Degree of unsaturation

The degree of unsaturation (also known as the index of hydrogen deficiency (IHD) or rings plus double bonds^{[1]}) formula is used in organic chemistry to help draw chemical structures. The formula lets the user determine how many rings, double bonds, and triple bonds are present in the compound to be drawn. It does not give the exact number of rings or double or triple bonds, but rather the sum of the number of rings and double bonds plus twice the number of triple bonds. The final structure is verified with use of NMR, mass spectrometry and IR spectroscopy, as well as inspection.
Contents
General formula
The formula for degree of unsaturation is
where n_{i} is the number of atoms with valence v_{i}.^{[2]}
That is, an atom that has a valence of x contributes a total of x2 to the degree of unsaturation. The result is then halved and increased by 1.
Rings plus pi bonds formulation
For molecules containing only carbon, hydrogen, monovalent halogens, nitrogen, and oxygen, the formula
where C = number of carbons, H = number of hydrogens, X= number of halogens and N = number of nitrogens,^{[3]} gives an equivalent result. Oxygen and other divalent atoms do not contribute to the degree of unsaturation, as (22) = 0.
The degree of unsaturation is used to calculate the number of rings and pi bonds, where
 Rings count as one degree of unsaturation
 Double bonds count as one degree of unsaturation
 Triple bonds count as two degrees of unsaturation
See also
References
 ^ David O. Sparkman. Mass Spectrometry Desk Reference. Pittsburgh: Global View Pub. p. 54. ISBN 0966081390.
 ^ Badertscher, M.; Bischofberger, K.; Munk, M.E.; Pretsch, E. (2001). "A Novel Formalism To Characterize the Degree of Unsaturation of Organic Molecules". Journal of Chemical Information and Modeling 41: 889. doi:10.1021/ci000135o
 ^ Organic structural spectroscopy, chapter 1.
Paul R. Young, Practical Spectroscopy ISBN 0534372309
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