1. Chapter 4 THE STUDY OF CHEMICAL REACTIONS The Study of Chemical Reactions

2. USEFUL CONCEPTS 1.MECHANISM – the complete, step-by-step process of bond breaking and bond formation, leading to the observed product(s). 2.THERMODYNAMICS – the study of the energy changes, accompanying chemical and physical transformations. It allows us to predict the direction of the chemical process. 3.KINETICS – the study of the rates at which the chemical reactions occur, thus enabling one to predict which product(s) forms fastest. The Study of Chemical Reactions

3. EXPERIMENTAL OBSERVATIONS ON METHANE CHLORINATION 1.The chlorination does not occur at room temperature in the absence of light. One needs to shine light upon the mixture or heat it, in order for the reaction to take place, i.e. the reaction has to be initiated. 2.If initiated by light, the most effective light “color” (i.e. light wavelength), is the blue color. This light is strongly absorbed by chlorine molecules. 3.The light-initiated reaction has a high quantum yield, which means that a large number of molecules of product are formed per each photon of light absorbed. The Study of Chemical Reactions

4. THE OVERALL REACTION OF CHLORINATION OF METHANE The Study of Chemical Reactions

5. SOME BDE BOND BDE (kcal/mol) H – H104 Cl – Cl58 Br – Br46 H – Cl103 CH 3 – H104 CH 3 – Cl84 CH 3 – Br70 BOND BDE (kcal/mol) CH 3 CH 2 CH 2 – H98 CH 3 CH 2 CH 2 – Cl81 CH 3 CH 2 CH 2 – Br68 (CH 3 ) 2 CH – H95 (CH 3 ) 2 CH – Cl80 (CH 3 ) 2 CH – Br68 (CH 3 ) 3 C – H91 The Study of Chemical Reactions

6. RELATIVE RATES AND TEMEPEARTURE DEPENDANCE OF HALOGENATION F. + CH 4 HF + CH 3. Cl. + CH 4 HCl + CH 3. Br. + CH 4 HBr + CH 3. I. + CH 4 HI + CH 3. REACTION E a (kcal/mole) REL. RATE AT 27 o C (300 K) REL. RATE AT 227 o C (500K) 1.2 ~ 110~ 230 4.01 ~ 14 18 7 x 10 -11 1.2 x 10 -5 34 1.5 x 10 -22 7 x 10 -12 The Study of Chemical Reactions

7. CHLORINATION VS. BROMINATION OF PROPANE – A COMPARATIVE STUDY 1.The first propagation step (the rate-determining step) is exothermic in case of chlorination and endothermic in case of bromination. 2.Both reactions lead to the formation of a mixture of a primary and a secondary radical, but the energy difference between the corresponding transition states in the case of bromination is larger than that in the case of chlorination, REGRADLESS OF THE FACT that the energy difference between the products (i.e. the radicals) is the same. The Study of Chemical Reactions

8. THE HAMMOND POSTULATE In an endothermic reaction the transition state (TS) is closer to the products in both energy AND structure. It is a PRODUCT-LIKE transition state. In an exothermic reaction the transition state is closer to the reactants in both energy AND structure. It is a REACTANT-LIKE transition state. OR IN A MORE GENERAL FASHION: RELATED SPECIES THAT ARE SIMILAR IN ENERGY ARE ALSO SIMILAR IN STRUCTURE. The Study of Chemical Reactions

9. REACTIVE INTERMEDIATES The Study of Chemical Reactions