Steriochemistry of organic compounds geometrical isomers : methods to distinguish cis and trans

methods to distinguish cis and trans

1. Stereochemistry of organic compounds Geometrical isomerism : Distinguishing geometrical isomers

2. Difference between cis and trans isomers Difference Between Between Cis and Trans Isomers Cis Trans Cis isomers are molecules with the same connectivity of atoms. They further consist of identical side groups usually on the same side. A trans isomer has molecules with two same atoms but in the opposite side of the double bond. It is most of the time a polar molecule. It is nonpolar or less polar molecule. Due to loosely packed molecules the melting point of cis isomer is comparatively low. Due to tightly packed molecules the melting point of trans isomer is high. Likewise, due to the presence of strong attraction forces between atoms cis isomers have a high boiling point. The boiling point is comparatively low for trans isomer as there are no strong attraction forces. Cis isomers have highly soluble in inert solvents. They are less soluble in inert solvents. These type of isomers have higher acidic strength. In trans isomers, the protons are not easily released. So it has less acidic strength.

3. Auwers-Skita Rule  the rule stated that in  cis -trans isomeric compounds , the  cis  had the higher density and refractive index and the lower molecular refractivity than trans

4. Physical methods for the determination of cis -trans isomers NMR (  1 H,  13 C  both) Vibrational (IR-Raman) spectra UV- visible spectra: not much reliable Refractive index Density Boiling points Dipole moments Less often mass spectra, X-ray , electron diffraction method, microwave spectra

5. Refractive index, Density, Boiling points, Dipole moments Property Cis Trans Eg : Dipole moment Mostly high Low or zero Trans isomer dpm =0 Polarity Polar Less polar or nonpolar Boiling point (most of the cases) High (due to intermolecular forces) Lower Melting point Low (due to loose packing) Higher Maleic acid=130 o C Fumaric acid= 300 o C

6. NMR (  1 H,  13 C  both) Cis and trans coupling appear differently on  1 H NMR spectrum Vicinal  - Coupling between hydrogens on adjacent carbons.  Trans: strongly coupled Cis : weakly coupled

7. Vibrational (IR-Raman) spectra no IR absorption for C=C (dipole moment zero and no change in μ during vibration— so no IR) strong Raman absorption Strong IR absorption corresponding to C=C

8. Chemical methods for the determination of cis -trans isomers Maleic anhydride Heat or isomerization