Molecular Formula and Index of Hydrogen Deficiency:
We have mentioned that in this unit you will learn as to how the information available from different types of molecular spectra can be used to elucidate the structure of a molecule. It is exciting to know that in addition to spectra, the molecular formula can also provide important structural leads. It is, therefore, worthwhile to know about the type of information available from the molecular formula.
You know that the molecular formula implies the elements constituting the molecule and the number of atoms of every element present in it. This can gives information about the unsaturation and/or the ring structures in the molecule. The information could be acquired through computing a parameter called Index of Hydrogen Deficiency (IHD). It is described as the number of pairs of hydrogen atoms which must be removed from the corresponding saturated formula to provide the molecular formula under consideration. Now let us learn about it with the help of some easy examples.
In case of ethane (C2H4), there are two that is, one pair of hydrogen atoms less than that needs for the corresponding saturated compound having molecular formula C2H6. Therefore, its IHD will be 1. Same, for ethyne (C2H2), the number of hydrogen atoms needs for the saturated hydrocarbon would be C2H6 C2H2 = 4. Therefore, the number of pairs of H atoms required would be 4/2= 2. So, IHD of ethyne is 2.