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=== Comparación das propiedades físicas ===
Os isómeros ''cis'' e ''trans'' a miúdo teñen diferentes propiedades físicas. As diferenzas entre isómeros, en xeral, orixínanse polas diferenzas de forma das moléculas ou do [[momento dipolar]] global.
''Cis'' and ''trans'' isomers often have different physical properties. Differences between isomers, in general, arise from the differences in the shape of the molecule or the overall [[Bond dipole moment|dipole moment]].
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TheseEstas differencesdiferenzas canpoden beser verymoi smallpequenas, ascomo inno thecaso casedo of[[punto thede boilingebulición]] pointdos ofalquenos straight-chainde alkenescadea liñal, suchcomo aso [[PentenePenteno|2-pentenepenteno]], whichque é isde 37°C inpara theo isómero ''cis'' isomere andde 36°C inno theisómero ''trans'' isomer.<ref>{{cite web|url=http://www.chemicalland21.com/info/Alkene%20Compound%20Boiling%20Points.htm |title=Chemicalland values |publisher=Chemicalland21.com |date= |accessdate=2010-06-22}}</ref> As Thediferenzas differencesentre betweenos isómeors ''cis'' ande ''trans'' isomerspoden canser bemaiores largerse ifestán polarpresentes bondsenlaces are presentpolares, ascomo in thenos [[1,2-dichloroethenedicloroeteno]]s. TheO isómero ''cis'' isomerneste incaso thisten caseun haspunto ade boilingebulición point ofde 60.,3°C, whilementres theque o isómero ''trans'' isomerteno has a boiling point ofde 47.5°C.<ref>CRC Handbook of Chemistry and Physics, 60th Edition (1979-80), p.C-298</ref> InNo theisómero ''cis'' isomeros thedous two[[momento polardipolar|momentos dipolares]] dos C-Cl [[bondpolares dipole moment]]scombinanse combinepara todar giveun anmomemnto overalldipolar molecularconxunto dipolede, so that there are intermolecular [[intermolecular forces#Dipole–dipole interactions|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).
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 degrees Celsius, making it a liquid at room temperature, while the ''trans'' isomer, elaidic acid, has the much higher melting point of 43 degrees Celsius, due to the straighter ''trans'' isomer being able to pack more tightly, and is solid at room temperature.
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