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. 

Definition of Saturation

The equation 2n + 2 (where n = #carbons) predicts the number of hydrogens that an organic molecule should contain to be considered saturated.

  • If the total number of hydrogens < 2n + 2, then the molecule is unsaturated. 

Problem: How many hydrogen atoms does the molecule contain?

 

2m

Hint: You can figure this out without using the 2n + 2 rule.  

Problem: How many hydrogen atoms does the molecule contain?

1m

Problem: How many hydrogen atoms does the molecule contain?

1m

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

Problem: What is the IHD of the molecule?

1m

Problem: What is the IHD of the molecule?

1m

Problem: What is the degree of unsaturation of the following compounds? 

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. 

Problem: What is the IHD of the compound? 

2m

Problem: What is the IHD of the compound? 

1m

Problem: What is the IHD of the compound? 

1m

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