Reactions of Alkenes

Addition Reactions

Addition reactions are reactions where a reagent adds to an alkene. Addition reactions are the opposite of HX{\rm {H{-}X}} eliminations.

Addition reactions involve two or more molecules that combine to form a new product. An addition reaction is a reaction in which two or more reactants combine to form products. It adds atoms or groups to the starting material. A reagent is added to the substrate, such as an alkene. The carbon-carbon double bond, the π\pi bond, is the source of electrons and acts as the Lewis base, or nucleophile. The reagent is a Lewis acid, an electron-pair acceptor, and is called an electrophile.

The simplest addition reaction is the electrophilic addition reaction of HX to an alkene. X can be bromine (Br), chlorine (Cl), or iodine (I). In this reaction the double bond of the alkene will act as a nucleophile and attack the hydrogen of HX to break the double bond. One carbon of the alkene will add the hydrogen, and the other side will form a carbocation. If possible, the carbocation will rearrange to form a more-stable carbocation, and then the halide (X) will attach to the carbocation. If the carbocation cannot rearrange, the halide will attach directly to that initial carbocation.
CH3CH=CH2+HBrCH3CHBrCH3{\rm{CH}_3{CH}}={\rm{CH}}_2+{\rm {HBr}}\rightarrow{\rm{CH}_3{CHBrCH}}_3
Addition reactions that add an HX{\rm {H{-}X}} can either be Markovnikov (more substituted) or anti-Markovnikov (less substituted). Markovnikov's rule states that in an addition of an asymmetric reagent to an alkene, the more-electronegative atom of the reagent will bond to the more-substituted carbon of the alkene. Addition of HBr to an alkene is an ionic addition that gives a Markovnikov product; reaction of an alkene with HBr/HOOH is a radical addition that gives an anti-Markovnikov product, which is where the more-electronegative atom of the reagent bonds to the less-substituted carbon of the alkene.

Hydrogen Bromide Peroxides

Addition of HBr without peroxides results in Markovnikov addition. Addition of HBr with peroxides results in anti-Markovnikov product.
In the presence of peroxide, HBr is a radical reaction that produces an anti-Markovnikov product. Free-radical reactions involve three main steps: chain initiation, chain propagation, and chain termination. Termination starts when two radicals bond with each other, which then arrests that process.

Radical Mechanism

The radical reaction of hydrogen bromide (HBr) and peroxide (HOOH) creates the most-stable (most-substituted) radical, leading to the anti-Markovnikov product.
Simple addition reactions of dihalides (X2) are straightforward. Addition of X2 will add an X to each carbon of an alkene. In a dihalide addition reaction, the halides are added to the alkene of a carbon chain. Addition of a dihalide will result in the halide being added to the alkene at the double bond location and a loss of the double bond. Addition of a dihalide leads to the anti addition of two halogens to an alkene. Anti addition (sometimes just called anti) is the addition of two substituents to opposite sides of a double bond or a triple bond.

Addition of Dihalide

Adding a dihalide, Br2, to an alkene results in both bromine atoms adding in the place of the double bond.

Hydration and Hydrogenation

Hydration is the process of adding water to an alkene. Hydrogenation is the process of adding hydrogen to an alkene.
Hydration, the addition of a hydrogen and a hydroxyl group to an alkene, is accomplished using H3O+ (Markovnikov with rearrangements possible), alkoxymercuration-demercuration (Markovnikov without rearrangements), or hydroboration-oxidation (Anti-Markovnikov and syn addition). The first hydration method involves adding water under acidic conditions, and the reaction results in an alcohol.

Hydration Reaction

Hydration of an alkene results in an alcohol being formed with the addition of a hydroxide ion to the carbon chain and a loss of the double bond.
The second method is alkoxymercuration-demercuration, which yields a Markovnikov product without rearrangements.


Alkoxymercuration-demercuration reactions result in a Markovnikov configuration without rearrangements.
Lastly, there is hydroboration-oxidation, when anti-Markovnikov and syn addition occurs. This is an anti-Markovnikov addition where the hydroxyl group attaches to the less-substituted carbon. The hydrogen and hydroxyl group add to the same side of the alkene. This is called syn addition.

