The Study of Chemical Reactions

1. The Study of Chemical Reactions • Writing and balancing the overall equation for the reaction barely scrapes the surface…You must understand the • mechanism, • thermodynamics, and • kinetics of the reaction as well.

2. The Chlorination of Methane • We will see how to investigate each of these areas by studying the following gas-phase reaction: CH4 + Cl2 HCl + CH3Cl (+ CH2Cl2 + CHCl3 + CCl4) ∆ or hν

3. The Chlorination of Methane • Does not occur at room temperature in the absence of light. • The most effective light for the reaction is blue and is absorbed by the Cl2. • The light-initiated reaction has a high quantum yield.

4. Free-Radical Chlorination of Methane • Proceeds by a chain reaction. • Steps in any chain reaction: • Initiation • the generation of a reactive intermediate • Propagation • Products form. Reaction continues until reactants or intermediates are depleted. • Termination • removes the reactive intermediates

5. Initiation: Generation of a Free Radical • With either heat (∆) or the appropriate wavelength of light (hν), Cl2 undergoes homolytic cleavage, one electron in the bond going to each of the Cl atoms:

6. Propagation • Propagation refers to the steps in the reaction that generate the products and regenerate the reactive intermediates. step 1 reactive intermediates step 2

7. Termination • Propagation continues until • a reactant is used up, or • the reactive intermediates get depleted by nonproductive reactions. These are some of the termination reactions.

8. Thermodynamics of the Free-Radical Chlorination of Methane • Thermodynamics tell a lot about a system at equilibrium. • ∆G°(25°C) = -108.6 kJ • KP= e-∆G°/RT = e108600/2477.7=e43.83=1.1x1019 CH4 + Cl2HCl + CH3Cl

9. Thermodynamics of the Free-Radical Chlorination of Methane • The large value of K and the large negative value of ∆G° say the reaction goes to completion. • In general, a reaction goes to >99% completion if its ∆G is < -12kJ.

10. Thermodynamics of the Free-Radical Chlorination of Methane • The free energy change depends on the enthalpy change for the reaction, the temperature at which the reaction takes place, and the entropy change: ∆G = ∆H - T∆S • Examining ∆H and ∆S will show what drives the reaction and, consequently, how the reaction will behave at different temperatures. • ∆H°(25°C) = -105 kJ The reaction is very exothermic, which favors products. • ∆S°(25°C) = 12.16 J/K The entropy change is positive, which also favors products. • At 25°C, the bigger influence on ∆G° is the fact that the reaction is so exothermic.

11. Bond-Dissociation Enthalpies • A measure of the strength of a bond is how much energy it takes to break it homolytically: CH3-H + 435* kJ/mol  ▪CH3 + H▪ • The weaker the bond, the less energy is needed to break it. Cl-Cl + 242 kJ/mol  2Cl▪ • This explains why blue light initiates the formation of Cl free radicals but not methyl radicals. *From Table 4-2

12. Bond-Dissociation Enthalpies • Since we know the mechanism of the chlorination reaction, we can calculate ∆H based on bond dissociation enthalpies (BDEs). To break bonds requires 435+242=677kJ/mol. 431 kJ 435 kJ 351 kJ To form bonds releases 431+351=782 kJ/mol. 242 kJ

13. Bond-Dissociation Enthalpies • ∆H° = BDE(bonds broken)-BDE(bonds formed)= 677-782 = -105 kJ/mol • This value tells us the reaction is very exothermic, primarily because the Cl-Cl bond is fairly weak.

14. Kinetics • Thermodynamics tell a lot about a system at equilibrium. • Kinetics tell how fast a system will reach equilibrium. • Reaction rates are determined experimentally.

15. Kinetics • rate = k[reactant 1]a[reactant 2]b … • [reactant 1] is the molarity of reactant 1. • a is the order of the reaction for reactant 1. • a+b+… is the overall order of the reaction. • k is the rate constant.

16. Kinetics • k is the rate constant. • k is given by the Arrhenius equation k=Ae-Ea/RT • A is a constant that incorporates collision frequency and orientation • Ea is the activation energy, the energy needed to form the transition state. • R is the gas constant (8.3145 J/mol K)

17. Reaction-Energy Diagram for a Single-Step Reaction transition state Ea, the activation energy ΔE, the energy change for the reaction

18. Reaction-Energy Diagram for the Two Propagation Steps of the Chlorination of Methane rate equation for step 1: rate=k1[CH4][Cl▪]

19. Temperature Dependence of the Rate Constant • k increases with T. • At a higher temperature, more reactant molecules will have kinetic energies ≥ Ea. • Estimation: Rate doubles for every 10°C the temperature increases. Activation energy Ea

20. Chlorination of Other Alkanes • For ethane and the cycloalkanes, the mechanism is very similar to that of methane. initiation propagation step 1

21. Chlorination of Ethane propagation step 2 termination overall reaction

22. Chlorination of Other Alkanes • Can you write the complete mechanism for the chlorination of cyclopentane? initiation propagation step 1 propagation step 2 termination overall reaction