Unit 13: Mass Spectrometry & IR Spectroscopy

These methods let students identify molecular formula and functional-group fingerprints before NMR fills in the rest of the structure.

• Interpret molecular ion, isotopic patterns, and common fragmentation clues in mass spectra.
• Use diagnostic IR stretches to recognize major functional groups quickly.
• Combine formula, IHD, MS, and IR to narrow structural possibilities efficiently.
• Explain the difference between a strong functional-group signal and a fingerprint-region pattern.

Checkpoint Questions

Q: What isotopic pattern suggests bromine in a mass spectrum?

A: A nearly equal-intensity M and M+2 pair, because 79Br and 81Br are present in roughly 1:1 abundance.

Q: What IR pattern strongly suggests a carboxylic acid?

A: A strong C=O stretch together with a very broad O-H band spanning roughly 2500–3300 cm−1.