Degree of unsaturation (or IHD, Index of Hydrogen Deficiency)

What is the degrees of unsaturation (or IHD, Index of Hydrogen Deficiency)?

The degrees of unsaturation (or IHD, Index of Hydrogen Deficiency) refers to the number of cycles or double or triple bonds (the latter count as two double bonds) in a compound. It can be useful in identifying the type of structure, and is calculated from the molecular formula using the following equation:

IHD = (2 × nºC – nºH + nºN) / 2 + 1

  • The expression does not include the number of O (oxygen) atoms or other trace elements.
  • S (sulfur) is not considered, although the number of instaurations may be distorted; there may be more instaurations but they will be on that atom.
  • If halogen is present, it is considered equivalent to H (hydrogen).
  • If P (phosphorus) exists, it is considered equivalent to N (nitrogen), but we must make the same consideration as in the case of S (sulfur).
  • If there were Si (silicon), it is considered equivalent to C (carbon).

Examples of how the IHD is calculated

The molecular formula C2H2 of acetylene with a triple bond has an IHD = 2.

IHD = (2 × 22) / 2 + 1 = 2

Another example: if we consider the compound with the molecular formula C6H5NO2, it has an IHD = 5

IHD = (2 × 65 + 1) / 2 + 1 = 5

Therefore, in the case of nitrobenzene, these five unsaturations are due to: the three double bonds of the aromatic ring (3), the ring (cycle) itself (1) and the double bond associated with the nitro group (1).