05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 200 200 CHAPTER 5 • ADDITION REACTIONS OF ALKENES PROBLEMS 5.13 Give the products (if any) expected from the treatment of each of the following compounds with ozone followed by dimethyl sulfide. (a) 3-methyl-2 pentene (b) A CH2 0 (c) cyclooctene (d) 2-methylpentane 5.14 Give the products (if any) expected when the compounds in Problem 5.13 are treated with ozone followed by aqueous hydrogen peroxide. 5.15 In each case, give the structure of an eight-carbon alkene that would yield each of the following compounds (and no others) after treatment with ozone followed by dimethyl sulfide. (a) O (b) CH3CH2CH2CHA O (c) O S S O A CH(CH2)5C CH3 L S O 5.16 What aspect of alkene structure cannot be determined by ozonolysis? FREE-RADICAL ADDITION OF HYDROGEN 5.6 BROMIDE TO ALKENES A. The Peroxide Effect Recall that addition of HBr to alkenes is a regioselective reaction in which the bromine is directed to the carbon of a double bond with the greater number of alkyl groups (Sec. 4.7A). For example, 1-pentene reacts with HBr to give almost exclusively 2-bromopentane: CH3CH2CH2CHA CH2 H Br CH3CH2CH2CHCH3 (5.41) 1-pentene + L "Br 2-bromopentane (79% yield) For many years, results such as this were at times difficult to reproduce. Some investigators found that the addition of HBr was a highly regioselective reaction, as shown in Eq. 5.41. Oth- ers, however, obtained mixtures of constitutional isomers in which the second isomer had bromine bound at the carbon of the double bond with fewer alkyl groups. In the late 1920s, Mor- ris Kharasch (1895–1957) of the University of Chicago began investigations that led to a solu- tion of this puzzle. He found that when traces of peroxides (compounds of the general structure R O O R) are added to the reaction mixture, the regioselectivity of HBr addition is re- versed!L LIn otherL words, 1-pentene was found to react in the presence of peroxides so that the bromine adds to the less branched carbon of the double bond: O O S S PhC O O C Ph LbenzoylL L peroxideL L (small amount used) CH3CH2CH2CHA CH2 H Br CH3CH2CH2CH2CH2 Br (5.42) 1-pentene + L 1-bromopentane L (96% yield) (Contrast this result with that in Eq. 5.41.) This reversal of regioselectivity in HBr addition is called the peroxide effect. 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 201 5.6 FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES 201 Because the regioselectivity of ordinary HBr addition is described by Markovnikov’s rule (p. 148), the peroxide-promoted addition is sometimes said to have anti-Markovnikov regioselectivity. This means simply that the bromine is directed to the carbon of the alkene double bond with fewer alkyl sub- stituents. It was also found that light further promotes the peroxide effect. When Kharasch and his colleagues scrupulously excluded peroxides and light from the reaction, they found that HBr addition has the “normal” regioselectivity shown in Eq. 5.41. The peroxide effect is observed with all alkenes in which alkyl substitution at the two car- bons of the double bond is different. In other words, in the presence of peroxides, the addition of HBr to alkenes occurs such that the hydrogen is bound to the carbon of the double bond bearing the greater number of alkyl substituents. Furthermore, the peroxide-promoted reac- tion is faster than HBr addition in the the absence of peroxides. Very small amounts of perox- ides are required to bring about this effect. H C HBr 3 peroxides $ CH CH Br faster $ 2 H3C L L $ H3C $C A CH2 (5.43) CH3 HBr H3C no peroxides H3C "C CH3 slower L L "Br The regioselectivity of HI or HCl addition to alkenes is not affected by the presence of per- oxides. For these hydrogen halides, the normal regioselectivity of addition predominates, whether peroxides are present or not. (The reason for this difference is discussed in Sec. 5.6E.) The addition of HBr to alkenes in the presence of peroxides occurs by a mechanism that is completely different from that for normal addition. This mechanism involves reactive interme- diates known as free radicals. To appreciate the reasons for the peroxide effect, then, let’s di- gress and learn some basic facts about free radicals. B. Free Radicals and the “Fishhook” Notation In all reactions considered previously, we used a curved-arrow notation that indicates the movement of electrons in pairs. The dissociation of HBr, for example, is written (5.44) H Br H| Br _ L 2 3 + 3 2 3 In this reaction, bromine takes both electrons2 of the H Br2 covalent bond to give a bromide ion, and the hydrogen becomes an electron-deficient species,L the proton. This type of bond breaking is an example of a heterolysis, or heterolytic cleavage (hetero different; lysis bond-breaking). In a heterolytic process, electrons involved in the process= “move” in pairs.