Nucleophilic Addition to Alkynes and Conjugated Enynes

as shown in the first and second examples in Table 6.8, the addition of ethanethiol (thioethane, ethyl mercaptan, CH3CH2SH) as well as ethanol (CH3CH2OH) across the triple bond of ethyne (acetylene, HOCH) is catalyzed by the corresponding metal salts and requires high temperature (at which high pressures will develop). These reactions are not generally observed in the presence of acid. 

TABLE 6.8. Representative Addition Reactions between Nncleophiles and Alkynes 

These are representative examples of nucleophilic addition to the triple bond of alkynes. If the triple bond bears an electron-withdrawing group the nucleophilic addition is much more facile (as was pointed out for the same reaction on the carbon-carbon double bond, q.v.) 

Through an apparently less complicated process that nonetheless requires com-plexation with an organometallic catalyst, ethyne (acetylene, HC CH) itself undergoes trimerization in the presence of dicobalt octacarbonyl [Co2(CO)g] (Equation 6.69) to benzene and (Equation 6.70) tetramerization in the presence of nickel(II) cyanide [Ni(CN)2] to cyclooctatetraene. 

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