5.5 OZONOLYSIS OF ALKENES

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1 196 PTE 5 ADDITIN EACTIN F AKENE As we just observed, neither method would yield a single compound with a 2-pentene as the starting alkene. ence 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 cisor trans--hexene C C 2 C 2 C CC or C C 2 C CC 2 C 2-hexene -hexene Because the double bond in -hexene is symmetrically located, either method in principle would give alcohol D as the only product. ence, either stereoisomer of -hexene is a satisfactory starting material. owever, the reaction of a 2-hexene would, like the reaction of a 2-pentene, give a mixture of constitutional isomers by either method.? C C 2 C 2 CC 2 C D PBEM 5.11 From what alkene and by which method would you prepare each of the following alcohols essentially free of constitutional isomers? (a (C 2 5 C (b (c 5. 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 ZNYI F AKENE zone,, adds to alkenes at low temperature to yield an unstable compound called a molozonide. 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. C C C + A ozone % _ C C C C a molozonide 2 78 C C 2 Cl the double bond is broken C C C C an ozonide 2 (5. 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.

2 5.5 ZNYI F AKENE 197 zone and Its Preparation zone is a colorless gas that is formed in the stratosphere, the part of the atmosphere that lies about 6 0 miles above the earth s surface, by the reaction of oxygen with short-wavelength ultraviolet radiation. 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 increase 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. zone can be formed from the reaction of oxygen in electrical discharges. An atmospheric example is the formation of ozone in a thunderstorm by lightning.the laboratory preparation of ozone involves a similar reaction: electrical discharge 2 2 zone is produced in the laboratory by passing oxygen through an electrical discharge in a commercial 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 A bond accepts p electrons from the alkene. A _ C C C C C C C (5.4 Further Exploration 5.2 Mechanism of zonolysis This reaction results in the formation of a ring because the three oxygens of the ozone molecule remain intact. Additions that give rings are called cycloadditions. Furthermore, the cycloaddition of ozone occurs in a single step. ence, this is another example, like hydroboration, 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. C C C C molozonide C C ozonide C C (5.5 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. zonides can be converted into aldehydes, ketones, or carboxylic acids, depending on the structure of the alkene starting material and the reaction conditions. When the ozonide is treated with dimethyl sulfide, (C 2, the ozonide is split: C + C 21 C 2 C C C C C + 21 dimethyl sulfide an aldehyde C _ 1 1 (5.6

3 198 PTE 5 ADDITIN EACTIN F AKENE Further Exploration 5. Mechanism of zonide Conversion into Carbonyl Compounds The net transformation resulting from the ozonolysis of an alkene followed by dimethyl sulfide treatment is the replacement of a C group by two groups: the double bond is completely broken C C C 1 2 (C 2 C + A C C (5.7 A here A here If the two ends of the double bond are identical, as in Eq. 5.7, then two equivalents of the same product are formed. If the two ends of the alkene are different, then a mixture of two different products is obtained: C (C 2 5 C 2 C 2 Cl 2 (5.8 If a carbon of the double bond in the starting alkene bears a hydrogen, then an aldehyde is formed, as in Eqs. 5.7 and 5.8. In contrast, if a carbon of the double bond bears no hydrogens, then a ketone is formed instead: C C C C (5.9 If the ozonide is simply treated with water, hydrogen peroxide ( 2 2 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. ence, the alkene in Eq. 5.9 would react as follows: C C C C (C (C (+ 2 2 C (C 2 5 heptanal (75% yield C + A C C C a ketone an aldehyde C C a ketone A C a carboxylic acid (5.40 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 tudy Problem A C 2 formaldehyde C tudy Problem 5. Alkene X of unknown structure gives the following products after treatment with ozone followed by aqueous 2 2 : A and C C 2 C cyclopentanone propionic acid What is the structure of X?

4 5.5 ZNYI F AKENE 199 olution The structure of the alkene can be deduced by mentally reversing the ozonolysis reaction. To do this, rewrite the double bonds as dangling double bonds by dropping the oxygen: and C C 2 C C C 2 C Next, replace the group of any carboxylic acid fragments with. This is done because a carboxylic acid is formed only when there is a hydrogen on the carbon of the double bond (see Table 5.1. C C 2 C C C 2 C Finally, connect the dangling ends of the double bonds in the two partial structures to generate the structure of the alkene: connect C C 2 C C 2 C C alkene structure TABE 5.1 ummary of zonolysis esults Under Different Conditions Conditions of ozonolysis Alkene carbon,then (C 2,then 2 2 / 2 ketone ketone aldehyde formaldehyde carboxylic acid formic acid

5 200 PTE 5 ADDITIN EACTIN F AKENE PBEM 5.1 Give the products (if any expected from the treatment of each of the following compounds with ozone followed by dimethyl sulfide. (a -methyl-2 pentene (b 0A C 2 (c cyclooctene (d 2-methylpentane 5.14 Give the products (if any expected when the compounds in Problem 5.1 are treated with ozone followed by aqueous hydrogen peroxide 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 (b C C 2 C 2 (c A C(C 2 5 C C 5.16 What aspect of alkene structure cannot be determined by ozonolysis? 5.6 FEE-ADI ADDITIN F YDGEN BMIDE T AKENE A. The Peroxide Effect ecall that addition of Br 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 (ec. 4.7A. For example, 1-pentene reacts with Br to give almost exclusively 2-bromopentane: C C 2 C 2 C 2 1-pentene + Br C C 2 C 2 CC Br 2-bromopentane (79% yield (5.41 For many years, results such as this were at times difficult to reproduce. ome investigators found that the addition of Br was a highly regioselective reaction, as shown in Eq thers, 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, Morris Kharasch ( of the University of Chicago began investigations that led to a solution of this puzzle. e found that when traces of peroxides (compounds of the general structure are added to the reaction mixture, the regioselectivity of Br addition is reversed! In other 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: C C 2 C 2 C 2 1-pentene + Br Ph C C Ph benzoyl peroxide (small amount used C C 2 C 2 C 2 C 2 Br 1-bromopentane (96% yield (5.42 (Contrast this result with that in Eq This reversal of regioselectivity in Br addition is called the peroxide effect.