14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 654 654 CHAPTER 14 • THE CHEMISTRY OF ALKYNES PROBLEMS 14.7 Give the product that results from the addition of one equivalent of Br2 to 3-hexyne. What are the possible stereoisomers that could be formed? 14.8 The addition of HCl to 3-hexyne occurs as an anti-addition. Give the structure, stereochem- istry, and name of the product. CONVERSION OF ALKYNES INTO 14.5 ALDEHYDES AND KETONES A. Hydration of Alkynes Water can be added to the triple bond. Although the reaction can be catalyzed by a strong acid, it is faster, and yields are higher, when a combination of dilute acid and mercuric ion (Hg2ϩ) catalysts is used. O 2 S Hg |, H2SO4 (dilute) C' CH H2O C CH3 (14.4) 0L + 0L L cyclohexylacetylene cyclohexyl methyl ketone (91% yield) The addition of water to a triple bond, like the corresponding addition to a double bond, is called hydration. The hydration of alkynes gives ketones (except in the case of acetylene itself, which gives an aldehyde; see Study Problem 14.1, p. 656). Let’s contrast the hydration reactions of alkenes (Sec. 4.9B) and alkynes. The hydration of an alkene gives an alcohol. H2SO4 R CH CH2 + H2O R CH CH3 (14.5a) an alkene OH an alcohol Because addition reactions of alkenes and alkynes are closely analogous, it might seem that an alcohol should also be obtained from the hydration of an alkyne: OH 2 H2SO4, Hg | RCC' H H2O R "C A CH2 (14.5b) LLan alkyne + Lan enol An alcohol containing an OH group on a carbon of a double bond is called an enol (pronounced en´-ôl).¯ In fact, enols are formed in the hydration of alkynes. However, most enols cannot be isolated because most enols are unstable and are rapidly converted into the corresponding aldehydes or ketones. OH O S R "C A CH2 R C CH3 (14.5c) L LL an enol a ketone 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 655 14.5 CONVERSION OF ALKYNES INTO ALDEHYDES AND KETONES 655 Most aldehydes and ketones are in equilibrium with the corresponding enols, but the equilib- rium concentrations of enols are in most cases minuscule—typically, one part in 108 or less. The relationship among aldehydes, ketones, and enols is explored in Chapter 22. The impor- tant point here is that, because most enols are unstable, if an enol is formed as the product of a reaction, it is rapidly converted into the corresponding aldehyde or ketone. The mechanism of alkyne hydration is very similar to that of the oxymercuration of alkenes (Sec. 5.4A). In the first part of the mechanism, mercuric ion reacts as an electrophile with the p electrons of the triple bond to form a carbocation, which could be in equilibrium with a cyclic mercurinium ion: R C CH R C CH R C CH (14.6a) 2 .. 2 Hg.. Hg Hg The carbocation is formed at the carbon of the triple bond that bears the alkyl substituent. (Re- call that alkyl substitution stabilizes carbocations; Sec. 4.7C.) This carbocation reacts with water, and loss of a proton to solvent water gives the addition product. As a result, the oxygen from water ends up on the carbon with the alkyl substituent. H .. .. .. .. .. O H OH .. .. OH2 OH2 .. R C CH R C CH R C CH + H3O (14.6b) Hg Hg Hg In the oxymercuration of alkenes, the reducing agent NaBH4 is the source of hydrogen that replaces the mercury. However, the use of NaBH4 is unnecessary in the hydration of alkynes. The reason is that the presence of a double bond makes possible removal of the mercury by a protonolysis reaction. This protonolysis occurs under the conditions of hydration; a separate procedure is not required. The first step in the mechanism of this protonolysis reaction is pro- tonation of the double bond. This protonation occurs at the carbon bearing the mercury be- cause the resulting carbocation is resonance-stabilized. | OH H OH2 OH |OH 3 2 L 3 2 S2 R" CA CH2 R" C CH2 RCCH2 OH2 (14.6c) L LL| LL + 3 2 Hg" | Hg" | Hg" | a resonance-stabilized carbocation (Recall that formation of a resonance-stabilized carbocation also explains the position of pro- tonation in HBr addition; Eq. 14.3b, p. 653.) Dissociation of mercury from this carbocation 2 liberates the catalyst Hg | along with the enol. OH OH A 2 R" C CH2 R" C CH2 Hg | (14.6d) LL L + | an enol Hg" | 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 656 656 CHAPTER 14 • THE CHEMISTRY OF ALKYNES Conversion of the enol into the ketone is a rapid, acid-catalyzed process. Protonation of