Alkyne vinylic carbocation from

Inspection of 92 shows that the chlorine is attached to the more substituted sp carbon, so the reaction with HCl may be termed a Markovnikov addition to the triple bond. If the alk5me reacts as a Brpnsted-Lowry base with HCl in a manner similar to alkenes from Section 10.2, the intermediate will be a carbocation. There are two 7i-bonds in an alkyne, and if only one Ji-bond reacts with HCl, the second 7i-bond of the C=C unit should remain, meaning that a C=C+ intermediate must be formed. When the cationic center is on an sp carbon, it is called a vinyl carbocation. In a vinyl carbocation, the positive charge resides on a carbon atom that is part of a C=C unit a vinyl unit. The two possible vinyl carbocations are secondary vinyl carbocation 93 and primary vinyl carbocation 94. As with any other carbocation, the secondary vinyl carbocation 93 is more stable than the primary vinyl carbocation 94, and the more stable carbocation is formed preferentially. 

The stability of vinyl carbocations generated from alkynes parallels that of the carbocations generated from alkenes secondary vinyl carbocation 96 is more stable than secondary carbocation 97 because there are more groups attached to the C=C unit, and 97 is more stable than primary vinyl carbocation 98. As with alkenes, the reaction of unsymmetrical alkynes with an acid will preferentially give the more stable carbocation, which reacts with the nucleophile to give the alkene product. 

When 1-hexyne is treated with a catalytic amount of sulfuric acid in an aqueous solvent, initial reaction with the acid gives the expected secondary vinyl carbocation 103, and the most readily available nucleophile in this reaction is water (from the aqueous solvent). Nucleophilic addition of water to 103 leads to the vinyl oxonium ion 104. Loss of a proton in an acid-base reaction (the water solvent is the base) generates a product (105) where the OH unit is attached to the C=C unit, an enol. Enols are unstable and an internal proton transfer converts enols to a carbonyl derivative, an aldehyde, or a ketone. This process is called keto-enol tautomerization and, in this case, the keto form of 105 is the ketone 2-hexanone (106). (Enols are discussed in more detail in Chapter 18, Section 18.5.) Note that the oxygen of the OH resides on the secondary carbon due to preferential formation of the more stable secondary carbocation followed by reaction with water, and tautomerization places the carbonyl oxygen on that same carbon, so the product is a ketone. When a disubstituted alkyne reacts with water and an acid catalyst, the intermediate secondary vinyl cations are of equal stability and a mixture of isomeric enols is expected each will tautomerize, so a mixture of isomeric ketones will form. 

Oxymercuration occurs with an alkyne as with an alkene, but differences in reactivity lead to a modification in the procedure. For reasons that will not be discussed, a mixture of mercuric sulfate (HgS04) and mercuric acetate [Hg(OAc)2] is used. When 1-heptyne is treated with this mixture in aqueous solvent, the initially formed enol (107) tautomerizes to 2-heptanone (108), which is isolated in 80% yield. Note that the ketone product mentioned in connection with vinyl chloride 92 in Section 10.4.5 results from formation of an enol. There is an important difference in the oxymercuration of alkynes and alkenes that is notable in this transformation. The mercury reacts with the alkyne, but the mercury is lost when the enol is formed and there is no need to add NaBH in a second step. This observation is general for oxymercuration of alkynes under these conditions. The more stable secondary vinyl carbocation is an intermediate, but the vinyl-mercury compound formed by reaction with the carbocation is unstable in the presence of water, so the enol is the product. 

Alkynes are considerably less reactive toward most electrophilic additions than are alkenes. The major reason for this difference is the instability of the sp-hybrid-ized vinylic carbocation intermediate formed from an alkyne compared with the ip -hybridized alkyl carbocation formed from an alkene. 

From this point there are two possible mechanistic paths that could be envisioned. The first follows Path A where the second alkyne group attacks the iodonium ion, giving a vinylic carbocation, which then is attacked by the iodide anion to afford the product However, this does not seem likely because the vinylic carbocation is a high energy intermediate. Path B is more likely because it avoids the formation of a vinyhc carbocation. After the initial formation of the... 

A reasonable mechanism for the bromination reaction is presented below. One of the alkyne groups reacts with bromine, resulting in the formation of a new C-Br bond, and a vinyl carbocation (see below for details on this). The n electrons from the other alkyne attack this carbocation, resulting in the formation of a new C-C bond, and a new vinyl carbocation. Nucleophilic attack of the bromide gives the product. 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their involvement in these additions has been questioned. For example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the corresponding additions to alkenes, the difference is not nearly as great as the difference in carbocation stabilities would suggest. 

If we consider the protonation of 2-butyne by HBr, then we obtain an unstable vinyl cation, 11.15. This is linear because the carbon atom is sp-hybridized, with an empty p-orbital. Thus, it can be attacked by bromide ion from either side, resulting in the formation of both E- and Z-alkene products (Figure 11.32). The instability of the vinyl cation intermediate means that alkynes react more slowly with electrophiles than do alkenes. The addition of a second mole of HBr to the alkene generally gives the geminal dibromide the intermediate carbocation is stabilized by interaction with the lone pair of electrons from bromine (Figure 11.33). 

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