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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.

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chlorination of propane
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

reactivity of hydrogens
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. =>
free radical stabilities
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 =>
chlorination energy diagram
Chlorination Energy Diagram

Lower Ea, faster rate, so more stable intermediate is formed faster.

=>

bromination of propane
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 hydrogens1
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. =>
bromination energy diagram
Bromination Energy Diagram
  • Note larger difference in Ea
  • Why endothermic?
  • BDE for HBr is 368 kJ, but 431 kJ for HCl

=>

hammond postulate
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.=>