Chapter 4 The Study of Chemical Reactions

1. Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 4The Study of Chemical Reactions Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003,Prentice Hall

2. Tools for Study • To determine a reaction’s mechanism, look at: • Equilibrium constant • Free energy change • Enthalpy • Entropy • Bond dissociation energy • Kinetics • Activation energy => Chapter 4

3. Chlorination of Methane • Requires heat or light for initiation. • The most effective wavelength is blue, which is absorbed by chlorine gas. • Lots of product formed from absorption of only one photon of light (chain reaction).=> Chapter 4

4. Free-Radical Chain Reaction • Initiation generates a reactive intermediate. • Propagation: the intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). • Termination: side reactions that destroy the reactive intermediate. => Chapter 4

5. Initiation Step A chlorine molecule splits homolytically into chlorine atoms (free radicals) => Chapter 4

6. Propagation Step (1) The chlorine atom collides with a methane molecule and abstracts (removes) a H, forming another free radical and one of the products (HCl). => Chapter 4

7. Propagation Step (2) The methyl free radical collides with another chlorine molecule, producing the other product (methyl chloride) and regenerating the chlorine radical. => Chapter 4

8. => Overall Reaction Chapter 4

9. Termination Steps • Collision of any two free radicals • Combination of free radical with contaminant or collision with wall. Can you suggest others? => Chapter 4

10. Equilibrium constant • Keq = [products] [reactants] • For chlorination Keq = 1.1 x 1019 • Large value indicates reaction “goes to completion.”=> Chapter 4

11. Free Energy Change • DG = free energy of (products - reactants), amount of energy available to do work. • Negative values indicate spontaneity. • DGo = -RT(lnKeq)where R = 1.987 cal/K-moland T = temperature in kelvins • Since chlorination has a large Keq, the free energy change is large and negative. => Chapter 4

12. Problem • Given that -X is -OH, the energy difference for the following reaction is -1.0 kcal/mol. • What percentage of cyclohexanol molecules will be in the equatorial conformer at equilibrium at 25°C? => Chapter 4

13. Factors Determining G • Free energy change depends on • enthalpy • entropy • H = (enthalpy of products) - (enthalpy of reactants) • S = (entropy of products) - (entropy of reactants) • G = H - TS => Chapter 4

14. Enthalpy • DHo = heat released or absorbed during a chemical reaction at standard conditions. • Exothermic, (-DH), heat is released. • Endothermic, (+DH), heat is absorbed. • Reactions favor products with lowest enthalpy (strongest bonds). => Chapter 4

15. Entropy • DSo = change in randomness, disorder, freedom of movement. • Increasing heat, volume, or number of particles increases entropy. • Spontaneous reactions maximize disorder and minimize enthalpy. • In the equation DGo = DHo - TDSo the entropy value is often small. => Chapter 4

16. Bond Dissociation Energy • Bond breaking requires energy (+BDE) • Bond formation releases energy (-BDE) • Table 4.2 gives BDE for homolytic cleavage of bonds in a gaseous molecule. We can use BDE to estimate H for a reaction.=> Chapter 4

17. 104 103 58 84 104 84 => 103 58 Which is more likely? Estimate DH for each step using BDE. Chapter 4

18. Kinetics • Answers question, “How fast?” • Rate is proportional to the concentration of reactants raised to a power. • Rate law is experimentally determined.=> Chapter 4

19. Reaction Order • For A + B  C + D, rate = k[A]a[B]b • a is the order with respect to A • a + b is the overall order • Order is the number of molecules of that reactant which is present in the rate-determining step of the mechanism. • The value of k depends on temperature as given by Arrhenius: ln k = -Ea + lnART => Chapter 4

20. => Activation Energy • Minimum energy required to reach the transition state. • At higher temperatures, more molecules have the required energy. Chapter 4

21. => Reaction-Energy Diagrams • For a one-step reaction:reactants  transition state  products • A catalyst lowers the energy of the transition state. Chapter 4

22. => Energy Diagram for a Two-Step Reaction • Reactants  transition state  intermediate • Intermediate  transition state  product Chapter 4

23. Rate-Determining Step • Reaction intermediates are stable as long as they don’t collide with another molecule or atom, but they are very reactive. • Transition states are at energy maximums. • Intermediates are at energy minimums. • The reaction step with highest Ea will be the slowest, therefore rate-determining for the entire reaction. => Chapter 4

