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Methods of Determination of Configuration of Geometrical Isomers

Melting point, Solubility, Dipole moment, Action of heat, E & Z notation for geometrical isomers

Melting Point

Melting points of trans isomers are generally higher than those of normal cis isomers. This is due to the fact that trans isomers have molecules that are symmetrical, thus fitting into the crystal lattice closer than cis isomers.

It is crucial that molecules pack together tightly in solids so that intermolecular forces operate effectively. Compared to the cis isomer, the trans isomer packs better. Cis isomers do not pack as well as trans isomers, since they have the shape of a U. In cis isomers, the packing is poorer, so the intermolecular forces are ineffective. As a consequence, there is less energy required to melt the molecule.




Solubility

Trans isomers are usually less soluble than their trans isomeric counterparts. Because cis isomers have molecules that are less tightly bound, the crystal lattice of a cis isomer is less rigid.



Dipole moment

Trans isomers have a smaller dipole moment than cis isomers.



Solubility

Due to steric hindrance, trans isomers have a greater stability than their cis counterparts. The close proximity of reacting groups facilitates intermolecular reactions. In comparison to its trans counterpart, the cis isomer forms more cyclic derivatives. This reaction can only occur in cis isomers whose substituents on two double-bonded carbons can react intramolecularly.



Action of heat

It is possible to convert cis and trans isomers under strong heating. Here is how the reaction proceeds:



E & Z notation for geometrical isomers

  • Geometrical isomers cannot be identified by merely using cis/trans descriptors A double bond may have more than two substituents. For stereochemical differentiation A new nomenclature to be used in them is the E & Z notation method.
  • As a result of this approach, when groups with higher priorities are positioned opposite one another E is the symbol for the isomer with the double bond. Throughout this sentence, E stands for Entgegen (the German word for "enemy" or "opposite").
  • On the other hand, if the group of higher priorities is on the same side of the double bond, then Z is used to represent that isomer. In German, zusammen means together and Z denotes it.
  • To distinguish the E and Z from the rest of the names, the letters are enclosed in parentheses.
  • The E and Z descriptors are used to label the geometrical isomers based on the E and Z configuration: As a result, the Z and E configurations are determined.
  • Assign a priority group to the end of the double bond with the higher priority.
  • In this case, E is an isomer represented by the groups that are higher in priority and opposite the double bond.
  • A double bond must be described by the Z descriptor if there are opposite sides.
Cahn-Ingold-Prelog rules are used to assign priorities:

Rule 1
A number is assigned to each atom that is directly attached to the olefinic carbon. Atoms with atomic numbers greater than 10 have the highest priority.



Rule 2
If two isotopes of the same element are found, the isotope with the larger atomic mass is given a higher priority. So, the Deuterium isotope (H2 or D) is considered more important than the Protium isotope (H1 or H). C13 is considered more important than C12.




Rule 3
When applying to the next atom or sequence of atoms in a group X is not possible, the relative priority of the group will be decided using the next atom or sequence of atoms in the group X. The longer the hydrocarbon carbon chain, the higher the priority of the grouping, such as in organic molecules with X more than one atom, e.g., -CH2CH2CH2CH3 > -CH3 > -CH3 > -H, where longer chains of hydrocarbon carbon are more important,



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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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