4 - Organic analysis

• Functional group + reagent : result

  Carboxylic acid + NaHCO3 (aq) : effervescence

  Acid chloride + water : misty fumes

  Alcohol + potassium dichromate : orange to green

  Aldehyde + tollens : silver ppt/mirror

  Aldehyde + fehlings : blue to orange/red ppt
• Mass spectrometry is used to find the molecular mass of an organic compound. The compound is ionised and becomes a negative ion.
• The time of flight depends on the M/Z value (mass to charge ratio). When the ToF of the ion is measured, the lower Mr ions (lighter) will hit the detector first.
• The relative abundance of the ions tells us the relative amounts of the different ions. This is detected by the size of the current at the detector.
• On a mass spectrum, the ion with the largest m/z value gives us the Mr of the compound and is called the molecular ion.
• High resolution mass spectrometry measures m/z values to high accuracy. Each molecular formula has a different Mr if measured to enough precison.
• All bonds vibrate at a characteristic frequency. These frequencies are in the IR region of the electromagnetic spectrum. This is used for IR spectroscopy.
• IR spec:

  IR radiation is passed through a sample

  Energy is absorbed by bonds that vibrate at a characteristic frequency

  Energy not absorbed passes through and is shown as 100% transmittance

  The radiation coming from the sample will miss the frequencies corresponding to bonds in the sample
• Do not assign peaks on IR specs below 1500cm-1.
• Peaks in the fingerprint region are unique to each compound and can be compared to known databases.
• Substances can be ionised in a mass spec by either electron guns or electron spray.

  Electron guns fire electrons in hopes of knocking an electron off the atoms.

  Electron spray causes the particles to gain an extra proton.