B.Sc.(H) Chemistry Semester - II Core Course - III (CC-III) Organic Chemistry - I II. Stereochemistry 3. Cahn Ingold and Prelog Rules (R and S Nomenclature)
Dr. Rajeev Ranjan University Department of Chemistry Dr. Shyama Prasad Mukherjee University, Ranchi
1 Syllabus & Coverage
Syllabus II Stereochemistry: Fischer Projection, Newmann and Sawhorse Projection formulae and their interconversions. Geometrical isomerism: cis–trans and syn-anti isomerism, E/Z notations with Cahn Ingold and Prelog (CIP) rules for determining absolute configuration. Optical Isomerism: Optical Activity, Specific Rotation, Chirality/Asymmetry, Enantiomers, Molecules with two or more chiral-centres, Distereoisomers, Meso structures, Racemic mixture. Resolution of Racemic mixtures. Relative and absolute configuration: D/L and R/S designations. Coverage: 1. Cahn Ingold and Prelog (CIP) rules for Determining Absolute Configuration 2. Relative and Absolute Configuration: D/L and R/S Designations 3. E and Z Nomenclature
2 Stereochemistry
Cahn-Ingold-Prelog (CIP) Rules for Determining Absolute Configuration: Labeling Stereogenic Centers with R or S: • Since enantiomers are two different compounds, they need to be distinguished by name. This is done by adding the prefix R or S to the IUPAC name of the enantiomer. • Naming enantiomers with the prefixes R or S is called the Cahn- Ingold-Prelog system. • To designate enantiomers as R or S, priorities must be assigned to each group bonded to the stereogenic center, in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1).
3 Dr. Rajeev Ranjan Stereochemistry
Labeling Stereogenic Centers with R or S: • If two atoms on a stereogenic center are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher atomic number determines the higher priority.
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Labeling Stereogenic Centers with R or S: • If two isotopes are bonded to the stereogenic center, assign priorities in order of decreasing mass number. Thus, in comparing the three isotopes of hydrogen, the order of priorities is:
5 Dr. Rajeev Ranjan Stereochemistry
Labeling Stereogenic Centers with R or S: • To assign a priority to an atom that is part of a multiple bond, treat a multiply bonded atom as an equivalent number of singly bonded atoms. For example, the C of a C=O is considered to be bonded to two O atoms.
• Other common multiple bonds are drawn below:
Dr. Rajeev Ranjan 6 Stereochemistry
Labeling Stereogenic Centers with R or S: Treatment of Multiple Bonds in R- or S- system:
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Labeling Stereogenic Centers with R or S: Examples of assigning priorities to stereogenic centers
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Labeling Stereogenic Centers with R or S:
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Labeling Stereogenic Centers with R or S:
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Labeling Stereogenic Centers with R or S:
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Labeling Stereogenic Centers with R or S: Examples: Orienting the lowest priority group in back
12 Dr. Rajeev Ranjan Stereochemistry Examples: When rotating to put the lowest priority group in the back, keep one group in place and rotate the other three. 3 1 OH CH2CH3 rotate C C 3 2 4 2 H CH2CH3 CH3CH2CH2 CH3CH2CH2 H OH 4 1 Clockwise (R)
3 CH3
1 4 H CH3CH2CH=CH 2 CH2CH2CH2CH3 Counterclockwise (S)
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Fischer-Rosanoff Convention: D and L
• Before 1951, only relative configurations could be known. • Sugars and amino acids with same relative configuration as (+)- glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L. • With X-ray crystallography, now know absolute configuration of : D is (R) and L is (S). • No relationship to dextro- or levo-rotatory. CHO H OH HO H COOH CHO H OH H N H H OH 2 * H OH * CH CH COOH * CH2OH 2 2 CH2OH D-(+)-glucose D-(+)-glyceraldehyde L-(+)-glutamic acid
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E and Z Nomenclature : Based on priority Higher priority at the opposite side of pi bond => E Higher priority at the same side of pi bond => Z
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E and Z Nomenclature and Geometric Isomerism:
16 Dr. Rajeev Ranjan Thank You
Dr. Rajeev Ranjan University Department of Chemistry Dr. Shyama Prasad Mukherjee University, Ranchi
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