Advices For Studying Organic Chemistry

C

H 3 C H

C

H H

H

H Br

C

H 3 C H

CH 3

H

H 3 C H H

CH 3

Br H 3 C H Br

CH 3

H

Top Bottom

(2S,4R)-2-Bromo-4-methylhexane (2R,4R)-2-Bromo-4-methylhexane

+ Br −

Figure 5. Attack of bromide ion on the 1-methylpropyl carbocation: Attack from
the top leading to S products is the mirror image of attack from the bottom leading
to R product. Since both are equally likely, racemic product is formed. The
dotted C−Br bond in the transition state indicates partial bond formation.

5.9B ENANTIOSELECTIVE SYNTHESES

1. Enantioselective:
   
   
   1. In an enantioselective reaction, one enantiomer is produced predominantly over
      its mirror image.
   
   
   2. In an enantioselective reaction, a chiral reagent, catalyst, or solvent must assert
      an influence on the course of the reaction.
   
   
   
   


2. Enzymes:
   
   
   1. In nature, where most reactions are enantioselective, the chiral influences come
      from protein molecules called enzymes.
   
   
   2. Enzymes are biological catalysts of extraordinary efficiency.
      i) Enzymes not only have the ability to cause reactions to take place much more
      rapidly than they would otherwise, they also have the ability to assert a
      dramatic chiral influence on a reaction.
      ii) Enzymes possess an active site where the reactant molecules are bound,
      momentarily, while the reaction take place.
      iii) This active site is chiral, and only one enantiomer of a chiral reactant fits it