Electrophilic addition reactions dominate the reactivity profile of alkenes. The presence of a pi bond not only makes alkenes electron-rich but also presents an accessible site for reactions to occur.
Mechanisms of Reactions between Symmetrical Alkenes and Halogens
When symmetrical alkenes react with halogens, the products are typically vicinal dihalides.
Reaction with Halogens (X₂)
- Mechanism:
- 1. Formation of Polarised Halogen: The electron-rich double bond of the alkene induces a dipole in the approaching halogen molecule. This polarisation causes one of the halogen atoms to become slightly positive (δ+) and the other slightly negative (δ-).
- 2. Formation of Cyclic Halonium Ion: The alkene's pi electrons attack the δ+ halogen atom, resulting in the formation of a cyclic halonium ion intermediate.
- 3. Attack by Halide Ion: The negatively charged halogen atom (X⁻) then attacks the more substituted carbon atom of this cyclic intermediate, leading to a vicinal dihalide.
Note: The resulting addition is anti-addition, meaning the halogens are added to opposite faces of the original double bond.
Image courtesy of Benjah-bmm27
Mechanisms of Reactions between Symmetrical Alkenes and Water
The addition of water to alkenes in the presence of an acid is termed hydration.
Hydration Reaction
- Mechanism:
- 1. Protonation of Alkene: The pi electrons of the alkene bond engage with a proton (H⁺) from a strong acid. This creates a carbocation intermediate at the less substituted carbon.
- 2. Nucleophilic Attack by Water: A water molecule approaches the carbocation and forms a bond, leading to an oxonium ion.
- 3. Formation of Alcohol: A second water molecule helps deprotonate the oxonium ion, yielding alcohol as the final product.
Image courtesy of JeanMi
Mechanisms of Reactions between Symmetrical Alkenes and Hydrogen Halides (HX)
Reaction with Hydrogen Halides
- Mechanism:
- 1. Protonation of Alkene: The alkene's pi electrons seize a proton from the hydrogen halide. This step is essential as it forms a carbocation intermediate.
- 2. Nucleophilic Attack by Halide: The halide ion (X⁻) swiftly approaches and forms a bond with the carbocation, yielding a haloalkane.
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Reactions with Unsymmetrical Alkenes
The reactivity of unsymmetrical alkenes is dictated by the stability of potential carbocation intermediates. The Markovnikov rule is an invaluable guide for predicting outcomes.
Reaction with Hydrogen Halides (HX)
- Mechanism:
- 1. Formation of Carbocation: The pi electrons of the alkene's double bond grab a proton from the hydrogen halide. The resulting carbocation is formed preferentially at the more substituted carbon due to increased stability.
- 2. Nucleophilic Attack by Halide: The halide ion (X⁻) then reacts with the carbocation, giving rise to a haloalkane.
Reaction with Water (Hydration)
- Mechanism:
- 1. Formation of Carbocation: The alkene interacts with a proton, forming a carbocation at the more substituted carbon.
- 2. Nucleophilic Attack by Water: Water acts as a nucleophile and combines with the carbocation, leading to an oxonium ion.
- 3. Deprotonation: Another water molecule assists in removing a proton, ultimately delivering an alcohol.
Reaction mechanism of hydration of 1-methylcyclohexene. Sulfuric acid dissociates completely in an aqueous solution and the hydronium ion (H3O+) generated participates in the reaction.
Image courtesy of Kwantlen Polytechnic University
Predicting the Major Product for Unsymmetrical Alkenes
When dealing with unsymmetrical alkenes:
- With hydrogen halides: Markovnikov's rule determines the major product. The halide typically binds to the carbon that has more substituents due to carbocation stability.
- With water: The hydroxyl group (OH) is expected to bond with the carbon that's more substituted, again owing to the stability provided by adjacent carbon atoms.
Detailed Example: Propene's interaction with HBr provides a perfect illustration. When they react, the major product is 2-bromopropane, not 1-bromopropane. This outcome is because the secondary carbocation that forms on the middle carbon atom of propene is more stable than a primary one.
Markovnikov rule demonstrated in the reaction of propene (alkene) with HBr.
