Stability of Allylic and Benzylic Cations An allylic carbon is an sp3 carbon adjacent to an sp2 carbon of an alkene These carbons can lose a basic substituent and become a carbocation - in the process they become sp2 hybridized Allylic and benzylic cations have delocalized electrons © Prentice Hall 2001 Chapter 6

Stability of Allylic and Benzylic Cations © Prentice Hall 2001 Chapter 6

Stability of Allylic and Benzylic Cations © Prentice Hall 2001 Chapter 6

Stability of Allylic and Benzylic Cations Alkyl substitution at the allylic carbon stabilizes allylic and benzylic cations © Prentice Hall 2001 Chapter 6

Stability of Allylic and Benzylic Radicals Allylic and Benzylic radicals also are stabilized by electron delocalization © Prentice Hall 2001 Chapter 6

Chemical Consequences of Electron Delocalization Markovnikov’s rule makes no prediction in this case The stability of the benzylic cation is such as to exclude the other possible product a secondary benzylic cation a secondary carbocation © Prentice Hall 2001 Chapter 6

Chemical Consequences of Electron Delocalization Rearrangements also are driven by electron delocalization The driving force for the rearrangement is the formation of a more stable secondary benzylic cation © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH) pKa = 4.76 pKa = 15.9 © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa Substituents that withdraw electrons from the oxygen of the OH group, such as (C=O), weaken the bond between O and H, and The resulting carboxylate ion is stabilized by electron delocalization © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa A protonated aniline is more acidic than a protonated cyclohexylamine pKa = 4.6 pKa = 11.2 © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa © Prentice Hall 2001 Chapter 6

Effect of Delocalization on pKa © Prentice Hall 2001 Chapter 6