Question

describe with words and pictures, how an Sn1 reaction on a chiral structure leads to a racemic product mixture

Answer 1

• SN1 reaction is a nucleophilic reaction, which goes through two step mechanism. First step is formation of carbocation and second step is attack of nucleophile. The first step is slower and therefore is called rate determining step (R.D.S.).

R-X + R+ +X-
    (R.D.S.)
                                       
R+ + Y- + R-Y
      (fast)

• Rate = K[R - X]
• Partial racemization occurs due to ion pair formation.

                                 
                                         (solvent separated system)        (some back attack occurs before complete detachment)

R-X + |R||X| → R+||X-

In the below image you may check the stereochemistry of the chiral molecule.

애 R OH R slow (Retention) filiu OH Inversion

As you can see, first step of the reaction is carbocation formation in which the leaving group Br leaves and then nucleophile can attack on either side on the carbocation. So, there will be two possibilities of the product formation (i.e. retention and inversion) which are shown in the image. In other words racemization has occured at the carbon center. So in SN1 reaction on one chiral centered molecule racemic mixture is obtained, but in this inversion is more than retention.