22.3 Reactions of Alkenes and Alkynes

Learning Objectives

By the end of this section, you will be able to:

  • Write equations for the addition reactions of alkenes and alkynes with hydrogen, halogens, and water
  • Describe Markovnikov’s Rule as it applies to addition reactions
  • Describe chemical tests to test for the presence of unsaturated hydrocarbons.

Addition Reactions

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. Examples of addition reactions include hydrogenation, halogenation and hydration.

Hydrogenation of Alkenes

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). An example of the addition reaction between ethylene and hydrogen is shown in Figure 22.3a.

The molecular structures showing an addition reaction of ethylene in the presence of hydrogen to produce ethane. The hydrogens are in red on the reactant side and it shows in red how they are added to the carbons in ethylene producing ethane.
Figure 22.3a. Addition reaction of ethylene with hydrogen to produce ethane. (Credit: Intro Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The product is an alkane having the same carbon skeleton as the alkene.

Halogenation of Alkenes

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 colour of the solution disappears because the alkene reacts with the bromine as shown in Figure 22.3b.

The structures involved in the halogenation of ethylene and bromine (in orange) to produce 1,2-dibromoethane. The bromines are depicted in orange. This reactions shows how bromine is added to the carbons of the double bond. The bromines are shown in orange.
Figure 22.3b. The halogenation of ethylene and bromine to produce 1,2-dibromoethane. (Credit: Intro Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Hydration of Alkenes

Another important addition reaction is that between an alkene and water to form an alcohol. This reaction, called hydration as shown in Figure 22.3c., requires a catalyst—usually a strong acid, such as sulfuric acid (H2SO4).

The structures involved in the hydration of ethylene and water (shown in a blue colour) to produce ethanol. A hydrogen (in blue) and an hydroxy group (in blue) are added to the carbons over the double bond.
Figure 22.3c. Hydration of ethylene and water to produce ethanol (Credit: Intro Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The hydration reaction is discussed in a later chapter, where we deal with this reaction in the synthesis of alcohols.

Example 22.3a

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.

a. 2-butene in the presence of hydrogen produces butane.

b. 2-butene in the presence of bromine produces 2,3-dibromobutane.

c. 2-butene in the presence of water produces 2-butanol.

Image credit: Intro Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0.

 

Exercise 22.3a

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)

Check Your Answers:[1]

Source: Exercise 22.3a is adapted from Fundamentals of GOB Chem, CC BY-NC-SA 4.0 with answer images drawn by Samantha Sullivan Sauer / Biovia Draw, CC BY-NC 4.0

Markovnikov’s Rule

In the addition any unsymmetrical hydrogen-based molecule (e.g. HX or H2O) to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. Examples of the rule is demonstrated in Figure 22.3d.

2-methylpropene reacts with hydrogen chloride in ether to form 2-chloro-2-methylpropane. 1-methylcyclohexene reacts with hydrogen bromide in ether to form 1-bromo-1-methylcyclohexane. X adds to more substituted carbon.
Figure 22.3d. Addition reactions using Markovnikov’s rule for the reactions of 2-methylpropene with HCl (top) and 1-methylcylcohexene with HBr (bottom). (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

When both double-bonded carbon atoms have the same degree of substitution, a mixture of addition products results as demonstrated in Figure 22.3e.

2-pentene reacts with hydrogen bromide in ether to form 2-bromopentane and 3-bromopentane. Text indicates each end of double bond has one substituent.
Figure 22.3e. Addition reaction when the double-bonded carbon has the same degree of substitution as shown when 2-pentene reacts with HBr. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Because carbocations are involved as intermediates in these electrophilic addition reactions, Markovnikov’s rule can be restated in the following way:

Markovnikov’s rule restated In the addition of HX or H2O to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one.

For example, in Figure 22.3f., addition of H+ to 2-methylpropene yields the intermediate tertiary carbocation rather than the alternative primary carbocation, and addition to 1-methylcyclohexene yields a tertiary cation rather than a secondary one. Why should this be?

First reaction shows 2-methylpropene reacting with hydrogen chloride to yield 2-chloro-2-methylpropane. 1-chloro-2-methylpropane is not formed. Second reaction shows 1-methylcyclohexene reacting with hydrogen bromide to yield 1-bromo-1-methylcyclohexane. 1-bromo-2-methylcyclohexane is not formed.
Figure 22.3f. Addition reaction involving the addition of H+ to 2-methylpropene to yield intermediate tertiary carbocation (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Example 22.3b

What product would you expect from reaction of HCl with 1-ethylcyclopentene?

