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Wize University Organic Chemistry Textbook > Structure Elucidation (IR Spectroscopy, NMR, Mass Spectrometry)

Hydrogen Deficiency Index

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Hydrogen Deficiency Index


The hydrogen deficiency index (HDI) determines how many "degrees of unsaturation" there are. Each HDI corresponds to a multiple bond or ring.


Wize Tip
When HDI = 1, this is a double bond or a ring. When HDI = 2, this is equal to 2 double bonds or one triple bond. When HDI = 4 this means there is a benzene ring!


HDI = [(2C +2+N−H−X)]2HDI\ =\ \frac{\left[\left(2C\ +2+N-H-X\right)\right]}{2}

where C = carbon, N = nitrogen, H = hydrogen, X = halogen (F, Cl, Br, I)


Example: For the molecular formula C6H9ClO, the HDI is:


HDI ={ [(2x6)+2]−1−9}2 = 2HDI\ =\frac{\left\{\ \left[\left(2x6\right)+2\right]-1-9\right\}}{2}\ =\ 2