Characteristic Reactions of Alkenes

2. Characteristic Reactions of Alkenes .

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; energy is plotted in the y-axis and progress of the reaction is plotted in the x-axis. • Reaction coordinate: A measure of the progress of a reaction. Plotted on the x-axis in an energy diagram reaction. • Heat of reaction H:The difference in energy between reactants and products. • Exothermic: Theproducts of a reaction are lower in energy than the reactants; heat is released. • Endothermic: The products of a reaction are higher in energy than the reactants; heat is absorbed.

5. Energy Diagram Figure 5.1 An energy diagram for a one-step reaction between C and A-B to give C-A and B.

6. Energy Diagram • Transition state:An unstable species of maximum energy formed during the course of a reaction; a maximum on an energy diagram. • Activation energy Ea:The difference in energy between the reactants and the transition state. • Eadetermines the rate of reaction. • If Ea is largevery few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow. • If Ea is smallmany collisions generate sufficient energy to reach the transition state, and the reaction is fast.

7. Energy Diagram Figure 5.2 An energy diagram for a two-step reaction involving formation of an intermediate.

8. Developing a Reaction Mechanism • Design experiments to reveal the details of a particular chemical reaction. • Propose a set or sets of steps that might account for the overall transformation. • A mechanism becomes established when it is shown to be consistent with every test that can be devised. • This doesn’t mean that the mechanism is correct, only that it is the best explanation we are able to devise.

9. Why Mechanisms? Mechanisms provide: • A theoretical framework within which to organize descriptive chemistry. • An intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems. • A tool with which to search for new information and new understanding.

10. Electrophilic Additions to Alkenes • Addition of hydrogen halides (HCl, HBr, HI) • Hydrohalogenation • Addition of water (H2O/H2SO4) • Acid-Catalyzed hydration • Addition of halogens (Cl2, Br2) • Halogenation

11. Addition of HX • Carried out with the 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 to a double bond, H adds to the carbon with the greater number of hydrogens bonded to it.

12. Addition of HCl to 2-Butene • A two-step mechanism • Step 1: Formation of a 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.

13. HCl + 2-Butene • Figure 5.4 An energy diagram for the two-step addition of HCl to 2-butene. The reaction is exothermic.

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

15. Carbocations • Bond angles about the positively charged carbon are approximately 120°. • Carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups. • The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons.

16. Carbocations • 3°carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation. • 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.

17. Relative Stability of Carbocations • Inductive effect: The polarization of the electron density of a covalent bond as a result of the electronegativity of a nearby atom. • The electronegativity of a carbon atom bearing a positive charge exerts an electron-withdrawing inductive effect that polarizes electrons of adjacent sigma bonds toward it. • the positive charge of a carbocation is not localized on the trivalent carbon, but rather is delocalized over nearby atoms as well. • The larger the area over which the positive charge is delocalized, the greater the stability of the cation.

18. Relative Stability of Carbocations • Figure 5.5 Delocalization of positive charge by the electron-withdrawing inductive effect of the positively charged trivalent carbon according to molecular orbital calculations.

19. Addition of H2O to an Alkene • Addition of H2O to an alkene is called hydration. • Acid-catalyzed hydration of an alkene is regioselective:hydrogen adds preferentially to the less substituted carbon of the double bond. Thus H-OH adds to alkenes in accordance with Markovnikov’s rule.

20. 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.

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

22. 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. The reaction occurs with • anti stereoselectivity.

23. Addition of Cl2 and Br2 • Step 1: Formation of a bromonium ion intermediate, • Step 2: Halide ion opens the three-membered ring.

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

25. Carbocation Rearrangements • Product of electrophilic addition to an alkene involves rupture of a pi bond and formation of two new sigma bonds in its place. In the following addition, however, only 17% of the expected product is formed. • Rearrangement: A reaction in which the product(s) have a different connectivity of atoms than that in the starting material.

26. Carbocation Rearrangements • Typically either an alkyl group or a hydrogen atom migrates with its bonding electrons from an adjacent atom to an electron-deficient atom as illustrated in the following mechanism. • The key step in this type of rearrangement is called a 1,2-shift.

27. Carbocation Rearrangements • Step 1: Proton transfer from the HCl to the alkene to give a 2° carbocation intermediate. • Step 2: Migration of a methyl group with its bonding electrons from the adjacent carbon gives a more stable 3°carbocation.

28. Carbocation Rearrangements • Step 3: Reaction of the 3°carbocation (an electrophile and a Lewis acid) with chloride ion (a nucleophile and a Lewis base) gives the rearranged product.

29. Carbocation Rearrangements • Rearrangements also occur in the acid-catalyzed hydration of alkenes, especially where the carbocation formed in the first step can rearrange to a more stable carbocation.

30. Carbocations-Summary • The carbon bearing a positive charge is sp2 hybridized with bond angles of 120° • The order of carbocation stability is 3°>2°>1°. • Carbocations are stabilized by the electron-withdrawing inductive effect of the positively charged carbon. • Methyl and primary carbocations are so unstable that they are never formed in solution. • Carbocations may undergo rearrangement by a 1,2-shift, when the rearranged carbocation is more stable than the original carbocation. The most commonly observed pattern is from 2° to 3°.

