Advices For Studying Organic Chemistry

finding a bromide ion in its vicinity is low. ⇒ These two factors slow the

addition so that allylic substitution competes successfully.

2. The use of a nonpolar solvent slows addition.
   
   
   1. There are no polar solvent molecules to solvate (and thus stabilize) the bromide
      ion formed in the first step, the bromide ion uses a bromine molecule as a
      substitute. ⇒ In a nonpolar solvent the rate equation is second order with respect
      to bromine.
   
   
   
   

C

C

C

C

Br 2 + +Br + Br 3 −

solvent

2 nonpolar

rate = k (^) C C [Br 2 ]^2
i) The low bromine concentration has a more pronounced effect in slowing the
rate of addition.

3. Why a high temperature favors allylic substitution over addition?
   
   
   1. The addition reaction has a substantial negative entropy change ⇒ At low
      temperatures, the T∆S° term in ∆G° = ∆H° – T∆S°, is not large enough to offset
      the favorable ∆H° term.
   
   
   2. At high temperatures, the T∆S° term becomes more significant, ∆G° becomes
      more positive ⇒ the equilibrium becomes more unfavorable.
   
   
   
   

13.3 THE STABILITY OF THE ALLYL RADICAL

13.3A MOLECULAR ORBITAL DESCRIPTION OF THE ALLYL RADICAL

1. As the allylic hydrogen atom is abstracted from propene, the sp^3 -hybridized
   carbon atom of the methyl group changes its hybridization state to sp^2.