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In hydrogenation reactions, H2 is added to the unsaturated (carbon-carbon double or triple bonds) bonds. The resulting product of hydrogenation of a pure alkene is an alkane.


Usually, catalysts like platinum, palladium, or nickel are used for these types of hydrogenation reactions. Hydrogenation reactions are exothermic. Thus heat is generated as a result of hydrogenation and is called heat of hydrogenation.


The greater the substitution of the carbon-carbon double bond is, the lesser the heat of hydrogenation, and the higher the stability of the alkene.

Alkenes with Hydrogen Halides

Alkenes undergo electrophilic addition reactions with hydrogen halides, to form alkyl halides.

Sample reaction


In the process, the hydrogen halide attacks the double bond in the alkene, and the pi electrons in the double bond are transferred to the electrophile, resulting in a carbocation intermediate. This is followed by the formation of the alkyl halide.

image\Ch 16 page 211, g1.png

If the alkene used is not symmetrical, the possibility of different products from hydrogen halide addition is an issue. In such cases, the hydrogen from the hydrogen halide adds to the double-bonded carbon that already has the greater number of hydrogens. This is Markovnikov's rule. Based on this rule, we can predict the major product in such reactions. Consider the following reaction involving 2-methyl-2-butene with hydrogen bromide.

image\Ch 16 page 211, g2.png

An example of a Markovnikov addition is shown below. Watch where the hydrogen and the bromine are added.

Sample reaction

Markovnikov addition of hydrogen and bromine

image\Ch 16 sec F, g 16-4.png


Sample reaction

image\Ch 16 sam 16-8.png

Anti-Markovnikov Addition

Hydrogen bromide in the presence of peroxides can add to an unsymmetrical alkene resulting in anti-Markovnikov products. The change in trend can be explained based on the mechanistic difference of HBr addition in the presence of peroxides. Peroxides can easily form free radicals, since the oxygen-oxygen bond in peroxides is weak. This type of addition is not seen with HCI or HI. The mechanism of HBr addition to an alkene in the presence of a peroxide is shown below.

image\Ch 16 page 213, g.png

Notice that when the bromine radical adds to the double-bonded carbon that contains the most number of hydrogens, the resulting alkyl radical is more stable (here, tertiary radical is formed). Remember that free radical stability parallels carbocation stability. A tertiary radical is more stable than a secondary radical which is more stable than a primary radical.

Alkenes with Halogens

We will consider the reaction of ethylene with chlorine.

Sample reaction

The overall reaction:



The mechanism of the reaction:

image\Ch 16 g 16-9.png

​This is an example of electrophilic addition of Cl2 to an alkene. The mechanism of this reaction involves the following steps. In the first step, the ethylene reacts with chlorine to form the cyclic ethylene chloronium ion (intermediate) and chloride ion.
Note that in this cyclic intermediate, the chlorine has a positive charge. This step is followed by the nucleophilic attack by chloride ion on the chloronium ion. The reaction is enhanced by electron-donating substituents such as alkyl groups on the carbon-carbon double bond, since such groups can further stabilize the formation of the transition state which results in the formation of the chloronium ion.
Halogen addition is usually an anti addition process. See the next reaction that exemplifies this aspect.

Sample reaction

image\Ch 16 sam 16-10.png

Alkenes with Halogens in Aqueous Medium

The organic product formed as a result of the reaction between an alkene and a halogen is called a halohydrin. An overall representative reaction is shown below:

Sample reaction

image\Ch 16 sam 16-11, g2.png

Sample reaction

image\Ch 16 sam 16-12.png


Sample reaction

image\Ch 16 sam 16-13.png

Alkenes react with peroxy acids to form epoxides and carboxylic acids as products.


Alkenes react with O3 (Ozone) to form ozonides, which on hydrolysis with water form aldehydes or ketones or both, depending on the type of the reacting alkene. This is illustrated by the sample reactions.

Sample reaction

image\Ch 16 sam 16-14.png

Hydroxylation using Osmium Tetroxide

Sample reaction

image\Ch 16 sam 16-16.png

Osmium tetroxide can undergo reaction with alkenes to form a cyclic osmate, which in the presence of hydrogen peroxide results in a glycol (diol). Hydrogen peroxide oxidizes the osmium back to osmium tetroxide, while hydrolyzing the cyclic osmate to glycol. The predominant product is a syn addition product.

Acid Catalyzed Reaction

Sample reaction

image\Ch 16 sam 16-17.png

Alkenes react with aqueous acidic solutions to form alcohols. The reaction intermediate is a carbocation. There is possibility of rearrangement of the intermediates. The reaction follows Markovnikov's rule.


Sample reaction

image\Ch 16 sam 16-18.png

Oxidation followed by hydroboration of alkenes results in alcohols. This reaction takes place in an anti-Markovnikov fashion. Notice that the hydrogen atom, instead of attaching to the carbon contained in the double bond with the highest number of hydrogen substituents, attaches to the carbon with the least number of hydrogen substituents.
Diborane (B2H6), a dimer of borane (BH3), is usually used complexed together with tetrahydrofuran (THF) since diborane by itself is a toxic, and flammable gas. Borane entity actually adds to one of the double-bonded carbons resulting in an alkylborane. GH3 is a strong electrophile and adds to the least highly substituted double-bonded carbon. This preference makes sense because in the transition state, the electron deprived boron pulls electrons from the pi cloud resulting in a partial positive charge to the other carbon atom. This partial positive charge is better stabilized on the more highly substituted carbon. Hydrogen peroxide under basic conditions oxidizes the alkylborane to an alcohol. In effect, the addition in the hydroboration-oxidation is anti-Markovnikov.

Sample reaction

image\Ch 16 sam 16-19.png

The addition of hydrogen and boron is simultaneous, and they must add to the same side of the double bond. Hence, this addition reaction involves syn addition or same-side addition. Study the next reaction.

Sample reaction

image\Ch 16 sam 16-20.png


Sample reaction

image\Ch 16 sam 16-21.png

Alkenes can be converted into alcohols by oxymercuration-demercuration. The addition of H and OH is in accordance with the Markovnikov's rule. There is no rearrangement of the intermediates in this process.
In the oxymercuation process, the electrophilic addition of the mercuric species occurs resulting in a mercurinium ion which is a three-membered ring. This is followed by the nucleophilic attack of water and, as the proton leaves, an organomercuric alcohol (addition product) is formed. The next step, demercuration, occurs when sodium borohydride (NaBH4) substitutes the mercuric acetate substituent with hydrogen. If an alkene is unsymmetric, Oxymercuration-demercuration results in Markovnikov addition. The addition of mercuric species and water follows an anti (opposite side) addition pattern. This reaction has good yield, since there is no possibility of rearrangement unlike acid-catalyzed hydration of alkenes.

image\Ch 16 sam 16-22.png

The Diels-Alder Reaction

The Diels-Alder Reaction is an addition reaction involving an alkene and a diene. Let's look at the representative Diels-Alder reaction involving 1,3-butadiene and an alkene (dienophile means diene-lover). The reaction involves a cyclic transition state. The product is usually a cyclic addition product. Study the representative reaction given below. Pay close attention to how the new bonds are formed in relation to the reactants.

image\Ch 16 page 221,g1.png

Notice that the substituents in the alkene remain the same way in the product. In other words, cis substituents remain cis in the cycloaddition product. Hence, the Diels-Alder reaction is stereospecific.

Sample reaction

image\Ch 16 sam 16-23.png

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