Strategy

When solving a problem that asks you to predict a reaction product, begin by looking at the functional group(s) in the reactants and deciding what kind of reaction is likely to occur. In the present instance, the reactant is an alkene that will probably undergo an electrophilic addition reaction with HCl. Next, recall what you know about electrophilic addition reactions to predict the product. You know that electrophilic addition reactions follow Markovnikov’s rule, so H+ will add to the double-bond carbon that has one alkyl group (C2 on the ring) and the Cl will add to the double-bond carbon that has two alkyl groups (C1 on the ring).

Solution

The expected product is 1-chloro-1-ethylcyclopentane.

Example 22.3c

Synthesizing a Specific Compound

What alkene would you start with to prepare the following alkyl halide? There may be more than one possibility.

Strategy

When solving a problem that asks how to prepare a given product, always work backward. Look at the product, identify the functional group(s) it contains, and ask yourself, “How can I prepare that functional group?” In the present instance, the product is a tertiary alkyl chloride, which can be prepared by reaction of an alkene with HCl. The carbon atom bearing the −Cl atom in the product must be one of the double-bond carbons in the reactant. Draw and evaluate all possibilities.

Solution

There are three possibilities, all of which could give the desired product according to Markovnikov’s rule.

Exercise 22.3b

Draw the major product formed from these reactions.

At the top a) 2-methyl-2-pentene in the presence of water produces? At the bottom b) methylcyclohexene in the presence of water produces?

Check Your Answers:[2]

Activity source: Exercise 22.3b is created by Samantha Sullivan Sauer, using images from Biovia Draw, licensed under CC BY-NC 4.0

For more details on alkene addition reactions including the concept of Markonikov’s rule, watch Alkene Addition Reactions below.

Watch Alkene Addition Reactions: Crash Course Organic Chemistry #16 (youtube.com) (13 min).

Video Source: Crash Course. (2020, Nov 11). Alkene Addition Reactions: Crash Course Organic Chemistry #16 (youtube.com) [Video]. YouTube.

Polymerization

The most important commercial reactions of alkenes are polymerizations, reactions in which small molecules, referred to in general as monomers (from the Greek monos, meaning “one,” and meros, meaning “parts”), are assembled into giant molecules referred to as polymers (from the Greek poly, meaning “many,” and meros, meaning “parts”). For more information on polymerization see Chapter 27: Polymers.

Sourcing of Alkenes

In summary, recall that organic functional groups can be converted into other functional groups through reactions.  To look at the sourcing of alkenes, refer to the map of some of the more common reactions to convert functional groups can be found in Section 19.6 – General Reactions of Carbon in Infographic 19.6a.

Reactions of Alkynes

Chemically, the alkynes are similar to the alkenes. Alkynes can undergo addition, hydration and hydrogenation (or reduction) reactions.

Halogenation of Alkynes

Since the functional group has two π bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example as shown in Figure 22.3g.
This diagram illustrates the reaction of ethyne and two molecules of B r subscript 2 to form 1 comma 1 comma 2 comma 2 dash tetrabromoethane. In this reaction, the structural formula of ethyne, an H atom bonded to a red C atom with a red triple bond to another red C atom bonded to a black H atom, plus B r bonded to B r plus B r bonded to B r is shown to the left of an arrow. On the right, the form 1 comma 1 comma 2 comma 2 dash tetrabromoethane molecule is shown. It has an H atom bonded to a C atom which is bonded to another C atom which is bonded to an H atom. Each C atom is bonded above and below to a B r atom. Each B r atom has three pairs of electron dots. The C and B r atoms, single bond between them, and electron pairs are shown in red.
Figure 22.3g. Addition reaction involving acetylene and bromine (credit: Chemistry: Atoms First 2e (OpenStax), CC BY 4.0)

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.

As a general rule, electrophiles undergo addition reactions with alkynes much as they do with alkenes. Take the reaction of alkynes with HX, for instance. The reaction often can be stopped with the addition of 1 equivalent of HX, but reaction with an excess of HX leads to a dihalide product. For example, reaction of 1-hexyne with 2 equivalents of HBr yields 2,2-dibromohexane. As the following examples indicate, the regiochemistry of addition follows Markovnikov’s rule, with halogen adding to the more highly substituted side of the alkyne bond and hydrogen adding to the less highly substituted side. Trans stereochemistry of H and X normally, although not always, occurs in the product.

