 # How to use IHD with molecular formula

When you’re given a molecular formula to calculate
IHD, you don’t have the same clues that you had let’s say if it was a structure. If you remember, we could use pi bonds or
rings to figure out how many hydrogens were missing from a molecule. But when you’re just given a molecular formula,
let’s say like, this example, C4H7Cl, I have no clue if there’s rings there, if there’s
double bonds there. Really, all I have is the atoms. So for this, we’re going to have to use our
own rule and our own formula because that’s really the only way to solve it. Now your professor may have used a different
rule in class, and in fact, most of the rules that I’ve seen are different from mine. The only reason I’ve settled on this one is
because it worked for a lot of my students and they like it better. But if you have a rule that your professor
taught you that you like better, please, by all means, use that one. But this is the way it works, basically we
need to calculate IHD and the way we do that is by calculating the theoretical H’s. The way we do that is by using the formula
2n plus 2. So basically we calculate 2n plus 2. Then we subtract the actual number of hydrogens. But the thing is that for this we don’t just
use hydrogens, we also take into consideration other atoms that might act as hydrogen equivalents. For example, when I’m calculating the number
of hydrogens in a molecule, I don’t just look at H, I also look at X’s. Do you guys remember what X is? X would be a halogen and that counts as one
hydrogen. For this formula, I count a halogen and a
hydrogen exactly the same. Then oxygen, what about if I see an oxygen
in my compound? What do I could that as? Actually, I count that as zero. I ignore it. Whenever you see oxygen in this compound,
just ignore it. That doesn’t go into your equation. Then how about nitrogen? Nitrogen is a special case. Nitrogen is actually going to count as negative
one hydrogen for the overall compound. So that means that I’m not just counting H’s,
I’m also counting all halogens and all nitrogens and I’m ignoring the oxygens. Finally, I get that number. That number is going to go here. This is going to be number of H’s total. Then I’m going to take that number and I’m
going to divide it all by 2, which is what I have here and that’s going to be your IHD. Now I know that sounds a little bit complicated,
but once you get going with it, you can actually do it pretty fast.