Hydroboration Oxidation

In a hydroboration-oxidation reaction, both anti-Markovnikov and syn addition occur.
Catalytic hydrogenation with hydrogen (H2) and a catalyst (Ni, Pt, or Pd) will add two hydrogen atoms to an alkene. Syn addition (sometimes just called syn) is the addition of two substituents to the same side of a double bond. In this reaction, two hydrogen atoms are added to the same side of the alkene. Catalytic hydrogenation (sometimes just called hydrogenation) is the use of a heavy-metal catalyst to add hydrogen to an alkene or alkyne. Examples of heavy-metal catalysts are Ni, Pt, and Pd. Catalytic hydrogenation results in syn addition.

Catalytic Hydrogenation

Catalytic hydrogenation uses heavy-metal catalysts, such as nickel (Ni), platinum (Pt), or palladium (Pd), resulting in a syn addition.

Other Addition Reactions

The halohydrin reaction, syn- or anti-dihydroxylation, carbene reaction, and oxidative cleavage are other types of addition reactions of alkenes.
The halohydrin reaction is an anti addition of a halide and hydroxyl group to an alkene. The halohydrin reaction proceeds with the anti addition of a hydroxyl group (OH) and a halogen, with the hydroxyl group going to the more-substituted carbon of the alkene. The halogen atom bonds to both carbons of the double bond, forming an intermediate halonium ion in a three-membered ring. A hydroxide nucleophile then adds to the opposite face of the halogen in the intermediate, forming a bond with the more substituted carbon. An anti addition is the addition of two substituents to opposite sides of a double bond. Both syn and anti addition add two substituents, but with syn they are added to the same side of the double bond.

Mechanism of Halohydrin

A halogen and hydroxide are added to an alkene in a halohydrin reaction, resulting in the loss of the double bond.
Dihydroxylation is the addition of two hydroxyl (OH) groups to an alkene. The product of this reaction is called a diol. Either a syn- or anti-diol can be created depending on the reagent used. This allows for control of stereochemistry based on the conditions employed in the reaction. In a syn-dihydroxylation reaction, hydroxyl groups are added on the same side of the alkene. In an anti-dihydroxylation reaction, hydroxyl groups are added on opposite sides of the alkene. This results in a product with the hydroxyls attached opposite each other.
Two hydroxyls are added in a dihydroxylation reaction. The addition can lead to either a syn or anti product.
A cyclopropane ring is added via a carbene reaction. A carbene is a reactive molecule containing a neutral carbon atom with two bonds and an unshared pair of electrons. One example of a carbene reaction involves the addition of chloroform (CHCl3) and potassium hydroxide (KOH) to an alkene. The reaction creates a cyclopropane ring on the carbons of the alkene. The ring will have two chlorides attached to the one carbon from the chloroform. The synthesis of a cyclopropane ring from an alkene is a syn addition.

Carbene Reaction

In a carbene reaction, a new cyclopropane ring is attached to the existing structure with the loss of a double bond and two chlorines attached to the ring.
Oxidative cleavage is the cleavage of a carbon-carbon double or triple bond to form two carbonyl compounds. Oxidative cleavage with ozone or KMnO4 cleaves an alkene into aldehydes, ketones, or carboxylic acids (COOH). Oxidative cleavage with ozone and dimethyl sulfide (DMS) will cleave an alkene into two carbonyl compounds. A terminal alkene carbon cleaves into formaldehyde (CH2O). A monosubstituted carbon of an alkene will cleave into an aldehyde, and a disubstituted carbon of an alkene will cleave into a ketone. A terminal alkyne carbon cleaved with ozone or KMnO4 cleaves into carbon dioxide. All other alkyne carbons cleave into carboxylic acids.

Oxidative Cleavage

Aldehydes, ketones, or carboxylic acids (COOH) are created by the oxidative cleavage, cleavage with ozone, of an alkene.