Advices For Studying Organic Chemistry

Figure 8.1 Free-energy diagram for the addition of HX to an alkene. The free
energy of activation for step 1 is much larger than that for step 2.

2. Rate-determining step:
   
   
   1. Alkene accepts a proton from the HX and forms a carbocation in step 1.
   
   
   2. This step is highly endothermic and has a high free energy of activation ⇒ it
      takes place slowly.
   
   
   
   


3. Step 2:
   
   
   1. The highly reactive carbocation stabilizes itself by combining with a halide ion.
   
   
   2. This exothermic step has a very low free energy of activation ⇒ it takes place
      rapidly.
   
   
   
   

8.2B THEORETICAL EXPLANATION OF MARKOVNIKOV’S RULE:

1. The step 1 of the addition reaction of HX to an unsymmetrical alkene could
   conceivably lead to two different carbocations:

CH 2 CHo 3 CH CH+ 2 + X −

H

X H + CH 3 CH

1 Carbocation

(less stable)

CH 3 CH CH 2 + CH 3 CH CH 2 H +

+

H X X −

2 o Carbocation

(more stable)

2

1) arbocation is more stable ⇒ accounts for the correct predication of the

overall addition by Markovnikov’s rule.

. These two carbocations are not of equal stability.
The 2° c