Index of Hydrogen Deficiency

Hydrocarbons fall into two categories: Saturated and Unsaturated- and both of these terms describe the number of hydrogens that carbon is bonded to. 

Predicting IHD Structurally

*IHD is also known as the Degree of Unsaturation.

Basically the IHD is an easy way for us to compare how unsaturated a molecule is compared to another.

  • 1 IHD = Compound is missing 2 hydrogens.
  • Rings/Double bonds = 1 IHD  
  • Triple Bonds = 2 IHD

Example: What is the IHD of the following molecule? 

1m

Example: What is the IHD of the following molecule? 

1m

Example: What is the IHD of the following molecule? 

1m

Predicting IHD using Molecular Formula

Unfortunately, the structure of a compound will not always be drawn for us. In that case, we use the following technique.

*If your professor taught you a different method, that’s fine! Whatever helps you get the right answer. 

Awesome job. Now we know how to easily calculate and compare the number of hydrogens that are missing for all types of compounds.