Elucidation of the structures of organic compounds - a brief summary

Introduction Organic chemistry is so vast that the identification (characterisation) of a compound can be quite
involved. The characterisation takes place in a series of stages (see below). In earlier times relatively
large amounts of substance were required to elucidate the structure but, with the advance in technology
and the increased use of electronic instrumentation, only very small amounts are now required.
Elemental
composition One assumes that organic compounds contain carbon and hydrogen but it can be proved by letting the
compound undergo combustion. Carbon is converted to carbon dioxide and hydrogen to water.
Percentage
composition The percentage composition by mass is found by dividing the mass of an element present by the mass
of the compound present, then multiplying by 100. Elemental mass of C and H can be found by allowing
the substance to undergo complete combustion. From this one can find...
• mass of carbon = 12/44 of the mass of CO2 produced
• mass of hydrogen = 2/18 of the mass of H2O produced
Empirical
formula This gives the simplest ratio of elements present in the substance. It can be calculated by dividing the
mass or percentage mass of each element present by its molar mass and finding the simplest ratio
between the answers. Empirical formula is converted to the molecular formula using molecular mass.
Molecular
mass Molecular mass determination was traditionally carried out using a variety of techniques such as ...
• volumetric analysis or
• molar volume methods such as the Dumas, Victor-Meyer or gas syringe experiments.
Nowadays mass spectrometry is used. The position of the last m/z signal is due to the molecular ion
and gives the molecular mass. The fragmentation pattern also gives information about the compound.
Molecular
formula The molecular formula is an exact multiple of the empirical formula. Comparing the molecular mass with
the empirical mass allows one to find the true formula.
e.g. if the empirical formula is CH (relative mass = 13) and the molecular mass is 78
the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 .
Structural
formula Because of the complexity of organic molecules, there can be more than one structure for a given
molecular formula. To work out the structure, one can carry out different tests...
Chemical Use chemical reactions to identify the functional group(s) present.
Spectroscopy e.g. IR detects bond types due to absorbance of i.r. radiation
NMR gives information about the position and relative
numbers of hydrogen atoms present in a molecule
Confirmation By comparison of spectra and melting point or boiling point.