Alkene Reactions

THEORY REACTIONS OF ALKENES

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The characteristic reaction of alkenes is the addition of substances to the double bond, according to the equation:

electrophilic substitution alkenes 01

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Hydrogenation is the addition of hydrogen to a double bond to form alkanes.
Platinum and palladium are the most commonly used catalysts in the hydrogenation of alkenes. Palladium is used in the form of powder adsorbed on carbon (Pd/C). Platinum is used as PtO₂ (Adams Catalyst).

hydrogenation achenes 1

[1] 2-Butene

[2] Butane

[3] Cyclopentene

[4] Cyclopentane

Alkenes are hydrogenated on both sides, producing two enantiomers in equal proportion (racemic mixture).

hydrogenation achenes 2

[5] Racemic mixture

When the alkene has one face more hindered than the other, diastereoisomers are obtained in different proportions.

hydrogenation achenes 3

[6] Major product of hydrogenation. The underside of the alkene is less hindered.

[7] Minority product. Hydrogenation occurs on the methyl side, which prevents the approach of the reagent

In bicycles the most favorable face is the one on the side of the bridge:

hydrogenation achenes 4

[8] Major product. The bridge of the bicycle is the chain that exerts the least impediment.

[9] Minority product.

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Hydrogen halides add to alkenes, forming haloalkanes. The proton acts as an electrophile, being attacked by the alkene in the first stage. HF, HCl, HBr, HI can be used as reagents in this reaction.

Read more: HX addition

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When an alkene does not have the same substituents on its sp² carbons, it can form two types of products in electrophilic substitution reactions. The majority product is the one obtained by adding the proton to the sp ² carbon that generates the most stable carbocation.

For example, when propene reacts with HBr, the proton can be attached to carbon 1 , forming the carbocation on carbon 2 . This carbocation is secondary and hyperconjugation gives it significant stability.

If the proton is attached to carbon 2 , a primary carbocation is obtained on carbon 1 . The stability of the primary carbocation is much lower than that of the secondary and it will form more slowly, giving rise to a small percentage of 1-bromopropane.

[1]Majority product, its formation takes place through the most stable carbocation.

[2] Minority product

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Water is a very weak acid, with insufficient proton concentration to start the electrophilic addition reaction. It is necessary to add an acid (H₂SO₄ ) to the medium for the reaction to take place.

This reaction is also known as hydration of alkenes and generates alcohols.