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Spectroscopy · Part 3 of 4
6 exam-style questions with model answers, plus 8 quick multi-choice questions — every question on this part of the standard, grouped by the 2 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
State how many 13C NMR signals each of the following would give: (a) propanone, CH3COCH3; (b) butane, CH3CH2CH2CH3; (c) propan-1-ol, CH3CH2CH2OH.
A compound with molecular formula C4H8O shows a strong IR absorption at 1715 cm−1 and gives three 13C signals. Determine the structure, explaining how you eliminated the alternatives.
A student concludes that a compound with four 13C signals must contain exactly four carbon atoms. Justify why this reasoning is unsound, and evaluate what the signal count can and cannot establish about a structure.
State the approximate 13C chemical shift range for each of: an alkane carbon, a carbon bonded to oxygen, an alkene carbon, an ester carbonyl carbon and a ketone carbonyl carbon.
Explain why the carbonyl carbon of a ketone appears at a higher chemical shift than the carbonyl carbon of an ester, and explain why this is more reliable than using the IR carbonyl position to distinguish them.
A compound with molecular formula C4H8O2 gives three 13C signals at 14, 60 and 171 ppm, with an IR absorption at 1740 cm−1 and nothing above 3000 cm−1. Justify the structure by integrating all three techniques and eliminating every alternative.