in China. Hydration The hydration reaction of acetylene gives acetaldehyde. The reaction proceeds by formation of vinyl alcohol, which undergoes tautomerizes to form the aldehyde. This reaction was once a major industrial process but it has been displaced by the Wacker process. This reaction occurs in nature, the catalyst being acetylene hydratase. The hydration of phenylacetylene gives acetophenone, and the catalyzed hydration of 1,8-nonadiyne to 2,8-nonanedione: PhC#CH + H2O -> PhCOCH3 HC#C(CH2)5C#CH + 2H2O -> CH3CO(CH2)5COCH3 Tautomerism Terminal alkyl alkynes exhibit tautomerism. Propyne exists in equilibrium with propadiene: HC#C-CH3 <=> CH2=C=CH2 Cycloadditions and oxidation Alkynes undergo diverse cycloaddition reactions. The Diels–Alder reaction with 1,3-dienes give 1,4-cyclohexadienes. This general reaction has been extensively developed. Electrophilic alkynes are especially effective dienophiles. The "cycloadduct" derived from the addition of alkynes to 2-pyrone eliminates carbon dioxide to give the aromatic compound. Other specialized cycloadditions include multicomponent reactions such as alkyne trimerisation to give aromatic compounds and the [2+2+1]-cycloaddition of an alkyne, alkene and carbon monoxide in the Pauson–Khand reaction. Non-carbon reagents also undergo cyclization, e.g. Azide alkyne Huisgen cycloaddition to give triazoles. Cycloaddition processes involving alkynes are often catalyzed by metals, e.g. enyne metathesis and alkyne metathesis, which allows the scrambling of carbyne (RC) centers: RC#CR + R'C#CR' <=> 2RC#CR' Oxidative cleavage of alkynes proceeds via cycloaddition to metal oxides. Most famously, potassium permanganate converts alkynes to a pair of carboxylic acids. Reactions specific for terminal alkynes Terminal alkynes are readily converted to many derivatives, e.g. by coupling reactions and condensations. Via the condensation with formaldehyde and acetylene is produced butynediol: 2CH2O + HC#CH -> HOCH2CCCH2OH In the Sonogashira reaction, terminal alkynes are coupled with aryl or vinyl halides: This reactivity exploits the fact that terminal alkynes are weak acids, whose typical pKa values around 25 place them between that of ammonia (35) and ethanol (16): RC#CH + MX -> RC#CM + HX where MX = NaNH2, LiBu, or RMgX. The reactions of alkynes with certain metal cations, e.g. and also gives acetylides. Thus, few drops of diamminesilver(I) hydroxide () reacts with terminal alkynes signaled by formation of a white precipitate of the silver acetylide. This reactivity is the basis of alkyne coupling reactions, including the Cadiot–Chodkiewicz coupling, Glaser coupling, and the Eglinton coupling: 2R-\!{\equiv}\!-H ->[\ce{Cu(OAc)2}][\ce{pyridine}] R-\!{\equiv}\!-\!{\equiv}\!-R In the Favorskii reaction and in alkynylations in general, terminal alkynes add to carbonyl compounds to give the hydroxyalkyne. Metal complexes Alkynes form complexes with transition metals.