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Alkenes are commonly described as unsaturated hydrocarbons because they have the capacity to react with substances that add to them. Alkanes, on the other hand, are saturated hydrocarbons and are incapable of undergoing addition reactions. 6.1 Hydrogenation of Alkenes The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes. Hydrogenation is the addition of H 2 to a multiple bond, as illustrated in the conversion of ethylene to ethane. The reaction is exothermic and is characterized by a negative sign for ?H?. Indeed, hydrogenation of all alkenes is exothermic. The heat given off is called the heat of hydrogenation and cited without a sign. In other words, heat of hydrogenation = -?H?.
