analytical techniques

• Infrared spectroscopy


  Certain bonds in a molecule absorb infra-red radiation at characteristic frequencies causing the covalent bonds to vibrate
• Infrared spectroscopy


  • Complicated spectra can be obtained than provide information about the types of bonds present in a molecule
• Wavenumber range below 1500 cm-1


  "Fingerprinting" - Complicated and contains many signals, picking out functional group signals difficult. This part of the spectrum is unique for every compound, and so can be used as a "fingerprint".
• Wavenumber range above 1500 cm-1


  "Functional group identification" - Use an IR absorption table provided in exam to deduce presence or absence of particular bonds or functional groups
• Absorption or trough in 1640-1750 cm-1 range


  • Indicates presence of C=O bond
• Absorption or trough in 2500-3300 cm-1 range


  • Indicates presence of O-H bond in an acid
• Wavenumber ranges for bonds

  • C-O: 1000-1300 cm-1
  • C=O: 1640-1750 cm-1
  • C-H: 2850-3100 cm-1
  • O-H (Carboxylic acids): 2500-3300 cm-1 (very broad)
  • N-H: 3200-3500 cm-1
  • O-H (Alcohols, phenols): 3200-3550 cm-1 (broad)
• Carbon dioxide (CO2), methane (CH4) and water vapour (H2O) are all greenhouse gases. They trap the Earth's radiated infra red energy in the atmosphere.
• Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane.
• The Earth is thought to be getting warmer, and many scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere.
• Concentrations of carbon dioxide in the atmosphere have risen significantly in recent years due to increasing burning of fossil fuels.
• Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming.
• The 'Greenhouse Effect'


  Infrared radiation is absorbed by C=O, O–H and C–H bonds in H2O, CO2 and CH4. These absorptions contribute to global warming.
• The 'Greenhouse Effect' of a given gas

  Dependent both on its atmospheric concentration and its ability to absorb infrared radiation and also its residence time (Time it stays in atmosphere).
• Modern breathalysers measure ethanol in the breath by analysis using infrared spectroscopy.
• Infrared spectroscopy can be used to monitor gases causing air pollution (e.g. CO and NO from car emissions)
• Molecular ion


  The peak with the largest m/z, will be due to the complete molecule and will be equal to the Mr of the molecule.
• Molecular ion formation


  1. M → [M]+. + e–

  2. The molecule loses an electron and becomes both an ion and a free radical
• Fragmentation


  1. [M]+. → X+ + Y.

  2. The molecular ion fragments due to covalent bonds breaking, producing an ion and a free radical
• Relatively stable ions


  • Such as carbocations R+ such as CH3CH2+ and acylium ions [R-C=O]+ are common. The more stable the ion, the greater the peak intensity.
• A mass spectrum is essentially a fingerprint for the molecule that can be identified by computer using a spectral database.