Image courtesy of V8rik
Key Takeaway: Carbocation stability is paramount in electrophilic addition reactions involving unsymmetrical alkenes. Remembering the order of stability (tertiary > secondary > primary) is instrumental in predicting reaction products. The guiding principles are the Markovnikov rule and the stability of potential carbocation intermediates. Always consider both while analysing reactions.
FAQ
Stereospecificity in electrophilic addition reactions arises due to the specific orientation or approach of reactants leading to a single stereoisomer as a product. One classic example is the bromination of alkenes, where the intermediate bromonium ion forces the second bromine atom to attack from the opposite side, resulting in anti-addition and a specific stereochemical outcome. The nature of the intermediate and the spatial arrangement of atoms or groups in the reactants dictate the stereochemistry of the products.
Electrophilic and nucleophilic addition reactions involve entities with contrasting electronic characteristics. Electrophilic addition reactions typically involve a substance with electron-rich regions (like alkenes) reacting with electrophiles - electron-poor species. On the contrary, nucleophilic addition reactions involve compounds with electron-poor regions (like carbonyl compounds) reacting with nucleophiles - electron-rich species. Additionally, in electrophilic addition reactions, the reaction starts with the attack on the alkene by the electrophile, whereas in nucleophilic addition, the reaction initiates with the nucleophile attacking the electron-poor centre.
Electrophilic addition reactions play a fundamental role in the synthesis of addition polymers. These polymers are formed from unsaturated monomers, such as alkenes, undergoing an electrophilic addition mechanism which leads to the breaking of the double bond and subsequent chain growth. For instance, poly(ethene), commonly known as polyethylene, is manufactured from ethene monomers that join together through electrophilic addition reactions. The polymerisation process is initiated by a radical, and subsequent monomers add to this radical. The result is a long-chain macromolecule. Such polymers find extensive use in packaging, automotive components, textiles, and countless other applications.
Steric hindrance is a phenomenon where bulky groups around the reactive site can restrict or impede a reaction. In electrophilic addition reactions, if an alkene has large substituents near the double bond, they can physically hinder the approach of the electrophile. As a result, the rate of reaction may decrease. Furthermore, in the case of unsymmetrical alkenes, the steric hindrance can also influence the major product's formation. A bulkier substituent will often direct the electrophile to the less hindered carbon atom, potentially overriding electronic factors such as carbocation stability.
While all alkenes possess a double bond and are generally electron-rich, their reactivity in electrophilic addition reactions can differ based on substituents and their arrangement. Electron-donating groups, such as alkyl groups, increase the electron density of the double bond and make the alkene more reactive towards electrophiles. On the other hand, electron-withdrawing groups can decrease reactivity. Additionally, steric hindrance, as mentioned earlier, can also influence reactivity. However, in general, alkenes are reactive towards electrophilic addition, although the rate and preferred product can vary based on the specific alkene's structure and the electrophile in question.
Practice Questions
Electrophilic addition commences when the electron-rich double bond of ethene polarises the approaching bromine molecule. Due to this polarisation, one bromine atom becomes slightly positive (δ+) and the other slightly negative (δ-). The pi electrons of ethene attack the δ+ bromine atom, leading to the formation of a cyclic bromonium ion intermediate. Subsequently, the negatively charged bromide ion (Br⁻) attacks the more substituted carbon atom of this cyclic intermediate. The resulting compound is 1,2-dibromoethane, a vicinal dibromide. It's crucial to understand that this addition is an anti-addition, signifying the bromine atoms are added to opposite faces of the original double bond.
The unsymmetrical alkene in this scenario is but-1-ene. During the electrophilic addition with HCl, the pi electrons of but-1-ene capture a proton from the HCl, forming a carbocation intermediate. The placement of this carbocation is crucial. It preferentially forms at the secondary carbon (2nd position) rather than the primary carbon (1st position) because secondary carbocations are more stable than primary ones. Following this, the chloride ion (Cl⁻) approaches and forms a bond with this carbocation, giving rise to 2-chlorobutane. Hence, the product's formation is guided by the Markovnikov rule and the inherent stability of the carbocation intermediate.
