6. Organic analysis

• High res mass spec can distinguish between molecules which have only a small difference in their molecular masses
• Infrared spectroscopy is used to identify certain bonds within a compound
• Acidified potassium dichromate can be used to distinguish between primary, secondary and tertiary alcohols
• Acidified potassium dichromate oxidises primary and secondary alcohols but not tertiary
• Because primary and secondary alcohols give the same colour change, we use fractional distillation and then test to see if an aldehyde or ketone is formed
• Fehlings solution is an oxidising agent so oxidises aldehydes but not ketones
• How to make tollens:

  1. Add few drops of sodium hydroxide to silver nitrate solution
  2. Add a few drops of dilute ammonia until ppt dissolves
• Aldehydes and ketones are flammable so we dont use a bunsen burner
• Test for carboxylic acid is adding carbonate

  • CO2 gas and water is made
  • When bubbled through limewater, it turns cloudy
• mass spec is used to find the Mr of a compound
• m/z is the mass of a fragment divided by charge
• Most fragments have a 1+ charge so m/z = fragment mass
• Peaks show fragments of the original compound
• The last peak is the M+ peak or molecular ion peak

  • Its the same as relative molecular mass of the molecule
• High res mass spec measure the relative mass to several decimal places
• IR spectroscopy uses infrared radiation to increase the vibrational energy of covalent bonds in a sample
• The frequency of IR radiation absorbed by a covalent bond depends on:

  1. The atoms that are on either side of the bond
  2. The position of the bond in the molecule
• The fingerprint region allows us to identify specific molecules
• You can compare the fingerprint region against a known library of spectra to identify the molecule
• Extra peaks in the fingerprint region indicates you have impurities in the samle
• Green house gases in the atmosphere absorb infrared radiation and re emit it back towards earth - gh effect

  • it is usually the covalent bonds that absorb the IF radiation
• GHG include:

  CO2

  H2O

  CH4
• The precise relative atomic mass quoted in the table for the 12C isotope is exactly 12.00000 because it is the standard
• Two molecular ion peaks can be due to two isotopes
• Infrared radiation is absorbed by covalent bonds
• IR spectroscopy is thus valuable for checking compound purity.
• Uses of infrared spectroscopy

  • Breathalyser tests - The intensity of ethanol's C-H peak indicates how much is present in a person's breath. This reveals if a driver is over the legal alcohol limit.
  • Atmospheric monitoring - Peak intensities for gases like CO and NO show pollutant levels from car emissions and industrial processes.
• The principles of the greenhouse effect and global warming are:

  1. Some incoming solar radiation is absorbed by the Earth then re-emitted as longer-wavelength infrared radiation (heat).
  2. Greenhouse gases like carbon dioxide, methane and water vapor in the atmosphere absorb this infrared radiation through their covalent bonds.
  3. These gases then re-radiate some heat back to Earth, warming the surface temperature - the natural greenhouse effect.
• Infrared spectroscopy takes advantage of the fact that covalent bonds in molecules can vibrate at specific natural frequencies.
• When molecular ions break apart, they form fragment ions that appear as distinct peaks on the mass spectrum
• molecules containing a carbon-13 isotope causes the M+1 peak in a mass spectrum
• • CH3+ (m/z = 15)
  • C2H5+ (m/z = 29)
  • C3H7+ (m/z = 43)
  • OH+ (m/z = 17)
•  fragmentation is the term for the breaking apart of molecular ions into smaller fragment ions