Radical Chain Reactions in Foundational Organic Chemistry

Chapter 9

Downloaded by CHINESE UNIV OF HONG KONG on March 7, 2016 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1151.ch009

Radical Chain Reactions in Foundational Organic Chemistry Warren T. Ford* Department of Chemistry, Portland State University, 262 SRTC Building, 1719 SW 10th Avenue, Portland, OR 97207 *E-mail: [email protected]

In the foundational organic chemistry course radical chain reactions are introduced via the chlorination of alkanes. The mechanism of a chain reaction and its selectivity based on the homolytic C-H bond dissociation energies leading to methyl, primary, secondary, tertiary, allylic, and benzylic alkyl radicals are vital to understanding of material covered later in the course. Radical chain polymerization introduces polystyrene and poly(vinyl chloride) which, due to their everyday use, are more interesting to students than chloroalkanes. The polymerizations further support the importance of radical chain reactions, and the head-to-tail structures of the polymers reinforce the importance of selectivity of radical formation. Radical chain autoxidation of ethers and lipids and inhibition of both polymerization and autoxidation of foods by hindered phenols complete the discussion of radical reactions. The additional time devoted to polymerization and autoxidation is compensated by omitting topics such as radical addition of HBr to alkenes and multistep syntheses.

Introduction Radical chain reactions are fundamental to organic chemistry, although not as widely used in organic synthesis as heterolytic bond-breaking and metal-coordinated reactions. Most textbooks devote one chapter to radical chain reactions during the first term and then return to a few more specific examples in later chapters. The early chapter generally includes the mechanism of © 2013 American Chemical Society Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


chlorination of methane and higher alkanes, homolytic bond dissociation energies in terms of reactivity and selectivity of chlorination, the greater selectivity of bromination than of chlorination, the planar structure of alkyl radicals that prevents enantioselective reactions, the anti-Markovnikov radical addition of HBr to alkenes, and multistep syntheses that include radical chain halogenation. Some texts cover radical reactions of biologically important compounds. Some texts include a section on radical chain polymerization. Others defer polymerization to a separate special topics section or to a separate chapter near the end of the book. This article shows how polymerization can be incorporated into the radical chain reaction section of first term organic chemistry without devoting more than the usual amount of time to radical reactions. Of course this requires deletion of some of the standard topics in the radical reactions chapter. Students gain from the inclusion of polymerization because the familiarity with polymeric materials in everyday life arouses their interest and because understanding basic polymer chemistry prepares them for their future careers in chemistry, chemical engineering, molecular biosciences, and the health professions.

Organic Chemistry I This article is based on the course taught at Oklahoma State University Fall semester 2010. The textbook was Solomons and Fryhle, Organic Chemistry, tenth edition (1). Two fifty-minute class periods and part of a review period preceding an exam were devoted to radical chain reactions. For out-of-class study students were assigned selected sections of the text, a two-page handout supplement on polymers, some of the text exercises for which answers were available in the accompanying study guide, and a short weekly problem set to be graded. The mechanism of a radical chain reaction via initiation, propagation, and termination steps was introduced with the chlorination of methane. Experimental product distributions from monochlorination of propane and 2-methylpropane were compared with statistical distributions to illustrate selectivity of reaction of tertiary > secondary > primary C-H bonds. Using the large table of homolytic bond dissociation energies (DH°) in the text students calculated enthalpy changes of the overall reactions and of the individual propagation steps in the radical chain mechanism. The practical importance of alkanes from petroleum and natural gas and of the chlorination of alkanes in industrial organic synthesis were mentioned briefly in class. Industrial organic synthesis and nature produce polymers, the materials we know as plastics, elastomers, and fibers. Students were told to look around them to see examples, and then materials that they identified from each class of polymeric materials such as cotton, wool, nylon, and polyester clothing, polyethylene, polystyrene, and poly(vinyl chloride) plastics, and synthetic and natural rubber were discussed briefly. The word “polymer” refers both to a material comprised of long chain molecules and to those long chain molecules, which also are called macromolecules. Thus “polymer” may refer either to a material or to a macromolecule. Common names were used because of familiarity. The systematic names of polymers are rarely used. 106 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Relation of Polymer Structure to Monomer Structure


The handout material introduced the structural relations of alkene polymers to their monomers. It is essential at this point to understand the concept of a polymer repeat unit. Not all textbooks do this well. For structures we use monosubstituted alkenes and their polymers (eq 1). Polyethylene (X = H), poly(vinyl chloride) (X = Cl), and polystyrene (X = C6H5) are examples.

