Geometrical Isomerism- Nomenclature of Geometrical Isomers (Cis-Trans, E-Z, Syn-Anti Systems)

Geometrical Isomerism

It is called geometrical isomerism when two groups or atoms around a double bond are arranged differently spatially. In geometrical isomers the double bond and the groups around it is the same structurally, but the arrangements of groups around the double bond differ.

Nomenclature of Geometrical Isomers

Cis – trans
If two different substituents are positioned at either end of the C=C of an alkene, then they may exist as geometric isomers (stereoisomers). There is limited rotation of the double bond due to the pi bond, and therefore they are difficult to interconvert.

Examples

• Trans- and cis-isomers are not possible for terminal alkenes, i.e., those with a C=CH2 unit.
• Neither the cis- nor the trans-isomers of a 1,1-symmetrical disubstituted alkene with a C=CR2 unit can exist.
• There are cis- and trans-isomers of alkenes with the R1-CH=CH-R2 unit.

The following alkene unit shows that the cis-and trans-isomers can exist only if R1 and R3 are not equal.

E-Z

It is possible to name such a structure using the Cahn - Ingold - Prelog priority sequence. On both double-bonded carbon atoms, the group with the highest priority is selected first. A double bond will be formed if both groups of highest priority are on either side. When the groups of highest priority are on the same side of the structure, our alphabet is E (from the German, Enttaggen, meaning opposite) and Z (from Zussamen meaning together), if they are on the opposite side of the structure. E represents the opposite side and Z represents the same side. As an example,

Cis isomers are generally described as Z forms while trans isomers are described as E forms, but due to sequence rules, there are some exceptions. Dichloro-1,2,1-bromoethene, for instance

A CI atom is present on the opposite side of the chain, but it has the Z form according to sequence rule.

Determine the geometrical isomer configurations

• A trans-isomer generally has a higher molecular weight and a lower boiling point than a cis isomer.
• The cis isomer is more soluble. Maleic acid, for example, is soluble in water at a solubility of 3.0g/100 mL at 293 K, and fumaric acid is soluble in water at a solubility of 0.7g in 100 mL at 293 K.
• Trans isomers have zero dipole moment or low dipole moment compared to their cis counterparts since they are symmetrical.

µ = 0

Syn-anti Systems

Syn and anti are no longer encouraged to describe double bonds in the geometry, but you can still find older articles that use them. As Z(usammen) and E(ntgegen) are the same, syn and anti represent carbon-nitrogen double bonds. It is in these situations that the lone pair of electrons has the lowest priority, and the usual sequence rule is applied. As an example, in the following N-methyl imine, the imine on the left is attached to the carbon in the double bond through a group of highest priority (ethyl) while the nitrogen in the double bond is connected to the lone pair by a higher priority group than the lone pair (methyl). The synth isomer (which would be called the Z isomer now) is therefore the molecule.

The molecule on the right is the antithesis of the molecule on the left. The important point is that an individual atom has a lower priority than a lone pair.

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