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

electrophilic substitution alkenes 01

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.

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.
adición hx alquenos

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.
rule markovnikov01
[1]Majority product, its formation takes place through the most stable carbocation.
[2] Minority product

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.
Hidratación de alquenos

Chlorine and bromine add to alkenes to give 1,2-dihaloalkanes. For example, 1,2-dichloroethane is synthesized by adding chlorine to ethene.

Alkenes react with halogens in an aqueous medium to form halohydrins, compounds that contain a halogen and a hydroxyl group in neighboring positions.

formación de halohidrinas

Alkenes can be hydrated with aqueous mercury acetate followed by reduction with sodium borohydride. This reaction produces alcohols and follows the Markovnikov rule.

hidratación con acetato de mercurio

Hydroboration is a reaction in which a boron hydride [2] reacts with an alkene [1] to give an organoborane [3.

Hidroboración de alquenos

The dihydroxylation of an alkene consists of adding an -OH group to each carbon to form vicinal diols. This reaction can be carried out with osmium tetroxide in hydrogen peroxide, or with potassium permanganate in water.

oxidación con tetraóxido de osmio

Alkenes react with peracids (peroxyacids) to form epoxides. Epoxides are three-membered cycles that contain oxygen.

epoxidación de alquenos

Alkenes react with ozone to form aldehydes, ketones, or mixtures of both after a reduction step.

Ozonólisis de alquenos

In 1929, Professor S. Kharasch of the University of Chicago observed the anti-Markovnikov addition of HBr to an alkene due to the presence of peroxides in the reaction medium.

adición hbr con peróxidos

Alkenes, in the presence of concentrated sulfuric acid, condense to form chains called polymers. Let's see an example with 2-Methylpropene

polimerización de alquenos