hydrocarbons. Dehydrohalogenation and related reactions Alkynes are prepared from 1,2- and 1,1-alkyl dihalides by double dehydrohalogenation. The reaction provides a means to generate alkynes from alkenes, which are first halogenated and then dehydrohalogenated. For example, phenylacetylene can be generated from styrene by bromination followed by treatment of the resulting of styrene dibromide with sodium amide in ammonia: Via the Fritsch–Buttenberg–Wiechell rearrangement, alkynes are prepared from vinyl bromides. Alkynes can be prepared from aldehydes using the Corey–Fuchs reaction and from aldehydes or ketones by the Seyferth–Gilbert homologation. Vinyl chlorides are susceptible to dehydrochlorination. Vinyl chlorides are available from aldehydes using the reagent (chloromethylene)triphenylphosphorane. Reactions, including applications Featuring a reactive functional group, alkynes participate in many organic reactions. Such use was pioneered by Ralph Raphael, who in 1955 wrote the first book describing their versatility as intermediates in synthesis. Hydrogenation Being more unsaturated than alkenes, alkynes characteristically undergo reactions that show that they are "doubly unsaturated". Alkynes are capable of adding two equivalents of , whereas an alkene adds only one equivalent. Depending on catalysts and conditions, alkynes add one or two equivalents of hydrogen. Partial hydrogenation, stopping after the addition of only one equivalent to give the alkene, is usually more desirable since alkanes are less useful: The largest scale application of this technology is the conversion of acetylene to ethylene in refineries (the steam cracking of alkanes yields a few percent acetylene, which is selectively hydrogenated in the presence of a palladium/silver catalyst). For more complex alkynes, the Lindlar catalyst is widely recommended to avoid formation of the alkane, for example in the conversion of phenylacetylene to styrene. Similarly, halogenation of alkynes gives the alkene dihalides or alkyl tetrahalides: RCH=CR'H + H2 -> RCH2CR'H2 The addition of one equivalent of to internal alkynes gives cis-alkenes. Addition of halogens and related reagents Alkynes characteristically are capable of adding two equivalents of halogens and hydrogen halides. RC#CR' + 2 Br2 -> RCBr2CR'Br2 The addition of nonpolar bonds across is general for silanes, boranes, and related hydrides. The hydroboration of alkynes gives vinylic boranes which oxidize to the corresponding aldehyde or ketone. In the thiol-yne reaction the substrate is a thiol. Addition of hydrogen halides has long been of interest. In the presence of mercuric chloride as a catalyst, acetylene and hydrogen chloride react to give vinyl chloride. While this method has been abandoned in the West, it remains the main production method