First, the maximum number of hydrogens possible for a given compound (2C + 2) is calculated and then the actual number of hydrogens present in the compound (H) is subtracted. Understanding this relationship allows for the degrees of unsaturation of a compound to be calculated from its molecular formula. Because they also have fewer than maximum number of hydrogens possible, cyclic compounds are also considered unsaturated.Ĭalculating the Degree of Unsaturation (DoU)Īs noted above, every degree of unsaturation causes the loss of two hydrogens from a compound's molecular formula when compared to an alkane with the same number of carbons. Also, it is important to note that cycloalkanes with one ring have a general molecular formula of C nH 2n just like alkenes. While the unsaturated compounds propene (C 3H 6) and propyne (C 3H 4) both have fewer hydrogens. The relationship between the number of carbons (n) and hydrogens in the molecular formula for alkanes, alkenes, and alkynes are listed below.įor example, the three carbon alkane, propane has the molecular formula of C 3H 8. Collectively, compounds which have fewer hydrogen atoms than an alkane with the same number of carbon atoms are called unsaturated hydrocarbons. Likewise, compounds containing a carbon-to-carbon triple bonds (R–C≡C–R) called alkynes ( Discussed in Chapter 9), also have fewer hydrogens than the corresponding alkane. The presence of a double bond causes alkenes to have less hydrogens than an alkane with the same number of carbons. \)īecause alkanes have the maximum number of H atoms possible according to the rules of covalent bonds, alkanes are also referred to as saturated hydrocarbons.
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