Chemistry Exam > Chemistry Notes > Organic > Cram’s rule and Prelog Rule
Cram’s rule and Prelog 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.
Topic relation ship of ligands and faces
The document Cram’s rule and Prelog Rule is a part of the Chemistry Course Organic Chemistry.
All you need of Chemistry at this link: Chemistry
FAQs on Cram’s rule and Prelog Rule
| 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.
About this Document
5.3K Views
4.73/5 Rating
Apr 26, 2026 Last updated
Related Exams
Document Description: Cram’s rule and Prelog Rule for Chemistry 2026 is part of Organic Chemistry preparation. The notes and questions for Cram’s rule and Prelog Rule have been prepared according to the Chemistry exam syllabus. Information about Cram’s rule and Prelog Rule covers topics like and Cram’s rule and Prelog Rule Example, for Chemistry 2026 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Cram’s rule and Prelog Rule.
Introduction of Cram’s rule and Prelog Rule in English is available as part of our Organic Chemistry for Chemistry & Cram’s rule and Prelog Rule in Hindi for Organic Chemistry course. Download more important topics related with notes, lectures and mock test series for Chemistry Exam by signing up for free. Chemistry: Cram’s rule and Prelog Rule
Description
Cram’s rule & Prelog Rule of Organic Chemistry covers all the important topics, helping you prepare for the Chemistry exam on EduRev. Start for free!
Information about Cram’s rule and Prelog Rule
In this doc you can find the meaning of Cram’s rule and Prelog Rule defined & explained in the simplest way possible. Besides explaining types of Cram’s rule and Prelog Rule theory, EduRev gives you an ample number of questions to practice Cram’s rule and Prelog Rule tests, examples and also practice Chemistry tests
Related Searches
Semester Notes, Summary, Exam, Cram’s rule and Prelog Rule, Previous Year Questions with Solutions, Sample Paper, Extra Questions, shortcuts and tricks, video lectures, past year papers, mock tests for examination, pdf , Free, Cram’s rule and Prelog Rule, Viva Questions, study material, Objective type Questions, practice quizzes, Important questions, ppt, MCQs, Cram’s rule and Prelog Rule;