Index of Hydrogen Deficiency

[thestrokes14]

Can someone confirm this formula:

IHD = [(2n+2) - (# H) - (#X) + (#N)]/2

where n is the number of carbons, H is the number of hydrogens, X is the number of halogens, and N is the number of nitrogens.

I noticed formulas can take on different forms, so I just want to make sure that this formula is correct.

Thanks!

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[TwoPaddles]

Can someone confirm this formula:

IHD = [(2n+2) - (# H) - (#X) + (#N)]/2

where n is the number of carbons, H is the number of hydrogens, X is the number of halogens, and N is the number of nitrogens.

I noticed formulas can take on different forms, so I just want to make sure that this formula is correct.

Thanks!

I don't know where you got this information but it is incorrect.
First of all, you add the # of halogens to the # of hydrogens and you subtract the # of nitrogens from the number of hydrogens.
The formula is = [(2n+2) - (#H + #X)] / 2
or
= [(2n+2) - (#H - #N)] / 2

Notice how the formulas I wrote differ than the one you wrote: You add/subtract the nitrogen/halogen to #hydrogens before you subtract it from # of carbons (and yeah the order matters or you will get a different answer).

Here's an example:
C5H9N
= [(2n+2) - (#H - #N)] / 2
= [(2*5+2) - (9 - 1)] / 2
= (12 - 8) / 2 = 2

Another example: C4H6Br2...you can do this one... the answer is 1.

Hope this helps.

 

[Rabolisk]

TwoPaddles, what you wrote and what he wrote are the same thing. Let us do some algebra.

a - b - c = a - (b + c)
a - b + c = a - (b - c)

I can recall at least 3 instances where you misread the OP's posts. Please read carefully before posting.

 

[dougkaye]

TwoPaddles, what you wrote and what he wrote are the same thing. Let us do some algebra.

a - b - c = a - (b + c)
a - b + c = a - (b - c)

I can recall at least 3 instances where you misread the OP's posts. Please read carefully before posting.

NOTE: Any SODAR (sum of double bonds and rings) of 4 or above is a strong indication of benzene ring. SODAR is what you get from the above "index of H deficiency".

 

[Rabolisk]

To be sure, index and SODAR are different ways of saying the same thing. SODAR is slightly more general (monovalent and trivalent as opposed to halogens and nitrogen), but index is applicable to organic compounds other than hydrocarbons (otherwise why would you include halogens and nitrogen?)

 

[dougkaye]

classically index of hydrogen deficiency is just for hydrocarbons but whatever source they're quoting is using SODAR and calling it "index". all the same thing. i just wanted to make sure folks learned the more general one so they didn't waste time.

 

[thestrokes14]

I'll give you all a big present from my o-chem professor. Forget the index and use SODAR (sum of double bonds and rings). Index can only be used with hydrocarbons, whereas SODAR can be used with all molecules. This should be your first step on H-NMR problems.

SODAR = (2C + 2 - H - mono + tri) / 2

C = carbon (of course!)
H = hydrogen
monovalents (halogens)
trivalent (nitrogen)

Any SODAR 4 or above is a strong indication of benzene ring.

You do realize that this is exactly what I wrote in my initial message?

 

[dougkaye]

ah you're so smart