= Thus, a heterolysis is a bond-breaking process that occurs with electrons “moving” in pairs. However, bond rupture can occur in another way. An electron-pair bond may also break so that each bonding partner takes one electron of the chemical bond. H Br H Br (5.45) L 2 3 8 + 8 2 3 In this process, a hydrogen atom and a2 bromine atom are produced.2 As you should verify, these atoms are uncharged. This type of bond breaking is an example of a homolysis, or homolytic cleavage (homo the same; lysis bond-breaking). In a homolytic process, electrons involved = = 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 202 202 CHAPTER 5 • ADDITION REACTIONS OF ALKENES in the process “move” in an unpaired way. Thus, a homolysis is a bond-breaking process that oc- curs with electrons moving in an unpaired fashion. A different curved-arrow notation is used for homolytic processes. In this notation, called the fishhook notation, electrons move individually rather than in pairs. This type of electron flow is represented with singly barbed arrows, or fishhooks; one fishhook is used for each electron: H Br H Br (5.46) L 2 3 8 + 8 2 3 Homolytic bond cleavage is not restricted2 to diatomic molecules.2 For example, peroxides, because of their very weak O O bonds, are prone to undergo homolytic cleavage: 1 1 1 L (CH3)3CO O C(CH3)3 2(CH3)3CO (5.47) L 1 L 1 L L 1 8 di-tert-butyl peroxide tert-butoxy radical The fragments on the right side of this equation possess unpaired electrons. Any species with at least one unpaired electron is called a free radical. The hydrogen atom and the bromine atom on the right side of Eq. 5.46, as well as the tert-butoxy radical on the right side of Eq. 5.47, are all examples of free radicals. Radicals Bound and Free The “R” used in the R-group notation (Sec. 2.9B) comes from the word radical. In the mid-1900s, R groups were called “radicals.”Thus, the CH3 group in CH3CH2OH might have been referred to as the “methyl radical.”Such R groups, when not bonded to anything, were then said to be “free radicals.” Thus, CH3 was said to be the “methyl free radical.”Nowadays we simply use the word group to refer to a group8 of atoms (for example, methyl group) in a compound; we reserve the word radical for a species with an unpaired electron. PROBLEMS 5.17 Draw the products of each of the following transformations shown by the fishhook notation. Br (a) (CH3)3C C(CH3)3 (b) L 8 12 3 (CH3)2C A CH2 (c) [H2C A CH CH A CH CH2 ] L L 8 (d) R CH2 CH2 L L 8 5.18 Indicate whether each of the following reactions is homolytic or heterolytic, and tell how you know. Write the appropriate fishhook or curved-arrow notation for each. (a) H2OH| _ OH 2 + 3 2 (b) CH23CH2OH CH3O 2 CH3CHOH CH3OH + 2 8 + 2 8 2 2 C. Free-Radical Chain Reactions Although a few stable free radicals are known, most free radicals are very reactive. When they are generated in chemical reactions, they generally behave as reactive intermediates—that is, they react before they can accumulate in significant amounts. This section shows how free rad- icals are involved as reactive intermediates in the peroxide-promoted addition of HBr to alkenes. This discussion will provide the basis for a general understanding of free-radical re- actions, as well as the peroxide effect in HBr addition, which is the subject of the next section. 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 203 5.6 FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES 203 The vast preponderance of free-radical reactions can be classified as free-radical chain re- actions. A free-radical chain reaction involves free-radical intermediates and consists of the following fundamental reaction steps: 1. initiation steps, 2. propagation steps, and 3. termination steps. Let’s examine each of these steps using the peroxide-promoted addition of HBr to alkenes as an example of a typical free-radical reaction. (The reason for the “chain reaction” terminology will become apparent.) Initiation In the initiation steps, the free radicals that take part in subsequent steps of the reaction are formed from a free-radical initiator, which is a molecule that undergoes homol- ysis with particular ease.