24. Rate, Ea,and Temperature => Chapter 4

25. Conclusions • With increasing Ea, rate decreases. • With increasing temperature, rate increases. • Fluorine reacts explosively. • Chlorine reacts at a moderate rate. • Bromine must be heated to react. • Iodine does not react (detectably). => Chapter 4

26. Chlorination of Propane 1 C • There are six 1 H’s and two 2 H’s. We expect 3:1 product mix, or 75% 1-chloropropane and 25% 2-chloropropane. • Typical product mix: 40% 1-chloropropane and 60% 2-chloropropane. • Therefore, not all H’s are equally reactive. => 2 C Chapter 4

27. Reactivity of Hydrogens • To compare hydrogen reactivity, find amount of product formed per hydrogen: 40% 1-chloropropane from 6 hydrogens and 60% 2-chloropropane from 2 hydrogens. • 40%  6 = 6.67% per primary H and60%  2 = 30% per secondary H • Secondary H’s are 30%  6.67% = 4.5 times more reactive toward chlorination than primary H’s. => Chapter 4

28. Predict the Product Mix Given that secondary H’s are 4.5 times as reactive as primary H’s, predict the percentage of each monochlorinated product of n-butane + chlorine. => Chapter 4

29. Free Radical Stabilities • Energy required to break a C-H bond decreases as substitution on the carbon increases. • Stability: 3 > 2 > 1 > methylDH(kcal) 91, 95, 98, 104 => Chapter 4

30. Chlorination Energy Diagram Lower Ea, faster rate, so more stable intermediate is formed faster. => Chapter 4

31. Bromination of Propane 1 C 2 C • There are six 1 H’s and two 2 H’s. We expect 3:1 product mix, or 75% 1-bromopropane and 25% 2-bromopropane. • Typical product mix: 3% 1-bromopropane and 97% 2-bromopropane !!! • Bromination is more selective than chlorination. => Chapter 4

32. Reactivity of Hydrogens • To compare hydrogen reactivity, find amount of product formed per hydrogen: 3% 1-bromopropane from 6 hydrogens and 97% 2-bromopropane from 2 hydrogens. • 3%  6 = 0.5% per primary H and97%  2 = 48.5% per secondary H • Secondary H’s are 48.5%  0.5% = 97 times more reactive toward bromination than primary H’s. => Chapter 4

33. Bromination Energy Diagram • Note larger difference in Ea • Why endothermic? => Chapter 4

34. Bromination vs. Chlorination => Chapter 4

35. Endothermic and Exothermic Diagrams => Chapter 4

36. Hammond Postulate • Related species that are similar in energy are also similar in structure. The structure of a transition state resembles the structure of the closest stable species. • Transition state structure for endothermic reactions resemble the product. • Transition state structure for exothermic reactions resemble the reactants.=> Chapter 4

37. Radical Inhibitors • Often added to food to retard spoilage. • Without an inhibitor, each initiation step will cause a chain reaction so that many molecules will react. • An inhibitor combines with the free radical to form a stable molecule. • Vitamin E and vitamin C are thought to protect living cells from free radicals. => Chapter 4

38. Reactive Intermediates • Carbocations (or carbonium ions) • Free radicals • Carbanions • Carbene => Chapter 4

39. Carbocation Structure • Carbon has 6 electrons, positive charge. • Carbon is sp2 hybridized with vacant p orbital. => Chapter 4

40. Carbocation Stability • Stabilized by alkyl substituents 2 ways: • (1) Inductive effect: donation of electron density along the sigma bonds. • (2) Hyperconjugation: overlap of sigma bonding orbitals with empty p orbital.=> Chapter 4

41. Free Radicals • Also electron-deficient • Stabilized by alkyl substituents • Order of stability:3 > 2 > 1 > methyl => Chapter 4

42. Carbanions • Eight electrons on C:6 bonding + lone pair • Carbon has a negative charge. • Destabilized by alkyl substituents. • Methyl >1 > 2  > 3  => Chapter 4

43. Carbenes • Carbon is neutral. • Vacant p orbital, so can be electrophilic. • Lone pair of electrons, so can be nucleophilic. => Chapter 4

44. End of Chapter 4 Chapter 4