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Cram’s rule and Prelog Rule | Organic Chemistry PDF Download
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.
Topic relation ship of ligands and faces
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FAQs on Cram’s rule and Prelog Rule - Organic Chemistry
| 1. What is Cram's rule and how is it used in chemistry? |  |
Ans. Cram's rule, also known as Cram's rule of preorganization or Cram's rule of selectivity, is a principle in organic chemistry that predicts the preferred conformation of a cyclic compound. It states that when a cyclic compound can exist in multiple conformations, the one that allows for the greatest degree of overlap between orbitals involved in the bonding is the most stable. This rule is particularly useful in understanding the stereochemistry of organic reactions and can help in predicting the outcome of reactions.
| 2. How does Cram's rule apply to determining the major product in reactions involving cyclic compounds? | |
Ans. Cram's rule can be used to determine the major product in reactions involving cyclic compounds by considering the preferred conformation of the reactant and the transition state. The preferred conformation is the one that allows for the greatest degree of orbital overlap, as per Cram's rule. The transition state, which represents the highest energy point along the reaction pathway, is also influenced by the preferred conformation. By analyzing the conformational preferences of the reactant and the transition state, one can predict the major product of the reaction based on Cram's rule.
| 3. What is the Prelog Rule and how is it used in stereochemistry? | |
Ans. The Prelog Rule, also known as the Cahn-Ingold-Prelog priority rules, is a set of rules used in stereochemistry to determine the priority of substituents attached to a stereocenter. According to the Prelog Rule, each substituent is assigned a priority based on the atomic number of the atoms directly attached to the stereocenter. The highest priority group is assigned the number 1, followed by the second highest priority group as number 2, and so on. This rule is essential in determining the stereochemical configuration (R or S) of chiral molecules.
| 4. How does the Prelog Rule help in determining the configuration of chiral centers? | |
Ans. The Prelog Rule is crucial in determining the configuration of chiral centers in organic molecules. By assigning priorities to the substituents attached to a chiral center using the Prelog Rule, one can establish a sequence of priorities. If the lowest priority group is pointing away from the viewer, the configuration is classified as "R" (right-handed). If the lowest priority group is pointing towards the viewer, the configuration is classified as "S" (sinister or left-handed). The Prelog Rule provides a systematic method for assigning configurations to chiral centers based on the priority of substituents.
| 5. Can Cram's rule and the Prelog Rule be applied together in organic chemistry? | |
Ans. Yes, Cram's rule and the Prelog Rule can be applied together in organic chemistry. Cram's rule is primarily used to predict the preferred conformation of cyclic compounds, while the Prelog Rule is used to determine the configuration of chiral centers. In some cases, the two rules can be utilized simultaneously to analyze the stereochemistry of organic reactions involving both cyclic compounds and chiral centers. By considering the preferred conformation of a cyclic compound and the configuration of the chiral center, one can make predictions about the outcome of reactions based on both rules.
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