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Enantiomers: R and S Nomenclature. Enantiomers To distinguish between enantiomers, chemists use the R and S classification system.

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Presentation on theme: "Enantiomers: R and S Nomenclature. Enantiomers To distinguish between enantiomers, chemists use the R and S classification system."— Presentation transcript:

1 Enantiomers: R and S Nomenclature

2 Enantiomers To distinguish between enantiomers, chemists use the R and S classification system.

3 The Cahn-Ingold-Prelog (CIP) System The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog Their rules for ranking groups were devised in connection with a different kind of stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene stereochemistry.

4 (1)Higher atomic number outranks lower atomic number Br > FCl > H higher lower Br F Cl Hhigherlower C C Table 5.1 CIP Rules

5 (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. —CH 2 CH 3 outranks —CH 3 —C(C,H,H) Table 5.1 CIP Rules —C(H,H,H)

6 (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. —CH(CH 3 ) 2 outranks —CH 2 CH 2 OH —C(C,C,H) —C(C,H,H) Table 5.1 CIP Rules

7 (4) Evaluate substituents one by one. Don't add atomic numbers within groups. —CH 2 OH outranks —C(CH 3 ) 3 —C(O,H,H) —C(C,C,C) Table 5.1 CIP Rules

8 Determination of R or S configuration can be applied in three steps 1.Order the substituents coming off the stereogenic carbon atom using the Cahn- Ingold-Prelog rules. 2.Rotate the molecule until the lowest priority (number 4) substituent is in the back 3.Draw a curve from number 1 to number 2 to number 3 substituent. If the curve is clockwise, the stereocenter is of R configuration. If the curve is counterclockwise, the stereocenter is of S configuration.

9 (S)-bromo, chloro, fluoro, iodomethane, (S)-bromo, chloro, fluoro, iodomethane,

10 clockwise = R counterclockwise = S

11

12 5.4 Naming Steroisomeric Alkenes by the E-Z Notational System

13 Stereochemical Notation cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond) cis and trans are ambiguous when analogies are not obvious C C CH 3 (CH 2 ) 6 CH 2 CH 2 (CH 2 ) 6 CO 2 H H H Oleic acid

14 Example What is needed: 1) systematic body of rules for ranking substituents 2) new set of stereochemical symbols other than cis and trans C C H FClBr

15 C C E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower The E-Z Notational System

16 C C E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower The E-Z Notational System

17 C C E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side Entgegen higher higherlower lower The E-Z Notational System

18 C C C C E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side EntgegenZusammen higher higherlower lower lower higher lower higher The E-Z Notational System

19 C C C C Answer: They are ranked in order of decreasing atomic number. EntgegenZusammen higher higherlower lower lower higher lower higher Question: How are substituents ranked? The E-Z Notational System

20 A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text. Table 5.1 CIP Rules


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