The methods of determination of configuration of geometrical isomerism are classified as:
(I) Chemical methods and (II) Physical methods
These methods include:
(a) Absolute method: This method is based on the following observations.
(i) Functional groups in a cyclic compound located cis to each other can be converted into cyclic lactones, anhydrides, or amides. e.g., maleic acid containing two –COOH groups cis to each other forms anhydride easily. Hence, it can be identified as cis-(maleic acid). Similarly in fumaric acid the two –COOH groups are on the opposite side, it can not form anhydride easily. Hence, it can be identified as trans-form.
(ii) Cis-isomers can be synthesized from the small rings but trans isomers can not be synthesized from small rings.
(b) Through chemical reaction not affecting the configuration of the double bond: The synthesis of trisubstituted alkene of known configuration is possible by syn addition of organo-copper reagent to alkyne followed by alkylation.
(c) If we synthesize a product from a starting material of known configuration, then the configuration of the product remains the same as that of starting material.
(d) The stereoselective reaction helps to predict the configuration of the resulting product. One such stereoselective reaction is the Wittig reaction.
The geometrical isomers differ from each other in their physical properties which include:
(a) Boiling point, melting point, density, refractive index, and dipole moment
(b) Acid strength
(c) UV-visible spectra
(d) Vibrational (IR-Raman) spectra
(e) NMR (1H, 13C both)
(f) X-ray, microwave spectra, and electron diffraction methods.
(a) The parameters like boiling point, melting point, density, and refractive index are not very reliable for the prediction of the configuration of the isomers. Dipole moment is variable for cis and trans isomer, sometimes higher for trans and at times for cis isomer. Similarly, the trans isomer has greater symmetry than the cis. Therefore, trans has usually a higher melting point. e.g.,
(b) Acid Strength: The acid strength is strongly dependent on the configuration of the compound e.g. pKa of cis and trans isomers of crotonic acid are.
The cis form (maleic acid) is more acidic in its first dissociation than transform (fumaric acid) but the acidity of the second proton is reversed. This is because of intramolecular H-bonding formed within the conjugate anion of maleic acid. It is stabilized to a greater extent than fumaric after the first dissociation of the proton. In the second dissociation, in cis-two negative anion species close to each other is not favorable as in fumaric, the transform (the negative species further away). Hence, the transform is more acidic than cis in the second dissociation.
(c) UV-visible Spectra: Cis isomer has two bulky groups on the same side. Hence, internally the molecule is extremely crowded and thus has less resonance energy and is less stable than trans isomer. The cis-isomer suffers distortion and is forced to be non-coplanar and thus has absorption maxima at a slightly shorter wavelength than the trans isomer.
(d) Infrared and Raman Spectra: The difference in the IR spectra of the two isomers may be pointed out in the following regions.
1650 cm-1 (C = C), and
970 – 690 cm-1 (= C – H out of plane vibration).
Similar for trans 1,2-dichloroethylene, the dipole moment is zero, due to its symmetrical nature.
Cis-isomer shows no IR absorption but shows Raman absorption at 1577 cm-1. While trans-isomer shows strong IR absorption at 1590 cm-1 but shows no Raman absorption.
(e) NMR Spectra: Not only it gives you information regarding which functional groups are present, but NMR spectra are also capable of giving information about the position and configuration of atoms (environment) in the molecule. NMR spectra can differentiate chemically unlike protons. In disubstituted ethylene, RHC = CHR’, where R and R’ differ significantly in the way they influence the magnetic environment of the olefinic protons, thereby these protons experience a resonance condition at different field strengths. These olefinic protons are typically found in the low field of the NMR spectrum and the hydrogens are said to be deshielded.
Trans isomer is strongly coupled and hence has a coupling constant of 17-18 c.p.s. (cycles per second). While the coupling constant of cis-isomer ranges from 8-11 c.p.s. Similarly the difference in chemical shifts of cis- and trans- isomers may be used to identify the configuration of the isomer.
(f) X-ray and Electron Diffraction: Single-crystal X-ray diffraction is the most powerful tool for detailed structural characterization of crystalline compounds. It reveals the spatial atomic arrangement providing an image of the internal structure of the crystal. Single crystal X-ray diffraction is the main source of information on the geometrical structure of the molecules including bond distances, bond angles, conformations of flexible molecules as well as intermolecular contacts.
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