General Methods of Preparation of Alkenes

(1) Dehydration of an alcohol: heating an alcohol with excess concentrated H2SO4 at 430-440 K removes a water molecule (an elimination reaction) to give the alkene, e.g. ethanol --conc. H2SO4, 430-440K--> ethene; the same conversion can also be run as a catalytic (vapour-phase) dehydration over Al2O3 at 623-723 K. (2) From alkynes: alkynes are reduced to CIS-alkenes using Lindlar's catalyst (palladium on CaCO3, partially deactivated with sulphur or quinoline) -- a stereospecific syn addition giving only the cis-alkene (e.g. 2-butyne --H2, Pd-CaCO3--> cis-2-butene); alternatively, sodium in liquid ammonia reduces an alkyne stereospecifically to the TRANS-alkene (e.g. 2-butyne --Na/NH3--> trans-2-butene). (3) Dehydrohalogenation of haloalkanes: a haloalkane heated with alcoholic KOH eliminates HX to give the alkene (e.g. 1-bromopropane --alc. KOH--> propene + KBr + H2O). (4) From vicinal dihalides: a vicinal dihalide (two halogens on adjacent carbons) warmed with granulated zinc in methanol loses ZnX2 to give the alkene (e.g. 1,2-dibromoethane --Zn/CH3OH, heat--> ethene + ZnBr2). (5) Kolbe's electrolytic method: electrolysing an …