Index of Hydrogen Deficiency (IHD)
||Chapter 13: Spectroscopy
IHD = 0.5 * [2c+2-h-x+n]
- The Index of Hydrogen Deficiency (IHD), is a count of how
many molecules of H2 need to be added to a structure in order to
obtain the corresponding saturated, acyclic species.
- Hence it takes a count of how many rings and multiple bonds are present
in the structure.
- So, IHD can also be thought of as (multiple bonds + rings) or (π +
- When you look at a structure, just count rings and π-bonds up (but
take care not to count any rings twice !)
- If you have a molecular formula, CcHhNnOoXx,
then the following equation can be derived:
Where does this equation come from ?
Determining the IHD for molecules can be useful
for the following reasons:
- Well, the maximum number of hydrogen atoms for "c" carbon
atoms is 2c+2 (think of the formulae of saturated hydrocarbons such as ethane,
- From this number, subtract the "h" hydrogens that you have.
- Since, like hydrogen, a halogen only forms one bond, then they can be treated
as if they are hydrogens, so subtract them as well.
- Oxygen forms two bonds, therefore it has no impact (compare H count
for methane, CH4, and methanol, CH3OH).
- Nitrogen forms three bonds. This means for "n" nitrogens, "n" extra hydrogen
atoms are needed (compare the H count for methane, CH4, and methyl
amine, CH3NH2), therefore, add "n".
- The factor of 0.5 accounts for us counting H atoms, but adding
hydrogen, H2 , molecules. OK ?
- Seeing what types of structural units maybe possible
- Quickly checking structures to see if they fit the molecular formula rather
than simply counting H (when a mistake is easily possible)
- What is the IHD for each of the following molecular formulae ?