Chapter 8 - Reactions of Alkenes. This chapter include objectives: Explain why electrophilic additions are among the most common reactions of alkenes; Predict the products of the reactions of alkenes, including the orientation of the reaction (regiochemistry) and the stereochemistry; propose mechanisms to explain the observed products of alkene reactions; use retrosynthetic analysis to solve multistep synthesis problems with alkenes as reagents, intermediates, or products. | Chapter 8 © 2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Reactions of Alkenes Chapter 8 Bonding in Alkenes Electrons in pi bond are loosely held. The double bond acts as a nucleophile attacking electrophilic species. Carbocations are intermediates in the reactions. These reactions are called electrophilic additions. Chapter 8 Electrophilic Addition Step 1: Pi electrons attack the electrophile. Step 2: Nucleophile attacks the carbocation. Chapter 8 Types of Additions Chapter 8 Addition of HX to Alkenes Step 1 is the protonation of the double bond. The protonation step forms the most stable carbocation possible. In step 2, the nucleophile attacks the carbocation, forming an alkyl halide. HBr, HCl, and HI can be added through this reaction. Chapter 8 Mechanism of Addition of HX Step 1: Protonation of the double bond. Step 2: Nucleophilic attack of the halide on the carbocation. Chapter 8 Regioselectivity Markovnikov’s | Chapter 8 © 2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Reactions of Alkenes Chapter 8 Bonding in Alkenes Electrons in pi bond are loosely held. The double bond acts as a nucleophile attacking electrophilic species. Carbocations are intermediates in the reactions. These reactions are called electrophilic additions. Chapter 8 Electrophilic Addition Step 1: Pi electrons attack the electrophile. Step 2: Nucleophile attacks the carbocation. Chapter 8 Types of Additions Chapter 8 Addition of HX to Alkenes Step 1 is the protonation of the double bond. The protonation step forms the most stable carbocation possible. In step 2, the nucleophile attacks the carbocation, forming an alkyl halide. HBr, HCl, and HI can be added through this reaction. Chapter 8 Mechanism of Addition of HX Step 1: Protonation of the double bond. Step 2: Nucleophilic attack of the halide on the carbocation. Chapter 8 Regioselectivity Markovnikov’s Rule: The addition of a proton to the double bond of an alkene results in a product with the acidic proton bonded to the carbon atom that already holds the greater number of hydrogens. Markovnikov’s Rule (extended): In an electrophilic addition to the alkene, the electrophile adds in such a way that it generates the most stable intermediate. Chapter 8 Markovnikov’s Rule The acid proton will bond to carbon 3 in order to produce the most stable carbocation possible. Chapter 8 Free-Radical Addition of HBr In the presence of peroxides, HBr adds to an alkene to form the “anti-Markovnikov” product. Peroxides produce free radicals. Only HBr has the right bond energy. The HCl bond is too strong, so it will add according to Markovnikov’s rule, even in the presence of peroxide. The HI bond tends to break heterolytically to form ions, it too will add according to Markovnikov’s rule. Chapter 8 Free-Radical Initiation The peroxide bond breaks homolytically to form the first
