Chlorination of Propane 1 C 2 C There

Chlorination 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 chloropropane and 25% 2 -chloropropane. • Typical product mix: 40% 1 -chloropropane and 60% 2 -chloropropane. • Therefore, not all H’s are equally reactive. =>

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 and 60% 2 = 30% per secondary H • Secondary H’s are 30% 6. 67% = 4. 5 times more reactive toward chlorination than primary H’s. =>

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

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

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. =>

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 and 97% 2 = 48. 5% per secondary H • Secondary H’s are 48. 5% 0. 5% = 97 times more reactive toward bromination than primary H’s. =>

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

Bromination vs. Chlorination =>

Endothermic and Exothermic Diagrams =>

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. =>