Chapter 18: Infrared Spectroscopy and Functional Group Identification

Recognizing Molecular Signatures

Introduction

Different chemical bonds vibrate in characteristic ways.

These vibrations interact with infrared radiation and produce distinctive absorption patterns.

Infrared spectroscopy allows chemists to identify functional groups and gain insight into molecular structure.

Although IR spectra may initially appear complicated, they often provide valuable clues about the presence or absence of specific functional groups.

Why Infrared Spectroscopy Matters

IR spectroscopy is especially useful for identifying:

  • alcohols,
  • carbonyl compounds,
  • amines,
  • alkenes,
  • and other common functional groups.

Because many functional groups possess characteristic absorptions, IR spectra often serve as an important first step in structural analysis.

Functional Groups Revisited

Infrared spectroscopy highlights a recurring theme throughout organic chemistry: functional groups determine behavior.

Characteristic peaks correspond to familiar functional groups encountered earlier in the handbook.

Thus, spectroscopy reinforces the importance of functional group recognition.

Thinking About IR Spectra

Helpful questions include:

  • Is a carbonyl present?
  • Is an alcohol present?
  • Are N–H or O–H bonds present?
  • Which functional groups are absent?

Rather than providing complete answers, IR spectroscopy often narrows the possibilities.

Common Mistakes

Trying to Memorize Every Peak

Better approach: Focus on major functional groups.

Viewing IR as a Collection of Numbers

Better approach: Think about structure.

Self-Assessment

I can:

☐ Explain what an IR spectrum reveals about a molecule.

☐ Recognize characteristic absorptions for common functional groups.

☐ Use IR evidence to narrow down, rather than fully determine, molecular structure.

Looking Ahead

Infrared spectroscopy reveals functional groups.

Nuclear magnetic resonance spectroscopy provides even richer structural information.

Specific wavenumber ranges for the functional groups mentioned above are collected in Appendix D.