Cis isomers have identical side groups on the same side of a double bond, making them more polar and allowing for stronger intermolecular forces. This gives cis isomers lower melting points, higher boiling points, and greater solubility than trans isomers, which have side groups on opposite sides of the double bond, imparting less polarity. NMR spectroscopy can also distinguish cis and trans isomers based on vicinal coupling patterns between hydrogens on adjacent carbons.

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 ( 1H, 13C 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=130oC
Fumaric acid=300oC

6. NMR ( 1H, 13C both)
Cis and trans coupling appear differently on 1H
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