The empirical formula of the repeat unit of an addition polymer is identical to that of the monomer. The subscript n after the repeat unit is an integer that denotes the average number of repeat units in a chain. The average number of carbon atoms in the polymer chain is 2n. As a specific example let n = 8 and write out the 16-carbon atom structure (eq 2).

Polyethylene must have average lengths n of thousands, and polystyrene must have average lengths of hundreds to be a practical plastic. The dangling bonds at the ends of the repeat unit structure must be explained. Of course the polymer structure has end groups (EG in structure 1), but the end groups are not usually shown because when n is very large, the two end groups cannot be identified in the presence of hundreds or thousands of repeat units. Moreover the structures of the end groups generally have no effect on the important properties of the polymer as a hard plastic, a thin film packaging material, or a fiber.

Even though it is the simplest alkene polymer, polyethylene was not discussed because it is no longer produced by radical chain reactions. New polyethylene manufacturing plants for decades have been based on metal coordination polymerizations that produce linear high density polyethylene and branched lower density polyethylene from mixtures of ethylene and 1-butene or 1-hexene. Although radical chain polymerization of ethylene is of historical interest and is instructive, its branched structures require additional discussion of chain transfer steps in radical chain reactions. Polypropylene, like polyethylene, is produced by coordination polymerization, so only the structure was mentioned. 107 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Mechanism of Radical Chain Polymerization Why does polymerization occur at all? A reaction proceeds only if the free energy of the products is less than the free energy of the reactants (ΔG < 0). Comparing monomer to polymer structure the net result of polymerization is conversion of a π-bond of the monomer to a σ-bond of the polymer chain. Because C-C single bonds are stronger (~335 kJ/mol) than C=C π-bonds (~250 kJ/mol) polymerization is exothermic. Polymerization also results in a loss of entropy, but the entropy (ΔS) contribution is much less than the enthalpy (ΔH) contribution to the net free energy change (ΔG = ΔH - TΔS). The mechanism of polymerization of styrene is shown in shown in Scheme 1.

Initiation Most radical chain polymerizations are initiated by thermal decomposition of an initiator that forms radicals at a convenient temperature, such as dibenzoyl peroxide at 80 °C. Initiation is endothermic because it breaks an O-O bond and forms two electron deficient radicals. The O-O bond of peroxides is weak (for dibenzoyl peroxide DH° = 139 kJ mol-1). The value is given in the table of homolytic bond dissociation energies in the textbook (1). A benzoyloxy radical then adds to the C=C bond of styrene to form a benzylic styryl radical. Radicals are electron deficient. The Lewis structures have only 7 electrons in the valence shell of one oxygen atom of the benzoyloxy radical (2 in Scheme 1) and only 7 electrons in the valence shell of the benzoyloxystyryl radical (3). As in chlorination of methane in which a chlorine atom and an alkyl radical have 7 electrons in the valence shell, radicals are reactive intermediates, not stable compounds. The electron deficient styryl radical has an average lifetime on the order of a microsecond before it reacts by propagation or termination.

Propagation In the first propagation step the styryl radical adds to another molecule of styrene to form a new, longer styryl radical. Propagation is exothermic; a π-bond breaks and a σ-bond forms. This process continues to form longer and longer styryl radicals until the chain reaction terminates. Propagation occurs only “headto-tail” by addition of the styryl radical to the less hindered CH2 end of the styrene double bond to form the benzylic radical, which is considerably more stable than the alternative primary alkyl radical because of delocalization of unpaired electron density into the phenyl ring. The regioselectivity of polymerization is based on the same principles of DH° and radical stability as the selectivity of radical chain substitution of Cl for H of alkanes. For a methyl C-H bond of toluene DH° = 375 kJ mol-1 to form a benzylic radical vs. DH° = 421 kJ mol-1 for a C-H bond of ethane to form a primary alkyl radical. 108 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Scheme 1. Mechanism of Polymerization of Styrene

Termination Termination converts two radicals to stable molecules having completely filled valence shells of electrons. In the beginning class only termination by combination is shown, which is analogous to the combination of two methyl radicals to form ethane during the chlorination of methane. Combination of two polystyryl radicals having an average of n repeat units forms a polymer having 2n repeat units. Other termination mechanisms give polymer chain lengths of n 109 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


repeat units and different end group structures. Termination is highly exothermic because it forms a C-C single bond from two electron deficient radicals, but the probability of termination is much less than the probability of propagation because the concentration of styrene in the polymerizing mixture is more than a million times higher than the concentration of styryl radicals. A styryl radical collides more than a million times more often with a styrene molecule than with another styryl radical.

