2.4: Chemical Properties of Alkenes

Learning Objectives

• To write equations for the addition reactions of alkenes with hydrogen, halogens, HX, and water

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.

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):

Similarly, an alkene can react with HX where X is a halogen. This reaction is called hydrohalogenation and it follows the same format as hydration. 

When a homonuclear diatomic molecule such as H₂ or X₂ reacts with an alkene, there is only one possible product. However, when a molecule consisting of two or more elements, like water or HX, reacts with an alkene, there may be multiple possible products. Markovnicov's rule is used to predict the major product in these cases. 

Markovnicov's rule: When a small molecule reacts with an alkene in an addition reaction, the hydrogen add to the carbon that already has more hydrogen atoms. 

In the examples used above, the two carbon atoms of the alkene had the same number of hydrogen atoms. However, in the reaction of propene with water or HX there are two possible products.

In the reaction above between propene and HBr, the second product, 2-bromopropane, would predominate and a lesser amount of 1-bromopropane would form. 

Example \(\PageIndex{1}\)

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

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

Solution

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

1. 
2. 
3. 

Exercise \(\PageIndex{1}\)

Write the equation for each reaction.

1. CH₃CH₂CH=CH₂ with H₂ (Ni catalyst)
2. CH₃CH=CH₂ with Cl₂
3. CH₃CH₂CH=CHCH₂CH₃ with H₂O (H₂SO₄ catalyst)

Concept Review Exercises

1. What is the principal difference in properties between alkenes and alkanes? How are they alike?
2. If C₁₂H₂₄ reacts with HBr in an addition reaction, what is the molecular formula of the product?

Answers

1. Alkenes undergo addition reactions; alkanes do not. Both burn.
2. C₁₂H₂₄Br₂

Key Takeaway

• Alkenes undergo addition reactions, adding such substances as hydrogen, bromine, and water across the carbon-to-carbon double bond.

Exercises

1. Complete each equation.

   1. (CH₃) ₂C=CH₂ + Br₂ →
   2. \(\mathrm{CH_2\textrm{=C}(CH_3)CH_2CH_3 + H_2 \xrightarrow{Ni}}\)
   3. 

2. Complete each equation.

   1. \(\mathrm{CH_2\textrm{=CHCH=C}H_2 + 2H_2\xrightarrow{Ni}}\)
   2. \(\mathrm{(CH_3)_2\textrm{C=C}(CH_3)_2 + H_2O \xrightarrow{H_2SO_4}}\)
   3. 

Answer

1. 1. (CH₃)₂CBrCH₂Br
   2. CH₃CH(CH₃)CH₂CH₃
   3.