1-hexyne reacts with hydrogen bromide in acetic acid to give 2-bromo-1-hexene. This further reacts with hydrogen bromide and acetic acid to give 2,2-dibromohexane.
(credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).
3-hexyne reacts with hydrogen chloride in acetic acid to form (Z)-3-chloro-3-hexene. This further reacts with hydrogen chloride in acetic acid to form 3,3-dichlorohexane.
Figure 22.3h. HBr addition to 1-hexyne (top) and HCl addition to 3-hexyne (bottom). (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Bromine and chlorine also add to alkynes to give addition products, and trans stereochemistry again results as demonstrated in Figure 22.3i. below.

1-butyne reacts with bromine in chloromethane to give (E)-1,2-dibromo-1-butene. This further reacts with bromine in chloromethane to form 1,1,2,2-tetrabromobutane.
Figure 22.3i. Br2 addition to 1-butyne.  (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

The mechanism of alkyne addition is similar but not identical to that of alkene addition. When an electrophile such as HBr adds to an alkene, the reaction takes place in two steps and involves an alkyl carbocation intermediate. If HBr were to add by the same mechanism to an alkyne, an analogous vinylic carbocation would be formed as the intermediate as shown in Figure 22.3j.

The figure shows two mechanisms. First reaction is an alkene reacting with hydrogen bromide to form alkyl bromide. The second is an alkyne reacting with hydrogen bromide to form vinylic bromide.
Figure 22.3j. The mechanism of alkene and alkyne addition involving carbocation intermediate. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

A vinylic carbocation has an sp-hybridized carbon and generally forms less readily than an alkyl carbocation (Figure 22.3j.). As a rule, a secondary vinylic carbocation forms about as readily as a primary alkyl carbocation, but a primary vinylic carbocation is so difficult to form that there is no clear evidence it even exists. Thus, many alkyne additions occur through more complex mechanistic pathways.

Hydration of Alkynes

Hydration of alkynes also can take place. Alkynes don’t react directly with aqueous acid but will undergo hydration readily in the presence of mercury(II) sulfate as a Lewis acid catalyst. The reaction occurs with Markovnikov regiochemistry, so the −OH group adds to the more highly substituted carbon and the −H attaches to the less highly substituted one as demonstrated in Figure 22.3k.

The figure shows 1-hexyne reacting with water, sulfuric acid, and mercury sulfate to generate an enol. This further leads to the final product, 2-hexanone (78%).
Figure 22.3k. Hydration of 1-hexyne to produce 2-hexanone. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Interestingly, the actual product isolated from alkyne hydration is not a vinylic alcohol, or enol (ene + ol), but is instead a ketone.

Hydrogenation of Alkynes

Lastly, hydrogenation (reduction) of alkynes is another chemical reaction that can take place. Alkynes are reduced to alkanes by addition of H2 over a metal catalyst. The reaction in Figure 22.3l., occurs in two steps through an alkene intermediate, and measurements show that the first step in the reaction is more exothermic than the second.

Acetylene reacts with hydrogen and catalyst to form ethene. Ethene reacts with hydrogen and catalyst to form ethane. The standard enthalpy of hydrogen for both reactions is given.
Figure 22.3l. Reduction of alkyne with H2 over a metal with an alkene intermediate catalyst. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene stage if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry, giving a cis alkene product.

The reaction shows 4-octyne reacting with hydrogen and Lindlar's catalyst to give cis-4-octene. This reacts with hydrogen, and palladium catalyst to form octane. Quinoline is also used in the process.
Figure 22.3m. Complete reduction of 4-octyne. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

The alkyne hydrogenation reaction has been explored extensively by the Hoffmann–LaRoche pharmaceutical company, where it is used in the commercial synthesis of vitamin A. The cis isomer of vitamin A produced initially on hydrogenation is converted to the trans isomer by heating as shown in Figure 22.3n.

An alkyne reacts with hydrogen and Lindlar catalyst to give 7-cis-retinol (7-cis-vitamin A; vitamin A has a trans double bond at C7).
Figure 22.3n. The cis isomer of vitamin A produced initially on hydrogenation is converted to the trans isomer by heating (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

An alternative method for the conversion of an alkyne to an alkene uses sodium or lithium metal as the reducing agent in liquid ammonia as solvent. This method is complementary to the Lindlar reduction because it produces trans rather than cis alkenes. For example, in Figure 22.3o., 5-decyne gives trans-5-decene on treatment with lithium in liquid ammonia.

