Hydrocarbons fall into two categories: Saturated and Unsaturated- and both of these terms describe the number of hydrogens that carbon is bonded to.
Concept #1: The difference between saturated and unsaturated molecules.
The equation 2n + 2 (where n = #carbons) predicts the number of hydrogens that an organic molecule should contain to be considered saturated.
Are the following molecules saturated?
Hint: You can figure this out without using the 2n + 2 rule.
Example #1: Is the following molecule saturated? Is it missing any hydrogens? How many if so?
Example #2: Is the following molecule saturated? Is it missing any hydrogens? How many if so?
Example #3: Is the following molecule saturated? Is it missing any hydrogens? How many if so?
*IHD is also known as the Degree of Unsaturation.
Concept #2: What index of hydrogen deficiency is.
Basically the IHD is an easy way for us to compare how unsaturated a molecule is compared to another.
What is the IHD of the following molecules?
Example #4: What is the IHD of the following molecule?
Example #5: What is the IHD of the following molecule?
Example #6: What is the IHD of the following molecule?
Unfortunately, the structure of a compound will not always be drawn for us. In that case, we use the following technique.
Concept #3: How to use IHD with molecular formula.
*If your professor taught you a different method, that’s fine! Whatever helps you get the right answer.
Example #7: What is the IHD for C4H7Cl?
Example #8: What is the IHD for C6H7N?
Example #9: What is the IHD for C7H12O2?
Awesome job. Now we know how to easily calculate and compare the number of hydrogens that are missing for all types of compounds.