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Unformatted text preview: Alkenes: Reactions and Synthesis In this chapter youll learn about how alkenes are prepared, further examples of alkene addition reactions, and the wide variety of compounds that can be made from alkenes. o Alco hol o Alka ne o 1,2- diol o Carbonyl compound o Cyclopropane o Epoxide o Halide o 1,2-dihalide o Halohydrin 7.1 P rep a r a t ion of A l kenes: A P review of E l i m i n a t ion React ions Alkenes are readily available from simple precursors usually alcohols ( in biological systems ) and either alcohols or alkyl halides ( in the lab ). The chemistry of alkenes is dominated by addition reactions while the preparation of alkenes is dominated by elimination reactions. The two most common types of elimination reactions are: o Dehydrohalogenation the loss of HX from an alkyl halide Usually occurs by reaction of an alkyl halide with a strong base Cyclohexene + KOH (in ethanol solution) bromocyclohexane o Dehydration the loss of water from an alcohol Often carried out by treatment of an alcohol with a strong acid 1-methylcyclohexanol + heat + aqueous sulfuric acid in tetrahydrofuran (THF) solvent = loss of water and 1- methylcyclohexene In biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in which the OH is positioned two carbons away from a carbonyl group. 7.2 Add i t ion of H a logens to Al kenes Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides , a process called halogenation (ex; Cl 2 + ethylene ethylene dichloride PVC ). o Fluorine is too reactive o Iodine doesnt react with most alkenes When the halogenation reaction is carried out on a cycloalkane, only the trans stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. The reaction occurs with anti stereochemistry , meaning that the two halide atoms come from opposite faces of the double bond (top, bottom face). Bromonium ion postulate o The reaction intermediate is not a carbocation but a bromonium ion ( R 2 Br + ), formed by addition of Br + to the alkene. The ion is formed in a single step by interaction of the alkene with Br 2 and simultaneous loss of Br- . o Chloronium ion ( R 2 Cl + ) o (explanation of stereochemistry of halogen addition to alkenes; deductive logic) 7.3 Add i t ion of Hypoh a lous Acids to A l kenes: H a lohyd r i n Form a t ion Another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HOCl or HOBr to yield 1,2-halo alcohols, called halohydrins ....
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