Mass Spectroscopy

Overview

• Mass Spectroscopy: Analytical technique used to measure mass-to-charge ratio of ions.
• Process:
  • Sample vaporized and ionized using an electron beam.
  • Fragments into radicals and cations; only positively charged ions reach the detector.

Graph Interpretation

• Y-axis: Relative abundance of positively charged fragments.
• X-axis: Mass-to-charge (m/z) ratio.
• Fragment Analysis: Determine which fragments form and their corresponding m/z numbers.

Pentane Example

• Identification of Fragments:
  • Methyl Cation: Possible fragment; corresponds to peak at m/z 15.
  • Peaks:
    • Base Peak (m/z 43): Highest relative abundance; stable fragment.
    • Parent Peak (m/z 72): Total molecular weight of pentane.

Fragmentation Details

• Breaking Bonds:
  • C1-C2 Bond: Results in CH3 group and butyl group.
  • C2-C3 Bond: Produces ethyl and propyl fragments.

Fragment Weights

• CH3: 15
• CH2: 14
• Methyl Cation (m/z 15): Methyl radical counterpart.
• Ethyl Cation (m/z 29): Matches observed peak.
• Propyl Cation (m/z 43): Corresponds to base peak.

Stability and Abundance

• Stability Factors:
  • Base Peak: Propyl cation more stable due to easier bond breaking (C2-C3).
  • Stability Comparison: Methyl carbocation less stable than primary carbocation.

Rearrangement and Secondary Cation

• Carbocation Rearrangement:
  • Propyl Cation: Can rearrange to more stable secondary carbocation via hydride shift.
  • Mass-to-Charge (m/z 43): Applies to both primary and secondary propyl cations.

Additional Peaks

• Peak at m/z 41:
  • Formation: From propyl cation losing two hydrogen radicals.
  • Allylic Carbocation: Stabilized by resonance, leading to abundance.
  • Calculation: m/z 43 minus 2 equals m/z 41.

Understanding these principles allows identification of molecular structures using mass spectroscopy.