<Tr> <Td> Oleic acid </Td> <Td> Elaidic acid </Td> </Tr> <P> 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). </P> <P> The two isomers of butenedioic acid have such large differences in properties and reactivities that they were actually given completely different names . The cis isomer is called maleic acid and the trans isomer fumaric acid . Polarity is key in determining relative boiling point as it causes increased intermolecular forces, thereby raising the boiling point . In the same manner, symmetry is key in determining relative melting point as it allows for better packing in the solid state, even if it does not alter the polarity of the molecule . One example of this is the relationship between oleic acid and elaidic acid; oleic acid, the cis isomer, has a melting point of 13.4 ° C, making it a liquid at room temperature, while the trans isomer, elaidic acid, has the much higher melting point of 43 ° C, due to the straighter trans isomer being able to pack more tightly, and is solid at room temperature . </P> <P> Thus, trans alkenes, which are less polar and more symmetrical, have lower boiling points and higher melting points, and cis alkenes, which are generally more polar and less symmetrical, have higher boiling points and lower melting points . </P>

Identify the structure that cis-trans isomerism can occur in