the double bond gives another resonance-stabilized carbocation: | OH H OH2 OH |OH 3 2 L 3 2 S2 R" CA CH2 2 RCC" H3 RCCH3 OH2 (14.6e) L LL LL + 3 2 |a resonance-stabilized carbocation This carbocation is also the conjugate acid of a ketone. Loss of a proton gives the ketone product. H OH2 O| 3 O L 3S 2 3S2 R C CH3 R C CH3 H OH| 2 (14.6f) L L LL + L The hydration of alkynes is a useful way to prepare ketones provided2 that the starting ma- terial is a 1-alkyne or a symmetrical alkyne (an alkyne with identical groups on each end of the triple bond). This point is explored in Study Problem 14.1. Study Problem 14.1 Which one of the following compounds could be prepared by the hydration of alkynes so that it is uncontaminated by constitutional isomers? Explain your answer. (a) O (b) O S CH3CH CH3CH2CCH2CH3 acetaldehyde 3-pentanone Solution First, what alkyne starting materials, if any, would give the desired products? The equations in the text show that the two carbons of the triple bond in the starting material corre- spond within the product to the carbon of the CAO group and an adjacent carbon. Thus, for part (a), the only possible alkyne starting material is acetylene itself, HC'CH. For part (b), the only possible alkyne starting material is 2-pentyne, CH3C'CCH2CH3. Next, it remains to be shown whether hydration of these alkynes gives only the products in the problem. Remember, a good synthesis gives relatively pure compounds. The hydration of acetylene indeed gives only acetaldehyde. (In fact, acetaldehyde is the only aldehyde that can be prepared by the hydration of an alkyne.) However, hydration of 2-pentyne gives a mixture consisting of compa- rable amounts of 2-pentanone and 3-pentanone, because the carbons of 2-pentyne both have one alkyl substituent. Thus, there is no reason that the reaction of water at either carbon should be strongly favored. HO CH CH O 2 3 S H3O| $CCC A ) CH3CCH2CH2CH3 H C 2 3 ) $Hg| 2-pentanone H3O|, Hg | H3CCC' CH2CH3 (14.7) Hg CH2CH3 O | S H3O| $CCC A ) CH3CH2CCH2CH3 one alkyl substituent on each carbon H3C) $OH 3-pentanone 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 657 14.5 CONVERSION OF ALKYNES INTO ALDEHYDES AND KETONES 657 Hence, hydration would give a mixture of constitutional isomers that would have to be separated, and the yield of the desired product would be low. Consequently, hydration would not be a good way to prepare 3-pentanone. (However, 2-pentanone could be prepared by hydration of a different alkyne; see Problem 14.9a). PROBLEMS 14.9 From which alkyne could each of the following compounds be prepared by acid-catalyzed hydration? (a) O (b) O S S CH3CCH2CH2CH3 (CH3)3CC CH3 L L (c) O S CH3CH2CH2CH2 C CH2CH2CH2CH2CH3 L L 14.10 The hydration of an alkyne is not a reasonable preparative method for each of the following compounds. Explain why. (a) CH3CH2CH A O (b) O (c) S A O (CH3)3CC C(CH3)3 0 L L 14.11 (a) Draw the structures of all enol forms of the following ketone, including stereoisomers. O S CH3CH2 C CH(CH3)2 L L (b) Would alkyne hydration be a good preparative method for this compound? Explain. B. Hydroboration–Oxidation of Alkynes The hydroboration of alkynes is analogous to the same reaction of alkenes (Sec. 5.4B). CH2CH3 % CH3CH2 "C 3CH CH C' CCH CH BH C (14.8a) 3 2 2 3 3 THF ( + B "H 3 As in the similar reaction of alkenes, oxidation of the organoborane with alkaline hydrogen peroxide yields the corresponding “alcohol,” which in this case is an enol. As shown in Sec. 14.5A, enols react further to give the corresponding aldehydes or ketones. CH2CH3 % CH CH CH CH O 3 2 2 3 S CH3CH2 H O ͞OH "C 2 2 _ A C $CCC ) CH3CH2CH2 C CH2CH3 ( L L B H) $OH "H 3 an enol (14.8b) 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 658 658 CHAPTER 14 • THE CHEMISTRY OF ALKYNES Because the organoborane product of Eq. 14.8a has a double bond, a second addition of BH3 is in principle possible. However, the reaction conditions can be controlled so that only one addition takes place, as shown, provided that the alkyne is not a 1-alkyne. If the alkyne is a 1-alkyne (that is, if it has a triple bond at the end of a carbon chain), a sec- ond addition of BH3 cannot be prevented. RC ' CH + BH3 multiple addition reactions L a 1-alkyne However, the hydroboration of 1-alkynes can be stopped after a single addition provided that an organoborane containing highly branched groups is used instead of BH3.