These differences can be very small, as in the case of the boiling point of straight - chain alkenes, such as pent - 2 - ene, which is 37 ° C in the cis isomer and 36 ° C in the trans isomer. The differences between cis and trans isomers can be larger if polar bonds are present, as in the 1, 2 - dichloroethenes. The cis isomer in this case has a boiling point of 60.3 ° C, while the trans isomer has a boiling point of 47.5 ° C. In the cis isomer the two polar C - Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole -- dipole forces (or Keesom forces), which add to the London dispersion forces and raise the boiling point. In the trans isomer on the other hand, this does not occur because the two C − Cl bond moments cancel and the molecule has a net zero dipole (it does however have a non-zero quadrupole).
why is the boiling point of the cis isomers higher
the two polar C - Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole -- dipole forces (or Keesom forces), which add to the London dispersion forces and raise the boiling point