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POD02Key - =C-H bond stretching The terminal alkyne can be...

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Problem of the Day #02 – Answer Key Deadline : 3:00 p.m., Wednesday, 7/11/07 LATE WORK WILL NOT BE ACCEPTED OR GRADED!!! This problem is worth a total of 20 raw points. Each part is worth 10 points. In each part below, an infrared spectrum is shown. To the right of each spectrum, four structures are given. Circle the structure of the compound that produced the IR spectrum. You will earn 6 points for circling the correct structure. To earn the remaining 4 points for each part, explain briefly how you arrived at your answer (the amount of space provided should be enough for a suitable explanation). (a) Explanation : (b) Explanation : 4000 3000 2000 1500 1000 500 4000 3000 2000 1500 1000 500 O O OH O OH O OH Both alkenes can be eliminated, since there is no absorption between 1500 and 2000 cm –1 (corresponding to the C=C double bond stretch) or at 3100 cm –1 (corresponding to vinylic
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Unformatted text preview: =C-H bond stretching). The terminal alkyne can be eliminated since there is no absorption near 3300 cm –1 (corresponding to the acetylenic ≡C-H stretch). The only reasonable choice is circled – the C ≡C triple bond stretch does not appear near 2200 cm –1 because this vibration is often IR inactive for internal alkynes! The strong, broad absorption centered near 3400 cm –1 indicates the presence of a hydroxyl group, eliminated the choice at the top left. The absence of any absorptions in the range between 1500 and 2000 cm –1 suggests that there is no carbonyl group present (eliminating the carboxylic acid), nor is there an alkene double bond present (eliminating the choice on the bottom right). By process of elimination, the circled choice appears to the only reasonable one!...
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