Alkenes are valued mainly for addition reactions, 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 hydrogenation—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 halogenation—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 hydration, 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.

Example 4

Write the equation for the reaction between CH₃CH=CHCH₃ and each substance.

1. H₂ (Ni catalyst)
2. Br₂
3. H₂O (H₂SO₄ catalyst)

Show Answer

In each reaction, the reagent adds across the double bond.