1. 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

2. 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. =>

3. Free Radical Stabilities • Energy required to break a C-H bond decreases as substitution on the carbon increases. • Stability: 3 > 2 > 1 > methylDH(kJ) 381, 397, 410, 435 =>

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

5. 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. =>

6. 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. =>

7. Bromination Energy Diagram • Note larger difference in Ea • Why endothermic? • BDE for HBr is 368 kJ, but 431 kJ for HCl =>

8. Bromination vs. Chlorination =>

9. Endothermic and Exothermic Diagrams =>

10. 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. • Endothermic reaction: transition state is product-like. • Exothermic reaction: transition state is reactant-like.=>