Following nomenclature system are used for naming of geometrical isomers:
- cis and trans nomenclature,
- E and Z nomenclature, and
- Syn and Anti nomenclature.
cis and trans Nomenclature
In a molecule having a carbon-carbon double bond, with four groups or atoms directly attached to the two carbon atoms, where all atoms are present in the same plane thus their rotation around the double bond is prevented.
This phenomenon of restricted rotation around the double bond gives rise to stereoisomerism in molecule (such as abc=cxy, when a is not equal to x, and b is not equal to y).
In the below mentioned example two isomers are present, A and B. In such type of compounds when a is equal to x, A is known as cis-isomer and when a is not equal to x, B is known as trans-isomer.
Other examples of cis-trans isomerism are:
E and Z Nomenclature
The compounds in which all the four substituents attached to the carbon atoms of double bond are different and do not obey the cis-trans nomenclature, are designated by E-Z nomenclature.
In the E-Z nomenclature system, E is derived from a German word entgegen meaning opposite and Z from a German word Zusammen meaning together. It is a newer method, based on the Cahn-Ingold Prelog (CIP) system and can be applied to all cases.
The rules for E-Z nomenclature system are as follows:
In this system, the two groups on each carbon atom are assigned as priority number (1) and (2) on the basis of following rules:
Rule 1: The atom with higher atomic number is given the higher priority, it is assigned as 1 and the other one as 2 . If, the two atoms attached to the double bond are isotopes, then the higher priority is given to the isotope with higher mass number.
Rule 2: If, the two atoms attached to the double bond have same atomic number, then priority is decided by comparing the next elements in the group and so on.
Rule 3: An atom with double or triple bond is equivalent to two or three such atoms. Thus, carbonyl group is considered as a carbon with two single bonds to oxygen.
According to these rules, the following priority sequence is assigned to some of the substituents:
(I, Br, Cl, F, O, N, C, H) (-C(CH3)3, -CH(CH3)2, -CH2CH3, -CH3) (-COOCH3, -COOH, -CONH2 -CONH3, -CHO).
On the basis of relative priorities and position of groups/atoms, the E-Z designation can be assigned as:
The atoms/groups with higher priority (denoted by 1) on each double bonded carbon is selected. If such atoms/groups are on the same side of the double bond, the isomer is assigned Z configuration. While, if atoms/groups of higher priority are on the opposite sides of the double bond, the isomer is assigned E configuration.
For example, consider the following compound:
In this, one of the double bonded carbon atoms has CH3 and C6H5 groups. In C6H5 group, the first carbon is attached to two other carbons (by a double bond to one carbon atom and by a single bond to the other). As a result, the C, C, CH of phenyl is given higher priority over the H, H, and H of CH3. Thus, priority (1) is assigned to C6H5 and priority (2) is assigned to CH3.
The other double bonded carbon atom has CH2OH and CHO groups. In CH=O group, C and O are attached by a double bond, but in CH2OH group, single bond is present between C and O. Due to this reason, the O, and H of CHO is given higher priority over the O, H, and H of CH2OH group. Thus, priority (1) is assigned to CHO over CH2OH which assigned as (2).
Other examples of E-Z isomers are:
Syn and Anti Nomenclature
The compounds which have C=N and N=N double bonds show the geometrical isomerism.
For example,
In aldoximes, the isomer is named as syn when H atom and OH group are on same side, and anti when they are on opposite sides.
In ketoximes, prefixes syn- and anti represent which group of ketoxime is syn or anti to the hydroxyl group.
Following are the compounds having N= N bond.
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