Degree of Unsaturation Degree of Unsaturation The degree of unsaturation (also known as the index of hydrogen deficiency (IHD) or rings plus double bonds) 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 qualitative inspection. 1. Alkanes :CH3-CH3, molecular formula is C2H6 [saturated, contains 7 sigma (single) bonds] 2. Alkenes :In case of alkenes like C2H4 ethene, the structure of ethane shows the presence of one double bond and four sigma bonds. CH2= CH2 (unsaturated molecule of C2H4 contains one double bond).
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As one double bond have one sigma and one pi-bond, so the presence of one pi-bond makes the ethene molecule unsaturated. Cyclic molecule :In the case for cyclic molecules like cyclo-propane, the chemical formula of cyclopropane is C3H6 which is similar to propene C3H6 . But the ring does not contain any pi-bond. So the ring is also introduced one degree of unsaturation. Calculating Degrees of Unsaturation :There is a method for calculating the degree of unsaturation. The degree of unsaturation can be measured easily from the molecular formula of compounds. (2n+2) rule- For Hydrocarbons :The molecules having only hydrogen and carbon and multiple bonds, the degree of unsaturation is calculated by applying (2n+2) rule. In alkenes (H2C=CH2) has the molecular formula C2H4, the presence of a double bond makes the hydrogen atoms less than in alkanes (CH3CH3, has the formula C2H6). Similar the cyclo-alkanes contain two less hydrogen atom in their molecular formula than parent hydrocarbon because two valance are utilized for closing the ring. Like the molecular formula of cyclo-hexane is C6H12 in which contain two less hydrogen atom than the molecular formula of hexane C6H14. The degree of unsaturation in hydrocarbons can be calculated by the following steps; First calculate the number of hydrogen atom in the parent hydrocarbon using the (2n + 2) rule.
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The every missing two hydrogen represents one degree of unsaturation. For example, CH3CH2CH2CH=CH CH3 , the molecular formula is C6H12 and the molecular formula of parent hydrocarbon is C6H14. Thus the formula has two less hydrogen than parent molecular formula so the degree of unsaturation is equal to 1 DU. Similar in C4H6 has four less hydrogen than the parent hydrocarbon C4H10, so the degree of unsaturation is equal to 2 DU.
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