Structure Determination

Using Analytical Techniques Together and Chromatography

Using Analytical Techniques Together and Chromatography

If you have all the spectra available then the best analytical strategy is to use them all together. There are a number of steps you should take in order to deduce the structure.

If composition data is available:

  1. Figure out the empirical formula.
  2. Deduce the relative molecular mass and the molecular formula by using the mass spectrum.
  3. Check for C=O and O-H bonds using the infra-red spectrum in order to identify the functional group.
  4. List all the isomers which are consistent with the molecular formula an the functional group.
  5. Deduce the hydrogen environment number by using the peak number. Get rid of any isomers which are inconsistent with this number.
  6. Figure out the number of hydrogen atoms in each environment by using the integration factors on the proton nmr spectrum. Get rid of any isomers which are inconsistent with this distribution.
  7. Compare the splitting of peaks within the nmr spectrum with the expected splitting patterns of the possible isomers remaining. Get rid of any isomers which do not give this splitting pattern.
  8. If you need to, compare the chemical shifts of the peaks within the nmr spectrum with the expected values. Get rid of any isomers which are inconsistent with these chemical shifts.

If the composition data is not available then you will need to find the molecular formula by trial and error:

  1. Use the mass spectrum to deduce the relative molecular mass.
  2. Use the infra-red spectrum to deduce the functional groups present and, therefore, establish the oxygen atoms likely to be present.
  3. Deduce the molecular formula by using the relative molecular mass and the information you have on the hydrogen and oxygen atoms.

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Chromatography

Chromatography is used to separate different compounds in a mixture so that they can then be analysed. There are two main types:

  • Thin-layer chromatography: a liquid solvent flows up a piece of chromatography paper or a TLC plate. The mixture is put in a small region of the paper and left to flow up the paper with the solvent. As different substances flow at different speeds, the distance that is travelled by the substance can be compared with the solvent and thus the substance can be identified.
  • Gas-liquid chromatography: a gaseous mixture flows through a column which is lined with a solid. It is possible to vaporise volatile liquids and this way also allow them to flow through the column. As different gases flow through the column at different speeds, the gas or volatile liquid can be identified.

The substance mixture moving up the plate or through the column is called the mobile phase. The plate or substance lining the column is called the stationary phase.

The relative solubility in the two phases dictates the speed at which a substance will move up the plate or through the column. In gas-liquid chromatography, for example:

  • a substance which is strongly attracted to the stationary phase will move slowly and, therefore, take longer to move through the column.
  • a substance which is not strongly attracted, on the other hand, will move more quickly and, therefore, take less time to move through the column.