Reactions of Alkenes

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 $\pi$ 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. Addition reactions that add an ${\rm {H{-}X}}$ 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.
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.
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.

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. The second method is alkoxymercuration-demercuration, which yields a Markovnikov product 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. 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.

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. 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.
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. 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.