Experimental Determination of Structure Flashcards
What is elemental microanalysis used for and how is it done?
Elemental microanalysis is used to determine the empirical formula of an organic compound:
1) A small sample of the compound is accurately weighed and oxidised at high temperature.
2) The product mixture is then separated (e.g. gas chromatography for gases) and the masses of each component is measured.
3) The mass of each element in the sample can then be worked out from these masses. If oxygen is present, it is determined by subtracting the total mass of the other elements from the original mass. The empirical fomular can then be determined.
How do you calculate the empirical formula?
1) Calculate the masses of each element in the sample in grams. Do this by dividing the elements gfm by the total gfm of the compound it is part of and then multiply that number by the mass of the compound in the sample in grams (e.g. If the total mass for H2O in a sample is 1.43 mg, the mass of H would be
1. 43x10^-3g x 2/18 = 1.59x10^-4g.
2) Calculate the number of moles for each element (n=m/gfm).
3) Calculate the mole ratio by dividing each elements mole number (n) by the smallest mole number in the sample.
4) Round the mole ratios to whole numbers and find out the empirical formula.
What is Mass Spectrometry and how does it work?
A technique used to determine the molecular mass and structure of an organic compound:
1) A sample of the unknown compound is passed tjrough the mass spectrometer and vaporised.
2) The vaporised sample is bombarded with high energy electrons which ‘knock off’ electrons from the molecules in the sample. This causes the molecules to break into smaller positively charged ions.
3) These positively charged ions are then accelerated by a high voltage electric field to a strong magnetic field which sorts them into a series of separate ion paths according to their mass/charge ratio.
4) Positive ions with lower mass/charge ratios are deflected more than those with higher ratios. Each separate ion path is detected and a mass spectrum is automatically plotted.
How do you read a mass spectrum?
- The peak with the highest m/z ratio represents the molecular/parent ion (The original molecule with a knocked off electron) and shows the gfm of the original molecule.
- The peaks with smaller m/z ratios represent other fragments of the molecule and can be worked out by their gfm.
