Enantiomers: R and S Nomenclature

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

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.

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

(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)

(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

(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

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.

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

clockwise = R counterclockwise = S

5.4 Naming Steroisomeric Alkenes by the E-Z Notational System

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

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

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

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

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

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

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

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