E-Z & Syn-Anti Isomerism | Organic Chemistry

E-Z & Syn-Anti Isomerism

Stereoisomerism Beyond Cis-Trans | Organic Chemistry

1. Limitations of Cis-Trans Notation

The Cis-Trans system works well when there are two identical groups on double-bonded carbons (e.g., $CH_3-CH=CH-CH_3$). However, if all four groups attached to the double bond are different (e.g., $F(Cl)C=C(Br)I$), Cis-Trans notation fails. Here, we use the E-Z System.

2. The CIP Priority Rules

Proposed by Cahn, Ingold, and Prelog to assign priorities to groups attached to the stereocenter.

                    Rule 1 (Atomic Number): Higher atomic number gets higher priority.
                    
$I > Br > Cl > F > O > N > C > H$

                    Rule 2 (Isotopes): If atomic numbers are same, higher atomic mass gets priority.
                    
$T ( ^3H ) > D ( ^2H ) > H ( ^1H )$

                    Rule 3 (Next Atom): If the first atom is identical, proceed to the next attached atoms. Look for the first point of difference.
                    
$-CH_2Cl > -CH_2OH > -CH_3$

                    Rule 4 (Multiple Bonds): Treat multiple bonds as multiple single bonds to phantom atoms.
                    
$-C=O$ is treated as C bonded to two O's.
                    
Priority Order: $-COOH > -CHO > -CH_2OH$

3. The E-Z System

Determine the high-priority group on each carbon of the double bond separately.

Z (Zusammen = Together): If the High Priority (HP) groups are on the same side.
E (Entgegen = Opposite): If the High Priority (HP) groups are on opposite sides.

4. Syn-Anti Isomerism

This notation is commonly used for Nitrogen-containing compounds like Oximes ($C=N-OH$) and Azo compounds ($-N=N-$).

A. Aldoximes ($R-CH=N-OH$)

Syn: $-H$ and $-OH$ are on the same side.
Anti: $-H$ and $-OH$ are on opposite sides.

B. Ketoximes ($R_1-C(R_2)=N-OH$)

The name is written with respect to the alkyl group.
Example: If $-OH$ is on the same side as Methyl, it is "Syn-methyl...". If opposite, "Anti-methyl...".

C. Azo Compounds ($R-N=N-R$)

Syn: Groups are on the same side (cis-like).
Anti: Groups are on opposite sides (trans-like).

Practice Quiz

Test your ability to assign configurations.

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