Isomería cis-trans: Diferenzas entre revisións

Usually, ''trans'' isomers are more stable than ''cis'' isomers. This is due partly to their shape; the straighter shape of ''trans'' isomers leads to hydrogen intermolecular forces that make them more stable {{Citation needed|date=February 2008}}, and ''trans'' isomers also have a lower [[heat of combustion]], indicating higher [[thermochemistry|thermochemical]] stability.<ref name="march"/> In the Benson [[heat of formation group additivity]] dataset, ''cis'' isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the ''cis'' isomer is more stable than the ''trans'' isomer.<ref>''The stereochemical consequences of electron delocalization in extended .pi. systems. An interpretation of the cis effect exhibited by 1,2-disubstituted ethylenes'' and related phenomena Richard C. Bingham [[J. Am. Chem. Soc.]]; '''1976'''; 98(2); 535-540 [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1976/98/i02/f-pdf/f_ja00418a036.pdf Abstract]</ref> This phenomenon is called the '''''cis'' effect'''.<ref>{{citeCraig, N. C.; Chen, A.; Suh, K. H.; Klee, S.; Mellau, G. C.; Winnewisser, B. P.; Winnewisser, M. (1997). "Contribution to the Study of the Gauche Effect. The Complete Structure of theAntiRotamer of 1,2-Difluoroethane". Journal of the American Chemical Society 119 (20): 4789. [http://pubs.acs.org/doi|/abs/10.1021/ja963819e}} doi 10.1021/ja963819e]. </ref>