this, and briefly discuss the considerations which must be addressed prior to GC analysis (10 marks).

Answer 1

a, b) Given a: 6.25ppm: 1H, (d, 3Hz): means it has only one meta coupling partner. b: 6.38ppm: 1H, (dd, 8&3Hz): means it has ortho and meta coupling partner c: 7.75ppm: 1H, (d, 8Hz): means it has only one ortho coupling partner. There are three aromatic protons, which allow us to rule out compound-A Compound C is the only possiblity which gives given couppling pattern hence answer is compound-C j=3Hz, meta coupling Ha-Hb: coupling j-3Hz Hc-Hb: coupling j 8Hz Ha-Hb-Hb: coupling j 3 & 8Hz -8Hz, ortho b coupling R2 C R3 c) There are two ortho protons to -COOH group hence they are equivalent. There are two ortho protons to -OCH3 group hence they are equivalent. OH "H2 OCH3 2

d) Shielding occurs due to presence of more electron density on the respective protons. The electron donating groups (such as -OCH3) via resonance will make the ortho and para protons shielded. In the give compound we have -OCH3 group and that will make its ortho protons (H2) shielded due to resonance effect OH OH OH OH + Hас Нас Hас Hас