Learning Outcomes:
i. Comprehend the reactivity of alkenes due to the presence of a pi bond and its susceptibility to electrophilic and free radical addition reactions.
ii. Analyze the mechanisms of various reactions of ethene, including hydrogenation, hydrohalogenation, hydration, halogenation, halohydrin formation, epoxidation, ozonolysis, and polymerization.
iii. Identify the factors affecting the regioselectivity and stereoselectivity of these reactions.
iv. Apply the concepts of alkene reactivity to predict the products of various reactions of ethene and other alkenes.
v. Appreciate the importance of understanding alkene reactivity in organic synthesis and industrial processes.
Introduction
Alkenes, unsaturated hydrocarbons characterized by the presence of one or more carbon-carbon double bonds, exhibit diverse chemical reactivity due to the susceptibility of their pi bond to attack by electrophilic and free radical reagents. This lesson delves into the chemistry of alkenes, focusing on the reactions of ethene as a representative alkene.
i. Hydrogenation: Adding Hydrogen Across the Double Bond
Hydrogenation involves the addition of hydrogen (H2) across the carbon-carbon double bond of an alkene, resulting in the formation of an alkane. The reaction is typically catalyzed by a transition metal catalyst, such as nickel or platinum.
ii. Hydrohalogenation: Adding Hydrogen Halide (HX) Across the Double Bond
Hydrohalogenation involves the addition of a hydrogen halide (HX), such as hydrogen chloride (HCl), hydrogen bromide (HBr), or hydrogen iodide (HI), across the carbon-carbon double bond of an alkene. The reaction follows Markovnikov's rule, where the halogen atom preferentially attaches to the carbon atom with the most hydrogen atoms.
iii. Hydration: Adding Water Across the Double Bond
Hydration involves the addition of water (H2O) across the carbon-carbon double bond of an alkene, resulting in the formation of an alcohol. The reaction is typically catalyzed by an acid, such as sulfuric acid (H2SO4), and follows Markovnikov's rule.
iv. Halogenation: Adding Halogen (X2) Across the Double Bond
Halogenation involves the addition of a halogen molecule (X2), such as chlorine (Cl2), bromine (Br2), or iodine (I2), across the carbon-carbon double bond of an alkene, resulting in the formation of a vicinal dihaloalkane. The reaction is typically catalyzed by light or heat.
v. Halohydrin Formation: Adding Hypohalous Acid (HOX) Across the Double Bond
Halohydrin formation involves the addition of hypohalous acid (HOX), such as hypochlorous acid (HOCl), hypobromous acid (HOBr), or hypoiodous acid (HOI), across the carbon-carbon double bond of an alkene, resulting in the formation of a halohydrin. The reaction is typically catalyzed by an acid.
vi. Epoxidation: Adding an Oxirane Ring Across the Double Bond
Epoxidation involves the addition of an oxirane ring (epoxide) across the carbon-carbon double bond of an alkene. The reaction is typically catalyzed by a peroxyacid, such as m-chloroperoxybenzoic acid (MCPBA).
vii. Ozonolysis: Breaking the Double Bond and Forming Ozonides
Ozonolysis involves the reaction of an alkene with ozone (O3), resulting in the cleavage of the carbon-carbon double bond and the formation of an ozonide. The ozonide can be further decomposed to yield various carbonyl compounds, such as aldehydes and ketones.
viii. Polymerization: Forming Long Chains of Alkenes
Polymerization involves the repeated addition of alkene monomers to form long chains of polymer. The process can be initiated by free radicals, catalysts, or heat. Ethene, for instance, undergoes polymerization to form polyethylene, a widely used plastic.
ix. Selectivity in Alkene Reactions
The regioselectivity of alkene reactions, such as hydrohalogenation and hydration, is governed by Markovnikov's rule, where the halogen or hydroxyl group preferentially attaches to the carbon atom with the most hydrogen atoms. The stereoselectivity of alkene reactions, such as epoxidation, can be influenced by the presence of chiral centers or bulky substituents. For instance, epoxidation of a chiral alkene with MCPBA can produce either enantiomer of the epoxide.
Alkenes exhibit a rich and diverse range of reactions, demonstrating their versatile chemical reactivity. Understanding the mechanisms, regioselectivity, and stereoselectivity of these reactions is essential for predicting product formation, designing synthetic routes, and developing new materials with desired properties.