Problem Assignment To help the students learn by repetition and by analogous structures, the problem set they submitted asked questions such as those in Scheme 2.

Scheme 2. Problem Assignment

In order to limit the time devoted to the radical chain reaction section of the course, other alkene polymers were not covered at this time. Polyethylene and polypropylene were not discussed because they are made by metal-coordination polymerization. The 1,2- vs. 1,4-repeat units of poly(1,3-butadiene) from the propagation polybutadienyl radicals was covered in the chapter on dienes after electrophilic 1,2- vs. 1,4-addition of HBr. Poly(isobutylene) is made by cationic polymerization and was covered in the section on substitution and addition reactions that proceed via carbocations. 110 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Autoxidation and Antioxidants The radical chain reaction section of the course concluded with radical chain autoxidation and antioxidants. The mechanism of autoxidation was the third example of radical chain reactions. Radical chain autoxidation is important in the degradation of organic materials ranging from solvents such as diethyl ether, to plastics (which account for some of the reputation of plastics as cheap materials), to food as in the degradation of unsaturated lipids (butter turns rancid) during storage. The propagation steps are shown in Scheme 3. R-H represents a compound or polymer with an aliphatic C-H bond. In the first propagation step ground state triplet dioxygen adds to a carbon radical to form a new C-O bond of an alkylperoxy radical (ROO.). In the second propagation step a hydrogen atom transfers from a C-H bond of another molecule of diethyl ether, a polymer, or a lipid to the peroxy oxygen atom to form an alkyl hydroperoxide (ROOH) and a new carbon radical. The topic of autoxidation of lipids appears again near the end of the two-semester course.

Scheme 3. Propagation Steps of Autoxidaton

Antioxidants were discussed also. Students were asked to look for BHT (butylated hydroxytoluene) and “mixed tocopherols” (which also are phenols; α-tocopherol is Vitamin E) in the lists of ingredients on packages of preserved foods such as breakfast cereals and bread. See Scheme 4. BHT and tocopherols inhibit both autoxidation and radical chain polymerization by transferring a phenolic hydrogen atom to a propagating radical, which stops the radical chain reaction because the hindered phenoxy radical does not propagate further. Antioxidants inhibit the formation of explosive peroxides in ether solvents during storage, the spoilage of foods, and the polymerization of monomers during storage in the laboratory and in railroad tank cars before use.

Scheme 4. Inhibitors of Autoxidation and Polymerization 111 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Omissions How can radical chain polymerization and autoxidation be included without lengthening the radical reaction of the course? Several of the standard topics in the radical reaction chapter of the textbook are omitted, including selectivity of bromination of alkanes, lack of enantioselectivity at chiral centers formed by radical chain halogenation, radical addition of HBr to alkenes, and multi-step synthesis of small molecules that involve alkane halogenation. Which is more important to the world as we know it – polymerization of alkenes or anti-Markovnikov addition of HBr to alkenes? Which topic holds student interest better? Multi-step organic synthesis may captivate the instructor and students who are future organic chemists, who comprise less than 5% of the class, but synthesis is neither as important nor as interesting as polymerization and autoxidation to the vast majority of pre-health profession and engineering students.

Acknowledgments Both Chemistry 3053 and 3153 students and faculty colleagues at Oklahoma State University, who set high standards for research and teaching, have unknowingly contributed to my thinking about this article and the teaching of foundational organic chemistry.

References 1.

Solomons, T. W. G.; Fryhle, C. Organic Chemistry, 10th ed.; Wiley: New York, 2011; chapter 10.

112 Howell; Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Radical Chain Reactions in Foundational Organic Chemistry

Recommend Documents