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Addition of Hydrogen Halides to Alkenes

Addition of Hydrogen Halides to Alkenes. What about the following reaction?. Which sp 2 carbon gets the hydrogen and which gets the chlorine?. Addition of Hydrogen Halides to Alkenes. The more substituted carbocation is preferred. Stability of Carbocations.

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Addition of Hydrogen Halides to Alkenes

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  1. Addition of Hydrogen Halides to Alkenes • What about the following reaction? • Which sp2 carbon gets the hydrogen and which gets the chlorine? Chapter 3

  2. Addition of Hydrogen Halides to Alkenes • The more substituted carbocation is preferred Chapter 3

  3. Stability of Carbocations • Alkyl groups (“R”s) tend to stabilize the positive charge on the sp2carbon of a carbocation Chapter 3

  4. Stability of Carbocations • Alkyl groups are more polarizable than hydrogen (i.e. they tend to release electrons more easily than does hydrogen) • Also, alkyl groups can release electrons via hyperconjugation Chapter 3

  5. Stability of Carbocations • Alkyl groups bonded to the sp2carbon of a carbocation tend to spread out the positive charge, thereby stabilizing the carbocation Chapter 3

  6. The Hammond Postulate Carbocation formation is endergonic - hence transition state is closest to the carbocation Chapter 3

  7. The Hammond Postulate • Formation of a tertiary carbocation should be faster than formation of a primary carbocation Chapter 3

  8. “When a hydrogen halide adds to an asymmetrical alkene, the addition occurs such that the halogen attaches itself to the carbon atom of the alkene bearing the least number of hydrogen atoms” Regioselectivity - Markovnikov’s Rule Chapter 3

  9. Markovnikov’s Rule • Modern equivalent statement: • The electrophile adds to the sp2carbon that is bonded to the greater number of hydrogens Chapter 3

  10. Addition of Water Chapter 3

  11. Rearrangements of Carbocations 1,2-Hydride Shift Chapter 3

  12. Rearrangements of Carbocations 1,2-Methyl Shift Chapter 3

  13. Addition of Halogens • The atoms that make up the Br2 or Cl2 molecules are very electronegative • The halogen-halogen bond is weak Chapter 3

  14. Addition of Halogens • When  electrons approach a halogen molecule, one end of the halogen molecule accepts them releasing the shared bonding pair to the other halogen atom to form a halonium ion and a halide ion Chapter 3

  15. Addition of Halogens • The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion Chapter 3

  16. Addition of Halogens • Chlorine and bromine are the only useful halogen used for this reaction • Fluorine reacts explosively, and iodine doesn’t react • Addition reactions typically are carried out in CCl4 Chapter 3

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