The figure shows 5-decyne reacting with lithium and ammonia to form trans-5-decene (78%).
Figure 22.3o. 5-decyne gives trans-5-decene on treatment with lithium in liquid ammonia. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Spotlight on Everyday Chemistry: 2022 Nobel Prize in Chemistry

Alkynes were involved in the concept of “click” chemistry where an azide is added to an alkyne with a copper catalyst allowing the two molecules to click together forming a cyclic molecule. The click chemistry concept was awarded the 2022 Nobel Prize in Chemistry to Carolyn R. Bertozzi, Morten Meldal and K. Barry Sharpless. For more information refer to infographic 22.3a.

Infographic 22.3a.  Read more about “The 2022 Nobel Prize in Chemistry” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.3a [New tab].

Testing for Presence of Alkenes/Alkynes

There are several ways to test for the presence of carbon-carbon double bonds and triple bonds (unsaturated hydrocarbons).  One such method, as previously mentioned above, is the bromine test.  Here the organic compound containing a double or triple C-C bond is mixed with an aqueous solution of bromine (or chlorine).  With bromine, there is a visible colour change resulting when bromine is added to the double or triple bond (Figure 22.3p. and Figure 22.3q.). Before addition, the bromine is brownish-red.  After addition the solution is colourless. If the solution stays brownish-red, it is a negative result and the compound being tested is saturated.  This means there is no opportunity for addition.

An example of a bromine test. A pipette drops bromine into a test tube. If the results is positive result the test tube sample remains colourless; if negative result turns the sample a brownish-red colour.
Figure 22.3p. Bromine test for presence of double or triple carbon-carbon bond.  Positive result is a colourless solution.  Negative result is a brownish-red solution (credit: Samantha Sullivan Sauer, Created with Chemix, Chemix license)
Bromine test for the presence of unsaturated hydrocarbons. An alkene in the presence of bromine creates a di-bromo alkane. An alkyne in the presence of bromine creates a dibromo alkene.
Figure 22.3q. Bromine test for presence of unsaturated hydrocarbons (credit: Samantha Sullivan Sauer, using Biovia Draw, CC BY-NC 4.0)

A second such test to confirm the presence of a carbon-carbon double or triple bond (unsaturated hydrocarbon) is the oxidation or permanganate test (Figure 22.3s.).  This test is also known as the Baeyer test. Here, potassium permanganate, KMnO4, is used as an oxidizing agent to convert the alkene or alkyne to a diol (two alcohol functional groups in the same molecule).  The visual colour change is from dark purple (permanganate solution) to dark green (manganate solution) then to black precipitate (manganese dioxide) (Figure 22.3r.). If the solution stays purple, it is a negative result and the compound being tested is saturated. This means there is no opportunity for oxidation. This test can give conflicting results in that any other components in the molecule or solution that are mildly reducing will also give a positive result.

A permanganate test. Permanganate is added to a sample with a dropper. A negative test turns the sample a purple colour. A position test results in the formation of a black precipitate.
Figure 22.3r. Permanganate test for presence of double or triple carbon-carbon bond.  Positive result is a black precipitate.  Negative result is a purplish solution (credit: Samantha Sullivan Sauer, Created with Chemix, Chemix license)
Permanganate test for presence of unsaturated hydrocarbons. An alkene in the presence of permanganate solution creates a di-hydroxy alkane. An alkyne in the presence of permanganate solution creates a di-hydroxy alkene.
Figure 22.3s. Permanganate test for presence of unsaturated hydrocarbons (credit: Samantha Sullivan Sauer, using Biovia Draw, CC BY-NC 4.0)

Links to Enhanced Learning

Attribution & References

Except where otherwise noted, this page is written and adapted by David Wegman, Adrienne Richards and Samantha Sullivan Sauer from

  1. 3 reactions are shown: a) 1-butene in the presence of hydrogen produces butane; b) propene in the presence of chlorine produces 1,2-dichloropropane; and c) 3-hexene in the presence of water produces 3-hexanol.

    ↵

  2. At the top a) 2-methyl-2-pentene in the presence of water produces? 2-pentanol At the bottom b) methylcyclohexene in the presence of water produces? cyclohexanol ↵
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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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