13.4 Chemical Properties of Alkenes

Alkenes are valued mainly for addition reactionsA reaction in which substituent groups join to hydrocarbon molecules at points of unsaturation—the double or triple bonds., in which one of the bonds in the double bond is broken. Each of the carbon atoms in the bond can then attach another atom or group while remaining joined to each other by a single bond.

Perhaps the simplest addition reaction is hydrogenationA reaction in which hydrogen gas reacts at a carbon-to-carbon double or triple bond or a carbon-to-oxygen double bond to add hydrogen atoms to carbon atoms.—a reaction with hydrogen (H₂) in the presence of a catalyst such as nickel (Ni) or platinum (Pt).

The product is an alkane having the same carbon skeleton as the alkene. (The use of hydrogenation to convert unsaturated vegetable oils to saturated fats is discussed in Chapter 17 "Lipids", Section 17.2 "Fats and Oils".)

Alkenes also readily undergo halogenationA reaction in which a halogen reacts at a carbon-to-carbon double or triple bond to add halogen atoms to carbon atoms.—the addition of halogens. Indeed, the reaction with bromine (Br₂) can be used to test for alkenes. Bromine solutions are brownish red. When we add a Br₂ solution to an alkene, the color of the solution disappears because the alkene reacts with the bromine:

Another important addition reaction is that between an alkene and water to form an alcohol. This reaction, called hydrationThe addition of water to a substance; in organic chemistry, the addition of water across the carbon-to-carbon double bond of an alkene or the carbon-to-oxygen double bond of an aldehyde or ketone., requires a catalyst—usually a strong acid, such as sulfuric acid (H₂SO₄):

The hydration reaction is discussed further in Chapter 14 "Organic Compounds of Oxygen", Section 14.4 "Reactions That Form Alcohols", where we deal with this reaction in the synthesis of alcohols.