CHAPTER 11: Unit 13. Chemical Reactions: Alkenes and Alkynes

Learning Objective

1. Write equations for the addition reactions of alkenes with hydrogen, halogens, 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. (The use of hydrogenation to convert unsaturated vegetable oils to saturated fats is discussed in Chapter 7 “Lipids”, Section 7.2 “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 2 “Alcohols, Phenols, Thiols, Ethers””, Section 2.4 “Reactions That Form Alcohols”, where we deal with this reaction in the synthesis of alcohols.

Example

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.

Addition reactions of alkenes: Numerous substances including H₂, Cl₂,Br₂, HCl, HBr and H₂O  add to an alkene carbon-carbon double bond.

An addition reaction is a chemical reaction in which atoms or groups of atoms are added to each carbon atom of a carbon-carbon multiple bonds in a hydrocarbon or hydrocarbon derivative.

A general equation for the addition reaction is:

Addition reactions can be classified as symmetrical and unsymmetrical.

A symmetrical addition reaction is an addition reaction in which identical atoms ( or groups) are added to each carbon of a carbon-carbon double bond.

An unsymmetrical addition reaction is an addition reaction in which different atoms or groups of atoms are added across the C-C double bond.

Hydrogenation:

Addition is a reaction in which elements are added to a compound. Why does addition occur? A double bond is made of a strong bond and a weak bond. When a molecule is allowed to react with an alkene, weak bond is broken and a strong single bond is formed.  When H₂ molecule is added across the double bond in presence of a transition metal as catalyst, reaction is called hydrogenation. When halogen molecules like Cl2, Br2 are added, then it is called halogenation. When hydrogen halides are added, the reaction is named as hydrohalogenation.

The hydrogenation of vegetable oils is very important commercial process these days. Vegetable oils from sources like soybeans and cottonseeds are composed of long-chain organic molecules that contain several double bonds. When these oils are hydrogenated, they are converted to low-melting solids that can be used in margarines and shortenings.

Formation of alkane from alkene via hydrogenation is the reverse reaction of the formation of alkene from alkane via dehydrogenation.

Dehydrogenation of Cyclohexane

When both alkene and the reactant to be added are unsymmetrical, the reaction proceeds according to Markonikov’s rule. The carbon atom of the double bond that already has the greater number of H atoms gets one more. In general, the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom- that is the carbon that has more H’s to begin with.

For example, the addition of HCl to propene ( an unsymmetrical alkene) could produce either 1-chloropropane or 2-chloropropane, depending on whether the H from the HCl attaches itself to carbon 2 or 1.

When two isomeric products are possible, one product usually predominates. The dominant product can be predicted by Markonikov’s rule.

Markonokov’s rule states that when an unsymmetrical molecule of the form HX adds to an unsymmetrical alkene, the H-atom from HX becomes attached to the unsaturated carbon atom that already has maximum number of H atom.

IN CLASS PRACTICE PROBLEMAddition of water: Hydration:

Hydration results in the addition of water to an alkene. Two bonds are broken, one bond of C=C double bond and H-OH bond and new C-H and C-OH bonds are formed. When water is added to alkene, alcohol is formed.

Following the Markonikov’s rule, the addition of H₂O to an unsymmetrical alkene, the H atom bonds to the less substituted carbon- that is the carbon that has more H’s to begin with.Alkynes: All reactions that were shown above occur with alkyne only the amount of reactant is doubled. The first molecule converts the triple bond into double bond and second molecule is added across the double bond. Most important reaction is hydrogenation of alkyne.

Homework Exercises

1. Complete each equation.a. (CH₃) ₂C=CH₂ + Br₂ →b.c.
2. Complete each equation.a.b.c.

Answer

1. a. (CH₃)₂CBrCH₂Brb.  CH₃CH(CH₃)CH₂CH₃c.

Polymerization of Alkene: Addition polymers: A Polymer is a large molecule formed by the repetitive bonding together of many smaller molecules. The smaller repeating units of polymer are called monomers.

the process by which polymer is made is called polymerization.

An addition polymer is a polymer in which the monomers simply add together with no other side products are formed.

The formula of simplest repeating unit ( the monomer with the double bond changed to a single bond) is written in parentheses and subscript n is added after the parentheses. The value of n exceeds 100,000 and can sometimes exceeds a million.

Polythethylen terephthalate (polymer used in plastic water bottles)

IN CLASS PRACTICE PROBLEMS

1. What is a monomer? What is a polymer? How do polymer molecules differ from the molecules we have discussed in earlier sections of this chapter?
2. What is addition polymerization? What structural feature usually characterizes molecules used as monomers in addition polymerization?
3. What is the molecular formula of a polymer molecule formed by the addition polymerization of 175 molecules of vinyl chloride (CH₂=CHCl)?

Answers

1. Monomers are small molecules that can be assembled into giant molecules referred to as polymers, which are much larger than the molecules we discussed earlier in this chapter.
2. In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers.
3. C₃₅₀H₅₂₅Cl₁₇₅

Additional Exercise: What is the primary product of the reaction of the following compound with water?

HOMEWORK EXERCISE

1. Complete each equation.

a. (CH₃) ₂C=CH₂+ Br₂→

b.

c.2. Complete each equation.

a. 

Answer

1. (CH₃)₂CBrCH₂Br

1. CH₃CH(CH₃)CH₂CH₃

c.