Quick Tips: Cram's, Prelog Rules & Prochirality Relation | Organic Chemistry PDF Download

Quick tips for exam point of view.

Prochirality relations:-

• Groups or ligands which are interchangeable with any t ype of simple axis of rotation are ho motopic groups or ligands.
• Groups or ligands which are interchangeable wit h the help of centre of inversio n or effected by plane of symmetry are enantiotopic ligands
• Groups or ligands which are not related wit h any type o f symmetry elements are called diastereotopic ligands.

This is a prochiral mo lecule. On performing a C₂ operation on the first mo lecule we will get the second one.
Now check for the relationship among groups.
5-6,7-8,9-10,11-12 - Diastereotopic
5-10,6-9,8-12,7-11,1-2,3-4 – Homotopic
10-11,9-12,8-6,7-5 – Enantiotopic

Nomenclature of faces and ligands 

Faces are given Re or Si on the basis of the rules of CIP. Same rules are applied.

In the same way we can solve other problems related with nomenclature.

Cram’s rule:-During certain additions to the carbon-oxygen double bond of aldehydes and ketones containing an asymmetric α carbon cram’s rule predicts diastereoselectivity i.e. which diastereomer will predominate.
According to this rule the incoming nucleophilic group preferentially attacks on the side of the plane containing the small group.

By this rule, it can be predicted that the reaction(A) should preferred over (B).
When the size of incoming group increases the extent of diastereoselectivity also increases. As in the above shown example if we increases the size of incoming group i.e. nucleophile from Me<Et<Ph then ratio of the major product formed is also increased.

Prelog rule:-This rule is applied for α-keto esters. This reaction is enantioselective as one enantiomer is preffered which is formed by attack on α-carbonyl carbon from less hindered site.

From the above reaction two compounds are formed which are non-superimposable mirror images of each other i.e. enantiomers. According to this rule incoming nucleophile will attack from that side of the plane which contains group of lowest priority.