Organic Spectroscopy

• Infrared Spectroscopy

  Technique that records the frequencies of infrared EM radiation that are absorbed by a sample to determine the type of bonds within the sample
• Infrared spectrum

  Plot of transmission of infrared radiation against wavenumber (1 / wavelength)
• Fingerprint region

  
  

  • Region below 1500 cm-1 with many peaks that are difficult to assign but form a unique pattern for a particular compound
  • Pattern is unique to a compound
  • Substance may be identified by comparing the IR spectrum to a database of reference spectra
  • An exact match in the fingerprint region identifies a compound (since it is characteristic of a compound)
  • >1500 - stretches, tells you type of bond
  • <1500 - exact compound, overlapping due to rotation and vibration; doing lots of things
• • The peak due to C-H is the stronger of the two
  • More C-H bonds than C-C bonds
  • C-H towards LHS of C-C peak
  • Doesn't help with finding the functional group but still useful
• Covalent Bonds

  • Can absorb IR Energy (to vibrate)
  • Frequency of absorbed radiation is characteristic of a particular type of vibration for a particular molecule
• Types of vibrations

  • Symmetric/Asymmetric stretching
  • Scissoring
  • Rocking
  • Wagging
• How it works

  • Oscillating vector of IR EMR must match the frequency of the oscillating dipole moment in the molecule for energy exchange to occur
  • No dipole moment -> no energy exchange
  • Energy from IR is only transferred to a bond if the bond has a dipole that changes when it vibrates
  • Can have more or less than the required energy but would just do the vibration in a different way; translation
• Stretching

  • Symmetric - no change of dipole moment, IR inactive
  • Asymmetric - change in dipole moment, IR active
• Infrared Spectrometer

  Records the frequencies of infrared EMR that are absorbed by a sample. Can tell us about the type of bonds within a sample
• Absorption and peaks

  • Any absored wavelengths will transmit less than others, forming a dip/peak in the graph
  • None of them are caused by translation of a molecule
  • Double bonds have a sharper spike as they have a higher force constant
• Interpreting IR spectra

  • Check wavenumber at which there is absorption
  • Which bond is that? (check data sheet)
  • Longer peaks -> more absorption
  • Higher wavenumbers towards LHS
  • Absorption will be in a relatively small range
  • Exact type of absorption depends on the molecule in which the bond is found
• Advantage of using wavenumber instead of wavelength

  • Easier to work with than wavelength in centimeters or frequencies in Hz
  • Easier to remember
• Bonds

  • C-H : spiky spiky
  • O-H : broad
• Uses of IR spectroscopy

  Drunk driving - breathe into a breathalyser with an IR spectrometer inside. Calculates % of alcohol in breath by looking at size of absoprtion of due to C-H stretch which is not normal found in breath
• Qualitative tests

  
  

  Also known as test tube tests.

  
  

  Tests for:

  • Carboxylic acid
  • Aldehyde/Ketone [2]
  • 1° / 2°  alcohol

  Need to know:

  • Test
  • +ve/-ve observation
• Carboxylic acid

  • Add CO3 or a HCO3
  • Acid - fizz (CO2 gas, check by bubbling through limewater to see if it goes cloudy)
• Fehling's Test

  • Add substance to Fehling's solution in a water bath and heat
  • Aldehyde + α-hydroxy ketone : brick red ppt. (Cu2+ ions reduced to Cu+)
  • Ketone : no change
• Tollen's Reagent

  • Compound added to Tollen's Reagent and heated in a water bath
  • Aldehydes and α-hydroxy ketone : silver mirror (Ag+(aq) reduced to Ag(s))
  • Ketone : no change
• 1°/2° and 3° alcohol


  • 1° oxidises to an aldehyde with acidified potassium dichromate/H+
  • 2° oxidises to a ketone
  • 3° doesn't oxidise
  • 1/2° will turn green, (green Cr3+ ions formed) 3° will stay orange
  • All alcohols react slowly with Na metal
  • NEVER add a -COOH, known/suspected to Na!
• Alkenes

  • Orange Br2 solution (bromine water) turns colourless when added to a compound with C=C double bonds
  • Saturated compounds give no change
• False positives for aldehydes

  • Methanoic acid (HCOOH) and α-hydroxy ketones
  • Adding CO3 to HCOOH makes it fizz which rules it out