All About Alkenes

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THE REACTIVITY OF ALKENES

THE REACTIVITY OF ALKENES

(1) Hydrogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes with hydrogen in the presence of a metal catalyst such as platinum, nickel or either palladium.  This is called hydrogenation. It includes the manufacture of margarine from animal or vegetable fats and oils.

(2) Halogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine. This is called halogenation. Alkenes react rapidly with chlorine or bromine in CH2Cl2 at room temperatura to form vicinal dihalides.

Reactions where the chlorine or bromine are in solution (for example, “bromine water”) are slightly more complicated and are treated separately at the end.

Chlorine reacts faster than bromine, but the chemistry is similar. Iodine reacts much, much more slowly, but again the chemistry is similar. You are much more likely to meet the bromine case than either of these.

(3) Hydrohalogenation of Hydrocarbons

Unsaturated hydrocarbons (alkenes and alkynes) react with hydrogen halides (HX). In addition reactions producing halogenated compounds .

Markovnikov’s Rule:

–       In additions of HX to unsymmetrical alkenes, the H+ of HX goes to the double-bonded carbon that already has the greatest number of hydrogens

–       Hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.

Anti-Markovnikov’s Rule:

–       Hydrogen bromide can also be added to an alkene in an anti-Markovnikov fashion.

–       In anti-Markovnikov additions, the hydrogen atom of the hydrogen halide adds to the carbon of the double bond that is bonded to fewer hydrogen atoms.

–       For this to result, the reaction must proceed by a noncarbocation intermediate; thus in the presence of peroxide, the reaction proceeds via a free-radical mechanism, with the major product being generated from the more stable free radical.

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February 1, 2010 Posted by | Reactivity Of Alkenes | Leave a comment