Organic Chemistry 4th Edition Chapter 4 Paula Yurkanis Bruice Addition Reactions of Alkenes Electrophilic Addition of Alkenes 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 3 Electrophilic Addition • Step 1: Pi electrons attack the electrophile. • Step 2: Nucleophile attacks the carbocation. Chapter 8 4 Addition of Hydrogen Halides What is the product? Carbocation formation is the rate-limiting step a more stable carbocation Carbocation Stabilities Markovnikov’s Rule The electrophile adds to the sp2 carbon that is bonded to the greater number of hydrogens In a regioselective reaction, one constitutional isomer is the major or the only product tert-Butyl cation is formed faster and it is more stable 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 11 Addition of Water to Alkene Hydration of Propene Acid-Catalyzed Addition of Alcohol Rearrangement of Carbocation 1,2-hydride shift a more stable carbocation Rearrangement of Carbocation a more stable 1,2-methyl shift carbocation Carbocation Rearrangement Ring Expansion a more stable carbocation Addition of Halogens to Alkene As soon as you see a new reaction, you should immediately think to yourself, ‘Which reagent is the nucleophile; which reagent is the electrophile?’ Evidently, neither the alkene nor bromine is charged, but Br2 has a low-energy empty orbital (the Br–Br σ*), and is therefore an electrophile. The alkene must be the nucleophile, and its HOMO is the C=C π bond. Addition of Halogens to Alkene In the second step of the reaction, Br- attacks a carbon atom of the bromonium ion. This releases the strain in the three- membered ring and forms a vicinal dibromide. The cyclic bromonium ion is more stable than the carbocation would have been, since all the atoms (except hydrogen) in the bromonium ion have complete octets, whereas the positively charged carbon of the carbocation does not have a complete octet. Addition of Halogens to Alkene When Cl2 adds to an alkene, a cyclic chloronium ion intermediate is formed. The final product of the reaction is a vicinal dichloride. Addition of Halogens to Alkene F2 and I2 are halogens, but they are not used as reagents in electrophilic addition reactions. Fluorine reacts explosively with alkenes, so the electrophilic addition of F2 is not a synthetically useful reaction. The addition of I2 to an alkene is a thermodynamically unfavorable reaction: The vicinal diiodides are unstable at room temperature, decomposing back to the alkene and I2 . We ignored the issue of symmetry in the alkene when we discussed the bromination of alkenes, because even unsymmetrical alkenes give the same 1,2-dibromides whichever way the bromide attacks the bromonium ion. Addition of Halogens in the Presence of Water Addition of Halogens in the Presence of Water How can we explain the regioselectivity of the preceding addition reaction? In the transition state for the second step of the reaction, the breaking of the C−Br bond has occurred to a greater extent than has the formation of the C−O bond. As a result, there is a partial positive charge on the carbon that is attacked by the nucleophile. Consider the transition states … Addition of Halogens in the Presence of Other Nucloephiles When nucleophiles other than H2O are added to the reaction mixture, they, too, change the product of the reaction, just as water changed the product of Br2 addition from a vicinal dibromide to a vicinal bromohydrin. Because the concentration of the added nucleophile will be greater than the concentration of the halide ion generated from Br2 or Cl2, the added nucleophile will be the nucleophile most likely to participate in the second step of the reaction. Synthesis of Bromobutane Generation of Radicals Addition of Radicals to Alkene a bromine radical Addition of Radicals to Alkene Addition of Radicals to Alkene Relative Stabilities of Alkyl Radicals Free-Radical Addition of HBr • In the presence of peroxides, HBr adds to an alkene to form the “anti-Markovnikov” product. • 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 34 .