Describe the drawbacks in cram's rule and felkin's modifications in nucleophilic addition to carbonyls
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Answer:
A carbonyl carbon is sp2 hybridized. This means that the carbonyl carbon and the three other atoms attached to it would be in one plane. A nucleophile attacking the carbonyl carbon in molecules like 1A in Fig 1.2.1 could attack from either side of this plane with equal ease. Have a close look at the aldehyde shown in Fig 1.2.1.The phenyl group is also flat. Nucleophilic attack on benzaldehyde could take place from either side with equal ease. Since a new asymmetric center is now created by this reaction, both enantiomers could be formed with equal ease, resulting in a racemic mixture.
In complex organic molecules, this seldom happens. When the αα - carbon is asymmetric (see 1B), the nucleophile would experience more steric hindrance from one side, leading to unequal synthesis of the two enantiomers. This fact was first recognized by D. Cram in 1952. After analyzing several published reactions, he put forth an explanation based on one proposed conformation. He set forth to first define a Reactive conformation, as the least energy conformation in which the chemical reaction takes place. Thus, for 1B there could be several conformations, out of which chemical reactions prefer to proceed via the conformation having the least steric strain.