Lecture on Mass Spectrometry and Infrared Spectroscopy

Key Topics

• Mass Spectrometry
• Interpreting Spectra Data
• Identifying Organic Structures
• Functional Groups
• Infrared Spectroscopy
• Absorptions and Wave Number Data

Mass Spectrometry

• Purpose: Used to determine molecular mass and structure of organic molecules.
• Components of Mass Spectrum:
  • Molecular Ion Peak: Represents the relative molecular mass of the organic molecule.
  • Base Peak: Tallest peak, most abundant fragment.
• Fragmentation: Breaking of carbon-carbon bonds, resulting in positive ions and free radicals.
• Example:
  • Butane (C4H10) shows peaks at 15, 29, 43, and 58.
  • 58 is the molecular ion peak.
  • 43 is the base peak (most common fragment).

Analyzing Mass Spectra

• Common Fragment Peaks:
  • Methyl (CH3) at 15
  • Ethyl (CH2CH3) at 29
  • Propyl (CH2CH2CH3) at 43
• Examples:
  • Differentiating between propanal and propanone using peaks at 29 and 43.
  • Propan-1-ol vs. Propan-2-ol differentiation by peaks at 31 and 45.

Infrared (IR) Spectroscopy

• Purpose: To determine functional groups by analyzing absorptions in the IR spectrum.
• IR Radiation Effects:
  • Bond stretching and bending.
  • Focus on stretching vibrations at A-level.
• Factors Affecting Absorption:
  • Bond length, bond strength, and atom mass.
• Spectrum Axes:
  • Vertical: Transmittance (how much radiation absorbed).
  • Horizontal: Wave number (frequency).
• Characteristic Peaks:
  • Alcohols: Broad peak (3750 - 3200 cm⁻¹).
  • Aldehydes: Sharp peak (1740 - 1720 cm⁻¹).

Analyzing IR Spectra

• Functional Group Identification:
  • Based on wave numbers and peak shapes (broad vs. sharp).
• Fingerprint Region: 1500 - 500 cm⁻¹ (not typically analyzed in exams).
• Using Data Book: Important for matching wave numbers to functional groups during exams.

Worked Examples

1. Predicting Spectrum Peaks:
   • Propanal as an aldehyde with specific CH and C=O absorptions.
   • Propan-2-ol showing an alcohol's broad O-H peak.
2. Deducing Functional Groups:
   • Use given wave numbers to identify alcohols, carbonyls, etc.
3. Determining Structure:
   • Use of both wave numbers and molecular formulas.

Practical Application in Exams

• Example Question: Analyze mass spectrum and IR data to identify compounds.
• Important Tips:
  • Draw structures to envision possible fragments.
  • Use data book effectively to correlate peaks with functional groups.

Conclusion:

• Regular practice of interpreting spectra and memorizing common peaks can help in exams.
• Utilize all resources such as past papers, textbooks, and online examples for practice.