14.4 Introduction to Addition Reactions of the Triple Bond 653

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652 CHAPTER 14 • THE CHEMISTRY OF ALKYNES

wavelength, micrometers 2.6 2.83 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 1516 100

20 percent transmittance 0 3800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000 800 600 wavenumber, cm 1 _

Figure 14.6 The IR spectrum for Problem 14.4.

PROBLEMS 14.4 Identify the compound with a molecular mass of 82 that has the IR spectrum shown in Fig. 14.6 and the following NMR spectrum: d 1.90 (1H, s); d 1.21 (9H, s) 14.5 (a) Match each of the following 13C NMR spectra to either 2-hexyne or 3-hexyne. Explain. Spectrum A: d 3.3, 13.6, 21.1, 22.9, 75.4, 79.1 Spectrum B: d 12.7, 14.6, 81.0 (b) Assign each of the resonances in the two spectra to the appropriate carbon atoms. 14.6 A student consulted a well-known compilation of reference spectra for the proton NMR spectrum of propyne and was surprised to ﬁnd that this spectrum consists of a single unsplit resonance at d1.8. Believing this to be an error, he comes to you for an explanation. Explain to him why it is reasonable that propyne could have this spectrum.

INTRODUCTION TO ADDITION REACTIONS 14.4 OF THE TRIPLE BOND

In Chapters 4 and 5 we learned that the most common reactions of alkenes involve additions to the double bond. Additions to the triple bond also occur, although in most cases they are somewhat slower than the same reactions of comparably substituted alkenes. For example, HBr can be added to the triple bond. (C H ) N Br ' 2 5 4 | _ A (14.1) CH3(CH2)3C CH HBr CH Cl CH3(CH2)3CCH2 + 2 2 1-hexyne "Br 2-bromo-1-hexene (89% yield)

The regioselectivity of the addition is analogous to that found in the addition of HBr to alkenes (Sec. 4.7A): the bromine adds to the carbon of the triple bond that bears the alkyl substituent. As in alkene additions, the regioselectivity is reversed in the presence of peroxides because free-radical intermediates are involved (Sec. 5.6). 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 653

14.4 INTRODUCTION TO ADDITION REACTIONS OF THE TRIPLE BOND 653

peroxides ' A (14.2) CH3(CH2)3C CH HBr 0–5 ЊC, 1 h CH3(CH2)3CH CHBr + 1-hexyne 1-bromo-1-hexene; stereochemistry not determined (74% yield)

Because addition to an alkyne gives a substituted alkene, a second addition can occur in many cases.

Br Br HBr H3CC ' C CH3 HBr H3C"C A CH CH3 H3CC" CH2CH3 (14.3a) L L +(excess) L(not isolated)L LL 2-butyne "Br 2,2-dibromobutane (60% yield)

The regioselectivity of this addition reaction is determined by the relative stabilities of the two possible carbocation intermediates. One of the two possible carbocations (A in the following equation) is stabilized by resonance. By Hammond’s postulate (Sec. 4.8D), this carbocation is

formed more rapidly.

.. ..

Br ..

H3CCCHCH3 + HBr..

.. ..

......

Br Br Br ..

.. ..

......

.. H3CCCH2CH3 H3C C CH2CH3 Br.. H3CCH CHCH3 Br

resonance-stabilized carbocation A less stable carbocation B

...... Br .. Br

H3CCH CHCH3 (14.3b)

H3C C CH2CH3

.. ..

Br.. Br.. observed product not formed

In the addition of a hydrogen halide or a halogen to an alkyne, the second addition is usu- ally slower than the first. The reason is that the halogen that enters the molecule in the first addition exerts a rate-retarding polar effect (Sec. 3.6C) on carbocation formation in the second addition. In other words, both carbocations A and B in Eq. 14.3b are destabilized by the polar effect of bromine, and this polar effect is only partially counterbalanced by the resonance sta- bilization in carbocation A. Because the second addition is slower, it is possible to isolate the product of the first addition if one equivalent of HBr is used, as in Eq. 14.1. 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, stereochemistry, 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