Reactions of alkenes: A summary

Table of Contents

• Addition reactions

• Terminology used in alkene reactions 

  • Symmetrical vs unsymmetrical

  • Markovnikov vs anti-Markovnikov

  • Syn vs anti addition

• Reactions of alkenes

  • Carbocation pathway

    • Hydrohalogenation (Addition of -H and -X)

    • Acid-catalyzed hydration (Addition of -H and -OH)

  • Radical Pathway

  • Three-membered ring pathway

    • Halogenation (Addition of X2)

    • Halohydrin formation (Addition of -X and -OH) 

    • Anti-dihydroxylation (Addition of -OH and -OH)

    • Oxymercuration-demercuration (Addition of -H and - …

  • Concerted pathway

    • Hydrogenation (Addition of H2)

    • Hydroboration-oxidation (Addition of -H and -OH)

    • Addition of carbenes

    • Syn-dihydroxylation (Addition of -OH and -OH)

    • Epoxidation

    • Ozonolysis

Addition reactions

The reactions of alkenes are known as addition reactions. In addition reactions, two groups are added across the pi bond. 

Terminology used in alkene reactions 

Symmetrical vs unsymmetrical

Alkene structure can be described using two terms: symmetrical and unsymmetrical. 

A symmetrical alkene has equal or identical substituents on each carbon of the double bond. 

In an unsymmetrical alkene, the carbons of the double bond are not equally substituted. 

Markovnikov vs anti-Markovnikov

In a Markovnikov addition, the nucleophile adds to the more substituted carbon of an alkene. In an anti-Markovnikov addition, the nucleophile adds to the less substituted carbon.

Unsymmetrical alkenes form either the Markovnikov or the anti-Markovinkov addition product, but not both. As such these reactions are said to be regioselective when only one of two possible pathways is preferred.

Syn vs anti addition

Syn addition refers to the addition of two substituents to the same side (or face) of a double bond. In contrast, anti addition involves the addition of two substituents to opposite sides (or faces) of the double bond. 

Reactions that occur only through anti addition or only through syn addition are said to be stereoselective. Reactions that form both syn and anti addition products are not stereoselective.

Reactions of alkenes

The reactions of alkenes can be grouped into four main categories based on the reaction pathway:

• Carbocation pathway

• Radical pathway

• Three-membered ring pathway

• Concerted pathway

Carbocation pathway

Reactions in this category involve two main steps: a proton transfer followed by a nucleophilic attack. In the proton transfer step, a proton adds to the alkene, generating a carbocation intermediate.

Then, a nucleophile attacks the more stable carbocation. The reaction follows Markovnikov's rule because the nucleophile adds to the more substituted carbon. 

Additionally, since carbocations are formed, rearrangements are possible leading to the formation of multiple products. 

Reactions in this category include:

• Hydrohalogenation

• Acid-catalyzed hydration

Hydrohalogenation (Addition of -H and -X)

Alkenes react with hydrogen halides (HX, like HCl or HBr) to form alkyl halides.

• Regiochemistry: Markovnikov

• Stereochemistry: A mixture of syn + anti

• Rearrangements: Yes

Acid-catalyzed hydration (Addition of -H and -OH)

Alkenes react with water in the presence of an acid catalyst such as H₂SO₄ to form alcohols.

• Regiochemistry: Markovnikov

• Stereochemistry: A mixture of syn + anti

• Rearrangements: Yes

Radical Pathway

The primary reaction in this category is the radical addition of HBr. In the presence of peroxides, such as hydrogen peroxide (H₂O₂) or the commonly used meta-chloroperoxybenzoic acid (mCPBA), the addition of -H and -Br proceeds via an anti-Markovnikov addition. 

• Regiochemistry: anti-Markovnikov

• Stereochemistry: A mixture of syn + anti

• Rearrangements: No

In the radical addition of HBr to an alkene, peroxides generate a bromine radical. 

The bromine radical adds to the pi bond generating a carbon radical intermediate. 

The observed anti-Markovnikov regioselectivity arises because the reaction proceeds via the most stable carbon radical intermediate. 

Three-membered ring pathway

Reactions in this category involve a three-membered ring intermediate. 

These reactions follow a Markovnikov-like pattern, where the nucleophile attacks the more substituted carbon of the three-membered ring from the backside, resulting in the formation of anti-addition products.

However, oxymercuration-demercuration is an exception to this rule. There is ample evidence indicating that demercuration occurs through a radical process, forming both syn and anti addition products. 

Reactions in this category include:

• Halogenation

• Halohydrin formation

• Anti-dihydroxylation

• Oxymercuration-demercuration

Halogenation (Addition of X₂)

This reaction involves the addition of halogens such as Cl₂ or Br₂ across an alkene. 

• Regiochemistry: Markovnikov-like

• Stereochemistry: anti 

• Rearrangements: No

Halohydrin formation (Addition of -X and -OH) 

This reaction adds a halogen, -X, and -OH to an alkene, forming a halohydrin.

• Regiochemistry: Markovnikov-like

• Stereochemistry: anti 

• Rearrangements: No

Anti-dihydroxylation (Addition of -OH and -OH)

This two-step reaction converts an alkene into an epoxide, followed by an acid-catalyzed ring opening. The net result is the anti addition of two -OH groups across the alkene.

• Regiochemistry: Markovnikov-like

• Stereochemistry: anti 

• Rearrangements: No

Oxymercuration-demercuration (Addition of -H and -OH)

This reaction adds -H and -OH across an alkene without carbocation rearrangements.

• Regiochemistry: Markovnikov-like

• Stereochemistry: anti 

• Rearrangements: No

Concerted pathway

In a concerted reaction, all bond-breaking and bond-forming processes occur simultaneously in a single step. 

This is different from a stepwise mechanism, where intermediates are formed. In these reactions, two groups are added to the same side (or face) of the double bond in a concerted process, giving rise to syn addition products.

Reactions in this category include:

• Hydrogenaton

• Hydroboration-oxidation

• Carbene formation

• Syn-dihydroxylation

• Epoxidation

• Ozonolysis

Hydrogenation (Addition of H₂)

This reaction adds two hydrogen atoms across an alkene forming an alkane.

• Regiochemistry: N/A

• Stereochemistry: syn

• Rearrangements: No

Hydroboration-oxidation (Addition of -H and -OH)

This reaction adds -H and -OH across an alkene forming an alcohol without any carbocation rearrangements.

• Regiochemistry: anti-Markovnikov

• Stereochemistry: syn

• Rearrangements: No

Addition of carbenes

Alkenes react with carbenes to yield cyclopropane.

• Regiochemistry: anti-Markovnikov

• Stereochemistry: syn

• Rearrangements: No

Syn-dihydroxylation (Addition of -OH and -OH)

This reaction adds two -OH groups across an alkene in a syn fashion.

• Regiochemistry: N/A

• Stereochemistry: syn

• Rearrangements: No

Epoxidation

Alkenes are oxidized in the presence of peroxyacids forming epoxides.

• Regiochemistry: N/A

• Stereochemistry: syn

• Rearrangements: No

Ozonolysis

Powerful oxidizing agents such as ozone (O₃) are powerful enough to cleave the carbon-carbon double of an alkene and produce two carbonyl compounds.

• Regiochemistry: N/A

• Stereochemistry: syn

• Rearrangements: No