31. Carbocations-Summary • Carbocation intermediates undergo three types of reactions: 1. Rearrangement by a 1,2-shift to a more stable carbocation. 2. Addition of nucleophile (e.g halide ion, H2O). 3. Loss of a proton to give an alkene (the reverse of the first step in both the hydrohalogenation and theacid-catalyzed hydration of an alkene).

32. Hydroboration-Oxidation • The result of hydroboration followed by oxidation of an alkene is hydration of the carbon-carbon double bond. • Because -H adds to the more substituted carbon of the double bond and -OH adds to the less substituted carbon, we refer to the regiochemistry of hydroboration/oxidation as anti-Markovnikov hydration.

33. Hydroboration-Oxidation • Hydroboration is the addition of BH3 to an alkene to form a trialkylborane. • Borane is most commonly used as a solution of BH3 in tetrahydrofuran (THF).

34. Hydroboration-Oxidation • Hydroboration is both regioselective and syn stereoselective. • Regioselectivity:-H adds to the more substituted carbon and boron adds to the less substituted carbon of the double bond. • Stereoselectivity: Boron and -H add to the same face of the double bond (syn stereoselectivity).

35. Hydroboration-Oxidation • Chemists account for theregioselectivityby proposing the formation of a cyclic four-center transition state. • And for the syn stereoselectivity by steric factors. Boron, the larger part of the reagent, adds to the less substituted carbon and hydrogen to the more substituted carbon.

36. Hydroboration-Oxidation • Trialkylboranes are rarely isolated. Treatment with alkaline hydrogen peroxide (H2O2/NaOH), oxidizes a trialkylborane to an alcohol and sodium borate.

37. Ozonolysis of an Alkene • Ozonolysis of an alkene followed by suitable workup, cleaves the carbon-carbon double bond and forms two carbonyl (C=O) groups in its place. Ozonolysis is one of the few organic reactions that cleaves carbon-carbon double bonds.

38. Ozonolysis of an Alkene • Ozone (18 valence electrons) is strongly electrophilic. • Initial reaction gives an intermediate called a molozonide.

39. Ozonolysis of an Alkene • The intermediate molozonide rearranges to an ozonide. • Treatment of the ozonide with dimethylsulfide gives the final products.

40. 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.

41. Reduction of Alkenes • The most common pattern is syn addition of hydrogens; both hydrogens add to the same face of the double bond. • Catalytic reduction is syn stereoselectivity.

42. Catalytic Reduction of an Alkene • Figure 5.6 Syn addition of H2 to an alkene involving a transition metal catalyst. (a) H2 and the alkene are absorbed on the catalyst. (b) One H is transferred forming a new C-H bond. (c) The second H is transferred. The alkane is desorbed.

43. Heats of Hydrogenation • Table 5.2 Heats of Hydrogenation for Several Alkenes

44. Heats of Hydrogenation • Reduction involves net conversion of a weaker pi bond to a stronger sigma bond. • The greater the degree of substitution of a double bond, the lower its heat of hydrogenation. • The greater the degree of substitution, the more stable the double bond. • The heat of hydrogenation of a trans alkene is lower than that of the isomeric cis alkene. • A trans alkene is more stable than its isomeric cis alkene. • The difference is due to nonbonded interaction strain in the cis alkene.

45. Heats of Hydrogenation • Figure 5.7 Heats of hydrogenation of cis-2-butene and trans-2-butene. • trans-2-butene is more stable than cis-2-butene by 1.0 kcal/mol

46. The Acidity of Terminal Alkynes • One of the major differences between the chemistry of alkanes, alkenes, and alkynes is that terminal alkynes are weak acids. • Table 5.3 Acidity of Alkanes, Alkenes, and Alkynes.

47. The Acidity of Terminal Alkynes • Treatment of a 1-alkyne with a very strong base such as sodium amide, NaNH2, converts the alkyne to an acetylide anion. • Note that hydroxide ion is not a strong enough base to form an acetylide anion.

48. Acetylide Anions in Synthesis • An acetylide anion is both a strong base and a nucleophile. It can donate a pair of electrons to an electrophilic carbon atom and form a new carbon-carbon bond. • In this example, the electrophile is the partially positive carbon of chloromethane. As the new carbon-carbon bond is formed, the carbon-halogen bond is broken. • Because an alkyl group is added to the original alkyne, this reaction is called alkylation.

49. Acetylide Anions in Synthesis • The importance of alkylation of acetylide anions is that it can be used to create larger carbon skeletons. • For reasons we will discuss fully in Chapter 7, this type of alkylation is successful only for methyl and primary alkyl halides (CH3X and RCH2X).

50. Acid-Catalyzed Hydration of Alkynes • In the presence of concentrated sulfuric acid and Hg(II) salts, alkynes undergo hydration in accordance with Markovnikov’s rule. • The initial product is an enol, a compound containing an hydroxyl group bonded to a carbon-carbon double bond. • The enol is in equilibrium with a more stable keto form. This equilibration is called keto-enol tautomerism.