05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 196 196 CHAPTER 5 • ADDITION REACTIONS OF ALKENES As we just observed, neither method would yield a single compound with a 2-pentene as the start- ing alkene. Hence C cannot be prepared as a pure compound by either method. (Don’t worry— there are other ways to make this alcohol!) Finally, the possible alkene starting materials for alcohol D are cis- or trans-2-hexene and cis- or trans-3-hexene OH ? CH3CH2CH2CH CHCH3 or CH3CH2CH CHCH2CH3 CH3CH2CH2CHCH2CH3 2-hexene 3-hexene D Because the double bond in 3-hexene is symmetrically located, either method in principle would give alcohol D as the only product. Hence, either stereoisomer of 3-hexene is a satisfactory start- ing material. However, the reaction of a 2-hexene would, like the reaction of a 2-pentene, give a mixture of constitutional isomers by either method. PROBLEMS 5.11 From what alkene and by which method would you prepare each of the following alcohols essentially free of constitutional isomers? (a) (C2H5)3C OH (b) OH (c) L OH L 5.12 Which of the following alkenes would yield the same alcohol from either oxymercuration– reduction or hydroboration–oxidation, and which would give different alcohols? Explain. (a) cis-2-butene (b) 1-methylcyclohexene 5.5 OZONOLYSIS OF ALKENES Ozone, O3, adds to alkenes at low temperature to yield an unstable compound called a molo- zonide. The molozonide is spontaneously transformed into a second adduct, called simply an ozonide. Both carbon–carbon bonds of the double bond are broken in the formation of the ozonide. O H C CH A CH CH O A 2 O –78 °C 3 3 | % _ CH Cl L L + 1 2 1 2 3 2 2 ozone the double bond is broken O O L 1 2 L O O O 2 2 33 3 2 2 3 " " " " H3C HC CH CH3 H3C HC CH CH3 (5.33) L L L L O L a molozonide 1 2 an ozonide The reaction of an alkene with ozone to yield products of double-bond cleavage is called ozonolysis. (The suffix -lysis is used for describing bond-breaking processes. Examples are hydrolysis, “bond-breaking by water,” thermolysis, “bond-breaking by heat,” and ozonolysis, “bond-breaking by ozone.”) 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 197 5.5 OZONOLYSIS OF ALKENES 197 Ozone and Its Preparation Ozone is a colorless gas that is formed in the stratosphere,the part of the atmosphere that lies about 6–30 miles above the earth’s surface,by the reaction of oxygen with short-wavelength ultraviolet ra- diation. It is very important in shielding the earth from longer-wavelength “UV-B”radiation, which it absorbs. Although depletion of stratospheric ozone is a significant environmental concern, an in- crease in ozone near the earth’s surface is also an environmental issue.This ozone,formed in complex reactions from nitrogen oxides and unburned hydrocarbons, is a significant contributor to smog. Ozone can be formed from the reaction of oxygen in electrical discharges.An atmospheric exam- ple is the formation of ozone in a thunderstorm by lightning.The laboratory preparation of ozone in- volves a similar reaction: electrical discharge 3O2 2O3 Ozone is produced in the laboratory by passing oxygen through an electrical discharge in a com- mercial apparatus called an ozonator. Because ozone is unstable,it cannot be stored in gas cylinders and must be produced as it is needed. The first step in ozonolysis, formation of the molozonide, is another addition reaction of the alkene p bond. The central oxygen of ozone is a positively charged electronegative atom and therefore strongly attracts electrons. The curved-arrow notation shows that this oxygen can accept an electron pair when the other oxygen of the OAO bond accepts p electrons from the alkene. O O A O 2 L O O L L O | _ 1 2 2 1 2 1 3 3 2 2 3 " " H3C HC A CH CH3 H3C HC CH CH3 (5.34) L L L L L This reaction results in the formation of a ring because the three oxygens of the ozone mole- cule remain intact. Additions that give rings are called cycloadditions. Furthermore, the cy- cloaddition of ozone occurs in a single step. Hence, this is another example, like hydrobora- tion, of