1. © Prentice Hall 2001Chapter 31 Thermodynamics Consider the reaction If the products are more stable than the reactants, (i.e. at a lower standard free energy) then reaction favors the products and K eq > 1

2. © Prentice Hall 2001Chapter 32 Thermodynamics There is a quantitative relationship between the Gibbs standard free energy change and the equilibrium constant  G° = -RT lnK eq  G° = -2.303RT logK eq

3. © Prentice Hall 2001Chapter 33 Thermodynamics When  G° is negative the reaction is exergonic

4. © Prentice Hall 2001Chapter 34 Thermodynamics When  G° is positive the reaction is endergonic

5. © Prentice Hall 2001Chapter 35 Thermodynamics  G° =  H° - T  S°  H° is the standard change in enthalpy or heat exchanged at constant pressure  S° is the standard change in entropy or disorder Note the “standard” here usually refers to 1 molar concentration of dissolved molecules and ions 1 atm pressure for gases

6. © Prentice Hall 2001Chapter 36  H° from Bond Energies bonds being broken  bond of ethylene DF = 61 kcal/mol H - Br DF = 87 kcal/mol DF total = 148 kcal/mol bonds being formed C - H DF = 101 kcal/mol C - Br DF = 69 kcal/mol DF total = 170 kcal/mol  H° = DF (bonds broken) - DF (bonds formed)  H° = 148 kcal/mol - 170 kcal/mol = - 22 kcal/mol

7. © Prentice Hall 2001Chapter 37 Things to Consider When Using  H° as an Approximation for  G° If  H° is significantly negative, as in the case of the addition of HBr to ethylene (- 22 kcal/mol),  S° not likely to have much effect Such approximations are most reliable when considering gas phase reactions In solution there can be significant  S° effects as polar solvent molecules orient themselves around reactants and/or products

8. © Prentice Hall 2001Chapter 38 Solvation Effects

9. © Prentice Hall 2001Chapter 39 Kinetics Knowing the  G° of a reaction will not tell us how fast it will occur or if it will occur at all We need to know the rate of reaction The rate of a reaction is related to the height of the energy barrier for the reaction,  G ‡, the free energy of activation

10. © Prentice Hall 2001Chapter 310 Free Energy of Activation

11. © Prentice Hall 2001Chapter 311 Kinetics The rate of a reaction depends on The rate collisions take place between reactant molecules The fraction of collisions that occur with sufficient energy to react The fraction of collisions that occur with the proper orientation to react

12. © Prentice Hall 2001Chapter 312 Kinetics You must distinguish between reaction rate and rate constant rate constant

13. © Prentice Hall 2001Chapter 313 Kinetics Information relating to the energy barrier for a reaction is obtained from measurement of the rate constant at different temperatures E a must be distinguished from  G ‡ E a does not include entropic terms;  G ‡ does

14. © Prentice Hall 2001Chapter 314 Thermodynamics and Kinetics A B k1k1 k -1 At equilibrium the rate of the forward reaction equals the rate of the reverse reaction k 1 [A] = k -1 [B]

15. © Prentice Hall 2001Chapter 315 Reaction of 2-Butene with Hydrogen Bromide

16. © Prentice Hall 2001Chapter 316 Reaction of 2-Butene with Hydrogen Bromide

17. © Prentice Hall 2001Chapter 317 Rate-Determining Step Formation of the carbocation intermediate is the slower of the two steps It is the rate-determining step

18. © Prentice Hall 2001Chapter 318 Rate-Determining Step Carbocation intermediates are consumed by bromide ions as fast as they are formed The rate of the overall reaction is determined by the slow first step

19. © Prentice Hall 2001Chapter 319 Transition States and Intermediates It is important to distinguish between a transition state and a reaction intermediate A transition state is a local maximum in the reaction coordinate diagram has partially formed and partially broken bonds has only fleeting existence

20. © Prentice Hall 2001Chapter 320 Transition States and Intermediates An intermediate is at a local minimum energy in the reaction coordinate diagram may be isolated in some cases

21. © Prentice Hall 2001Chapter 321 Mechanism for Electrophilic Addition to Alkenes Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes The reaction starts with thee slow addition of an electrophile to an sp 2 carbon, resulting in formation of a carbocation The next step is the rapid addition of a nucleophile to the other sp 2 carbon

22. © Prentice Hall 2001Chapter 322 Addition of Hydrogen Halides to Alkenes

23. © Prentice Hall 2001Chapter 323 Addition of Hydrogen Halides to Alkenes What about the following reaction? Which sp 2 carbon gets the hydrogen and which gets the chlorine?

24. © Prentice Hall 2001Chapter 324 Addition of Hydrogen Halides to Alkenes The more substituted carbocation is preferred

25. © Prentice Hall 2001Chapter 325 Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the positive charge on the sp 2 carbon of a carbocation

26. © Prentice Hall 2001Chapter 326 Stability of Carbocations Alkyl groups are more polarizable than hydrogen (i.e. they tend to release electrons more easily than does hydrogen) Also, alkyl groups can release electrons via hyperconjugation

27. © Prentice Hall 2001Chapter 327 Stability of Carbocations Alkyl groups bonded to the sp 2 carbon of a carbocation tend to spread out the positive charge, thereby stabilizing the carbocation