Chlorination of Propane 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. => 1  C 2  C

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  H(kJ) 381, 397, 410, 435 =>

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

There are six 1  H’s and two 2  ’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. => 1  C 2  C Bromination of Propane

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. => Reactivity of Hydrogens

Bromination Energy Diagram Note larger difference in E a Why endothermic? BDE for HBr is 368 kJ, but 431 kJ 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. =>