Chapter 9: Spectroscopic Identification of Organic Compounds

Overview

• Introduction to a new topic: spectroscopic identification of organic compounds.
• Building on knowledge from a previous semester on IR spectroscopy and hydrogen deficiency.
• New techniques include Nuclear Magnetic Resonance (NMR) and Mass Spectrometry.
• Importance of reviewing IR spectroscopy and hydrogen deficiency from Organic Chemistry I.

Infrared (IR) Spectroscopy

Theory and Application

• IR spectroscopy is an instrumental method to identify functional groups in organic compounds.
• Limitations: cannot identify all functional groups.
• Focus on four functional groups easily identifiable in IR spectrum.
• Basic process:
  • Unknown compound is irradiated with electromagnetic radiation (infrared frequency).
  • Energy is absorbed if the vibrational frequency of a bond matches the IR frequency.

Vibrational Modes

• All organic compounds have carbon-carbon and carbon-hydrogen bonds, which show absorbance in IR.
• Functional groups absorb IR energy at unique wavelengths/frequencies, creating diagnostic bands.

Example of IR Spectrum

• Example organic compound: carbon-carbon, carbon-hydrogen, carbon-oxygen double bond, carbon-oxygen single bond, and oxygen-hydrogen single bond.
• Diagnostic absorbance bands:
  • Carbon-oxygen double bond: sharp band at ~1700 cm⁻¹.
  • Oxygen-hydrogen bond: broad band between ~3200-3500 cm⁻¹.

Interpretation Challenges

• Avoid over-interpreting IR spectra.
• Focus on key diagnostic bands related to specific functional groups.
• Use a correlation table for reference (provided during exams).

Hydrogen Deficiency

Concept and Calculation

• Hydrogen deficiency measures unsaturation in organic compounds.
• Based on the difference between calculated maximum hydrogens (2n + 2 rule) and actual hydrogens.
• Examples provided:
  • Pentane: saturated molecule example.
  • Introduction of double bond or ring reduces hydrogen count (hydrogen deficiency).

Interpretation

• HD = 2 corresponds to double bond; HD = 4 to triple bond or other combinations (e.g., two double bonds, one double bond and one ring).
• Benzene ring has an HD of 8.

Analyzing Unknowns

• Calculate max hydrogens for given carbons using formula.
• Compare to actual hydrogens to determine hydrogen deficiency.
• Consider all possible combinations of functional groups.

Summary

• Encouragement to review and understand IR and hydrogen deficiency as preparation for further learning in the course.
• Future classes will involve examples and practical applications in labs.