Advices For Studying Organic Chemistry

CH 3

CH

OH

CH 3

CH (^3) CH CH
3

• H+, heat
  (−H 2 O)
  2 o Carbocation
  CH (^3) CH CH
  3


• 
  

  CH 3
  CH 3
  H

  
  


• CH^3
  CH 3

7.8B REARRANGEMENTS AFTER DEHYDRATION OF A PRIMARY

ALCOHOL

1. The alkene that is formed initially from a 1° alcohol arises by an E2 mechanism.
   
   
   1. An alkene can accept a proton to generate a carbocation in a process that is
      essentially the reverse of the deprotonation step in the E1 mechanism for
      dehydration of an alcohol.
   
   
   2. When a terminal alkene protonates by using its π electrons to bond a proton at
      the terminal carbon, a carbocation forms at the second carbon of the chain (The
      carbocation could also form directly from the 1° alcohol by a hydride shift from
      its β-carbon to the terminal carbon as the protonated hydroxyl group departs).
   
   
   3. Various processes can occur from this carbocation:
      i) A different β-hydrogen may be removed, leading to a more stable alkene than
      the initially formed terminal alkene.
      ii) A hydride or alkanide rearrangement may occur leading to a more stable
      carbocation, after which elimination may be completed.
      iii) A nucleophile may attack any of these carbocations to form a substitution
      product.
      v) Under the high-temperature conditions for alcohol dehydration the principal
      products will be alkenes rather than substitution products.