© Prentice Hall 2001Chapter 31 Addition of Hydrogen Halides to Alkenes What about the following reaction? Which sp 2 carbon gets the hydrogen and which gets the chlorine?

© Prentice Hall 2001Chapter 32 Addition of Hydrogen Halides to Alkenes The more substituted carbocation is preferred

© Prentice Hall 2001Chapter 33 Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the positive charge on the sp 2 carbon of a carbocation

© Prentice Hall 2001Chapter 34 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

© Prentice Hall 2001Chapter 35 Stability of Carbocations Alkyl groups bonded to the sp 2 carbon of a carbocation tend to spread out the positive charge, thereby stabilizing the carbocation

© Prentice Hall 2001Chapter 36 The Hammond Postulate Carbocation formation is endergonic - hence transition state is closest to the carbocation

© Prentice Hall 2001Chapter 37 The Hammond Postulate Formation of a tertiary carbocation should be faster than formation of a primary carbocation

© Prentice Hall 2001Chapter 38 Regioselectivity - Markovnikov’s Rule “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”

© Prentice Hall 2001Chapter 39 Markovnikov’s Rule Modern equivalent statement: The electrophile adds to the sp 2 carbon that is bonded to the greater number of hydrogens

© Prentice Hall 2001Chapter 310 Addition of Water

© Prentice Hall 2001Chapter 311 Rearrangements of Carbocations 1,2-Hydride Shift

© Prentice Hall 2001Chapter 312 Rearrangements of Carbocations 1,2-Methyl Shift

© Prentice Hall 2001Chapter 313 Addition of Halogens The atoms that make up the Br 2 or Cl 2 molecules are very electronegative The halogen-halogen bond is weak

© Prentice Hall 2001Chapter 314 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

© Prentice Hall 2001Chapter 315 Addition of Halogens The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion

© Prentice Hall 2001Chapter 316 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 CCl 4