Advices For Studying Organic Chemistry

Figure 7.7 Free-energy diagrams for the formation of carbocations from
protonated tertiary, secondary, and primary alcohols. The relative
free energies of activation are tertiary < secondary « primary.

3. Hammond-Leffler postulate:
   
   
   1. There is a strong resemblance between the transition state and the cation product.
   
   
   2. The transition state that leads to the 3° carbocation is lowest in free energy
      because it resembles the most stable product.
   
   
   3. The transition state that leads to the 1° carbocation is highest in free energy
      because it resembles the least stable product.
   
   
   
   


4. Delocalization of the charge stabilizes the transition state and the carbocation.

C OH

H

+ OH

H

OH

H

C δ+ C+ +

++

δ+

Protonated alcohol Transition state Carbocation

1) The carbon begins to develop a partial positive charge because it is losing the

electrons that bonded it to the oxygen atom.

2) This developing positive charge is most effectively delocalized in the transition

state leading to a 3° carbocation because of the presence of three

electron-releasing alkyl groups.