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Reactions of Alkenes

Reactions of Alkenes . Addition of HOX; X: Cl, Br. CCl 4. Ozonolysis. Ozonolysis. Reaction Mechanism. A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order in which bond-breaking and bond-forming steps take place

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Reactions of Alkenes

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  1. Reactions of Alkenes

  2. Addition of HOX; X: Cl, Br CCl4 Ozonolysis Ozonolysis

  3. Reaction Mechanism • A reaction mechanism describes how a reaction occurs • which bonds are broken and which new ones are formed • the order in which bond-breaking and bond-forming steps take place • the role of the catalyst (if any is present) • the energy of the entire system during the reaction

  4. Energy Diagram • Energy diagram: a graph showing the changes in energy that occur during a chemical reaction • Reaction coordinate:a measure of the change in position of atoms during a reaction • Heat of reaction, H:the difference in energy between reactants and products • exothermic:products are lower in energy than reactants; heat is released • endothermic:products are higher in energy than reactants; heat is absorbed

  5. Energy Diagram • Transition state:an energy maximum • represents an unstable species formed during the course of a reaction • Activation Energy, Ea:the difference in energy between reactants and the transition state • determines the rate of reaction • if Ea is large, only a few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow • if Ea is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast

  6. Activation Energy • Activation energy • Is the difference in energy between the reactants and the transition state. • Is the minimum energy required to a reaction to occur or happen • This will determine the rate of the reaction • How fast the reaction occurs

  7. Energy Diagram • an energy diagram for a two-step reaction involving formation of an intermediate

  8. Nucleophile(nucleus loving): An electron rich species that seeks a region of low electron density. • Electrophile (electron loving): A low electron-density species that seeks a region of high electron density.

  9. Addition of HX (X=halogens)Hydrohalogenation • Carried out with pure reagents or in a polar solvent such as acetic acid • Addition is regioselective • regioselective reaction:a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions • Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

  10. HCl + 2-Butene • A two-step mechanism • Step 1: formation of the sec-butyl cation, a 2° carbocation intermediate • Step 2: reaction of the sec-butyl cation (an electrophile) with chloride ion (a nucleophile) completes the reaction

  11. Carbocations • Carbocation:a species in which a carbon atom has only six electrons in its valence shell and bears positive charge • Carbocations are: • classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge • electrophiles; that is, they are electron-loving • Lewis acids; that is, they are electron-pair acceptors

  12. Carbocations • a 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation • a 2° carbocation is, in turn, more stable than a 1° carbocation, and requires a lower activation energy for its formation • methyl and 1° carbocations are so unstable that they are never observed in solution

  13. Addition of H2O • Addition of H2O is called hydration • acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the moresubstituted carbon of the double bond

  14. Step 1: proton transfer to the alkene gives a carbocation Step 2: a Lewis acid/base reaction gives an oxonium ion Step 3: proton transfer to solvent gives the alcohol

  15. Carbon Rearrangements • Rearrangement: change in connectivity of atoms in a product compared to starting material • 1,2-shift • Hydride ion is migrating group • More stable carbocation is formed

  16. Addition of Cl2 and Br2 • Carried out with either the pure reagents or in an inert solvent such as CH2Cl2

  17. Addition of Cl2 and Br2 • addition is stereoselective • stereoselective reaction: a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed • addition to a cycloalkene, for example, gives only a trans product

  18. Addition of Cl2 and Br2 • Step 1: formation of a bromonium ion intermediate • Step 2: halide ion opens the three-membered ring

  19. Addition of Cl2 and Br2 • Anti coplanar addition to a cyclohexene corresponds to trans-diaxial addition

  20. Ozonolysis • Cleavage of a Carbon-Carbon double bond • Two Carbonyl groups in place

  21. Reduction of Alkenes • Alkenes react with H2 in the presence of a transition metal catalyst to give alkanes • the most commonly used catalysts are Pd, Pt, and Ni • the reaction is called catalytic reduction or catalytic hydrogenation

  22. Reduction of Alkenes • the most common pattern is syn addition of hydrogens; both hydrogens add to the same side of the double bond

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