Chapter 14: Infrared Spectroscopy and Mass Spectrometry

Infrared spectroscopy and mass spectrometry represent two complementary analytical techniques essential for determining molecular structure in organic chemistry. This chapter establishes the theoretical foundations and practical applications of these instrumental methods, enabling students to interpret spectroscopic data and deduce unknown compound structures. Infrared spectroscopy operates on the principle that molecules absorb electromagnetic radiation at frequencies corresponding to their vibrational modes, with each functional group producing characteristic absorption peaks at predictable wavenumbers. The chapter systematically presents the infrared absorption regions for major functional groups including alkanes, alkenes, alkynes, aromatic compounds, alcohols, ethers, amines, aldehydes, ketones, carboxylic acids, and esters, providing students with a reference framework for rapid functional group identification. Mass spectrometry ionizes organic molecules and fragments them into smaller ions, which are then separated by mass-to-charge ratio to produce a mass spectrum showing relative abundance patterns. The chapter covers ionization methods such as electron impact ionization and chemical ionization, explains the formation of molecular ions and fragment ions, and demonstrates how to interpret base peaks and isotope patterns to confirm molecular weight and infer structural features. Students learn to correlate specific fragmentation patterns with molecular structure, recognizing that fragmentation pathways often cleave bonds adjacent to heteroatoms or result from rearrangement reactions that stabilize carbocations. Integration of both techniques provides a powerful diagnostic approach where infrared data suggests functional groups present while mass spectrometry confirms molecular formula and provides structural clues through fragmentation analysis. The chapter emphasizes how these complementary methods, often used together with nuclear magnetic resonance spectroscopy, enable organic chemists to solve structure determination problems systematically and confidently.