Instrumental Methods

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Chapter: Organic Chemistry : Structure Determination of Organic Compounds

When the reaction product(s) is obtained in pure form, modern instrumental methods of structure determination, rather than traditional wet methods, provide the fastest way to determine the functionality and connectivity present.


INSTRUMENTAL METHODS

When the reaction product(s) is obtained in pure form, modern instrumental methods of structure determination, rather than traditional wet methods, provide the fastest way to determine the functionality and connectivity present. Today’s chemist has a large number of tools available with which to probe the structure of molecules, but for determining the structures of organic molecules the “big three” are nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and mass spectrometry (MS). The frequency of use of these techniques generally falls in the same order (NMR > IR > MS).

The first two of these methods (NMR and IR) are spectroscopic techniques in which the molecule is interrogated with electromagnetic radiation in a particular range of frequencies (NMR uses radio frequencies and IR spectroscopy uses infrared radiation). Only certain frequencies will be absorbed by a particular compound, and those frequencies that are absorbed can be used to infer structural details about the compound.

Mass spectrometry is not a spectroscopic technique because absorption of electromagnetic energy is not involved in any way. In MS the molecule is frag-mented into ions and these charged pieces are separated on the basis of their mass-to-charge ratio. Since the usual charge is +1, the masses of the pieces are determined. Knowledge of the masses of the pieces allows the structure of the compound to be reconstructed.

Another distinction between these techniques is the structural information they are capable of revealing. Both NMR spectroscopy and MS establish connectivity between atoms and groups in a molecule (albeit slightly differently). In addition the functional groups present are suggested. Infrared spectroscopy does not generally establish connectivity but is unmatched for identifying functional groups present in the molecule. By combining these three major methods, the functional groups, the molecular weight, and the connectivity of the atoms and groups can be established rapidly and efficiently. It is normally a trivial step then to write the structure of the molecule.

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