Spectroscopy — practice questions

Name the three spectroscopic techniques assessed in this standard and state which aspect of structure each is best at determining.

Explain why no single spectroscopic technique is sufficient to determine an organic structure, using an example.

Justify the distinction between determining a structure and justifying a structure in this standard, and explain what an Excellence answer must contain that a Merit answer does not.

A mass spectrum shows its highest significant peak at m/z 58 and its tallest peak at m/z 43. Identify the molar mass and explain the difference between the molecular ion peak and the base peak.

A compound has a molecular ion at m/z 60 and an IR spectrum showing a very broad absorption from 2500–3000 cm⁻¹ and a strong peak at 1715 cm⁻¹. Determine the molecular formula and structure, showing your reasoning.

A student reports that a compound with a molecular ion at m/z 45 must contain oxygen because 45 is an odd number. Justify whether this reasoning is sound, and explain what an odd molar mass actually indicates.

Explain how the mass spectrum distinguishes a compound containing chlorine from one containing bromine.

A compound with molecular ion at m/z 72 shows strong fragments at m/z 57 and m/z 43. Its IR shows a strong absorption at 1715 cm⁻¹ and nothing above 3000 cm⁻¹. Determine the structure, explaining how the fragments support your answer.

Two compounds both have a molecular ion at m/z 60. One shows a strong fragment at m/z 45 and one at m/z 31. Justify what each fragment indicates, and integrate this with what IR data would be needed to confirm each structure.

State the characteristic infrared absorptions used to identify an alcohol, a carboxylic acid, an amine and a ketone.

Explain how infrared spectroscopy distinguishes an alcohol from a carboxylic acid, and why the absence of an absorption is as useful as its presence.

Four compounds all show a strong C=O absorption. Justify how their exact carbonyl positions and other absorptions distinguish an aldehyde, a ketone, an ester and an amide, and evaluate how reliable the carbonyl position alone is as evidence.

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.

A compound has a molecular ion at m/z 46, an IR with a broad absorption at 3350 cm⁻¹ and no peak near 1700 cm⁻¹, and a single ¹³C signal region showing two signals at 18 and 58 ppm. Identify the molar mass, the functional group and the number of carbon environments.

Determine the structure of a compound with a molecular ion at m/z 58, an IR showing a strong absorption at 1715 cm⁻¹ and nothing above 3000 cm⁻¹ except C–H, and two ¹³C signals at 31 and 207 ppm.

A compound gives a molecular ion at m/z 78 and an M+2 peak at m/z 80 of about one third the height. Its IR shows C–H just below 3000 cm⁻¹ and nothing above 3000 or near 1700 cm⁻¹. Its ¹³C spectrum shows one signal at 25 ppm. Justify the structure fully, integrating all three techniques.