Online Review

Online Review - Click on any reaction for more information....

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Reactions of Alkynes Addition of Halogen Acids to Alkenes The addition of halogen acids to alkynes is a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the halogen acid donates a proton to the alkyne π -system, which is functioning as a Lewis base. The protonated π - system has a short lifetime and can rapidly revert to starting materials, or can rearrange from a (cationic) protonated π -bond, to an sp 2 sigma bond adjacent to an sp 2 carbocation center. If the alkyne is unsymmetrical, the protonated π -cloud intermediate can break down by two pathways to potentially form carbocations having differing ground-state energies. The reaction pathways leading from this intermediate to the two carbocations will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway. The resulting carbocation is formed on the carbon of the alkyne which is best able to stabilize the cationic center. In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbocations formed from alkyne protonation will be secondary > primary. Since secondary centers have no attached and primary centers have one, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the carbocation will form at that center and this is another example of Markovnikov's Rule , which was described for alkenes. Once the carbocation is formed, the most favorable reaction will involve the addition of a nucleophile to form a vinyl halide intermediate. This alkene can now undergo a second protonation step, just like any other alkene, except that the carbocation will always be formed on the carbon bearing the halogen, since this carbocation is now stabilized by resonance with the halonium ion. The final result of the addition is that two moles of halogen halide are added, to give a 1,1-dihalide. 3
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Addition of Halogen to Alkynes The addition of halogen to alkynes is a stepwise process involving a "halonium" ion intermediate. The formation of this intermediate is initiated through attack of halogen on the alkyne π -system, to form the cyclic halonium ion (i.e., bromonium or chloronium ion) and expel the halogen anion (i.e., bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system; that is, the halide anion expelled in the previous step. Attack by halide generates a vinyl halide, which is an alkene and can undergo a second addition of halogen. The final product of the reaction is therefore a 1,1,2,2-tetrahalide. Addition of Water to Alkynes
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This note was uploaded on 03/31/2008 for the course CHEM 262L taught by Professor Mbadugua during the Spring '08 term at UNC.

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Online Review - Click on any reaction for more information....

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