a concerted mechanism. The molozonide cycloaddition product is unstable, and spontaneously forms the ozonide. In this reaction, the remaining carbon–carbon bond of the alkene is broken. (The mechanistic details are given in Further Exploration 5.2.) O O L L O O " " Further Exploration 5.2 H3C HC CH CH3 H3C HC L L CH CH3 (5.35) Mechanism of L molozonideL L L L Ozonolysis O" O" ozonideL A few daring chemists have made careers out of isolating and studying the highly explosive ozonides. In most cases, however, the ozonides are treated further without isolation to give other compounds. Ozonides can be converted into aldehydes, ketones, or carboxylic acids, de- pending on the structure of the alkene starting material and the reaction conditions. When the ozonide is treated with dimethyl sulfide, (CH3)2S, the ozonide is split: CH3 O $ 1 H3C HC L L CH CH3 H3C S CH3 2 H3C CH A O $|S O1 _ (5.36) L L ++L 12 L L L 21 O" O" dimethyl sulfide an aldehyde CH3 L 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 198 198 CHAPTER 5 • ADDITION REACTIONS OF ALKENES The net transformation resulting from the ozonolysis of an alkene followed by dimethyl sul- fide treatment is the replacement of a C A C group by two C A O groups: Further Exploration 5.3 $ ) $ Mechanism of Ozonide Conversion ) $ ) into Carbonyl the double bond is Compounds completely broken 1) O3 2) (CH3)2S H3C CH A CH CH3 H3C CHOA OACH CH3 (5.37) L L L + L A O here O A here If the two ends of the double bond are identical, as in Eq. 5.37, then two equivalents of the same product are formed. If the two ends of the alkene are different, then a mixture of two dif- ferent products is obtained: O3 (CH3)2S CH3(CH2)5CH A CH2 CH3(CH2)5CH A O O ACH2 (5.38) CH2Cl2 heptanal +formaldehyde (75% yield) If a carbon of the double bond in the starting alkene bears a hydrogen, then an aldehyde is formed, as in Eqs. 5.37 and 5.38. In contrast, if a carbon of the double bond bears no hydro- gens, then a ketone is formed instead: H3C H 1) O3 H3C H 2) (CH3)2S $C A C) $C AO O A C) (5.39) + H3C) $CH3 H3C) $CH3 a ketone an aldehyde If the ozonide is simply treated with water, hydrogen peroxide (H2O2) is formed as a by-product. Under these conditions (or if hydrogen peroxide is added specifically), aldehydes are converted into carboxylic acids, but ketones are unaffected. Hence, the alkene in Eq. 5.39 would react as follows: H3C H 1) O3 H3C OH 2) H2O ( H2O2) $C A C) + $C AO O A C) (5.40) + H3C) $CH3 H3C) $CH3 a ketone a carboxylic acid The different results obtained in ozonolysis are summarized in Table 5.1. If the structures of its ozonolysis products are known, then the structure of an unknown alkene can be deduced. This idea is illustrated in Study Problem 5.3. Study Problem 5.3 Alkene X of unknown structure gives the following products after treatment with ozone followed by aqueous H2O2: O S A Oand HO C CH2CH3 L L propionic acid cyclopentanone What is the structure of X? 05_BRCLoudon_pgs5-0.qxd 12/5/08 2:47 PM Page 199 5.5 OZONOLYSIS OF ALKENES 199 Solution The structure of the alkene can be deduced by mentally reversing the ozonoly- sis reaction. To do this, rewrite the CAO double bonds as “dangling” double bonds by dropping the oxygen: O S S CA OC A and HO C CH2CH3 HO C CH2CH3 L L L L Next, replace the HO group of any carboxylic acid fragments with H . This is done because a carboxylic acid is formedL only when there is a hydrogen on the carbon ofL the double bond (see Table 5.1). O S S HO C CH2CH3 H C CH2CH3 L L L L Finally, connect the dangling ends of the double bonds in the two partial structures to generate the structure of the alkene: CH CH connect S 2 3 C A H C CH2CH3 C AC) L L $H alkene structure TABLE 5.1 Summary of Ozonolysis Results Under Different Conditions Conditions of ozonolysis Alkene carbon O3 ,then (CH3)2 SO3 ,then H2O2 /H2O R R R $C A $COA $COA R) R) R) ketone ketone R R R $C A $COA $COA H) H) HO) aldehyde carboxylic acid H H H $C A $COA $COA H) H) HO) formaldehyde formic acid 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.