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Index of Hydrogen Deficiency (IHD)



Alkanes are saturated organic compounds with number of hydrogens 2n + 2 (n is number of carbons).
All organic compounds that have less hydrogens than 2n+2 have a degree of unsaturation: 


The hydrogen deficiency index (HDI) or Index of hydrigen deficiency (IDH) is a measure of the number of degrees of unsaturation. A compound has one degree of unsaturation for every two hydrogen atoms that are missing.

There is a formula to determine IDH:

IDH = 0.5x(2C + 2 + N − H − X )
C is number of carbons
N is number of nitrogens
X is number of halogens

For compounds mentioned above:

C4H10
HDI =0.5x(2x4 + 2 + 0 – 10 – 0) = 0
C4H8
HDI = 0.5x(2x4 + 2 + 0 – 8 – 0) = 1
 C4H6
HDI = 0.5x(2x4 + 2 + 0 – 6 – 0) = 2
C4H4
HDI = 0.5x(2x4 + 2 + 0 – 4 – 0) = 3

Please pay attention that we can determine IDH as well from brutto formula as from structure. 

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