Useful Concepts |
The Index of Hydrogen Deficiency (or IHD) is also known as "units of unsaturation" and several other similar names.
The index of hydrogen deficiency is
a count of
how many molecules of H_{2} need to be added to a structure
in order to obtain the corresponding saturated, acyclic species. Hence, the IHD takes a count of how many rings and multiple bonds are present
in the structure, so the IHD can
also be thought of as (multiple bonds +
rings)
or
Note that the IHD for neutral molecules MUST be an integer (i.e. 0, 1, 2 etc.).
There are two ways in which the IHD can be used, both can be useful depending up on the situation:
(1) From a drawn structure
When you look at a structure, just count
the number of rings and π bonds present (but take care not
to count any rings twice !).
When counting π bonds, π bonds containing heteroatoms (e.g. O, N etc.) can be counted in exactly the same way as all C bonds.
(2) From the molecular formula.
The IHD can be calculated from a molecular formula... Consider the following generic molecular formula C_{c}H_{h}N_{n}O_{o}X_{x}, then the following equation can be derived:
IHD = 0.5 * [2c+2-h-x+n]
Where does this equation come from ?
© Dr. Ian Hunt, Department of Chemistry, University of Calgary |