View source for Cis-trans isomerism

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[[Image:Cis-2-butene.PNG|right|frame|Cis-2-butene]][[Image:Trans-2-butene.PNG|right|frame|Trans-2-butene]]

==Overview==

In [[chemistry]], '''cis-trans isomerism''' or '''geometric isomerism''' is a form of [[stereoisomerism]] describing the orientation of [[functional group]]s within a molecule. Such isomers typically contain [[double bond]]s which cannot rotate, but they can also arise from ring structures where the rotation of bonds is greatly restricted. 

The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by [[IUPAC]]. It is sometimes used as a synonym for general stereoisomerism (e.g. [[optical isomerism]] being called geometric isomerism); the correct term for non-optical stereoisomerism is [[diastereomer|diastereomerism]].

There are two forms of a cis-trans isomer, the ''[[cis]]'' and ''[[trans]]'' versions. When the [[substituent|substituent groups]] are oriented in the same direction the diastereomer is referred to as ''cis'',  while when the substituents are oriented in opposing directions the diastereomer is referred to as ''trans''. An example of a small hydrocarbon displaying cis-trans isomerism is [[2-butene]]. 

[[Alicyclic compound]]s can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

{| class="wikitable" style="margin: 1em auto 1em auto"
|[[Image:Trans-1,2-dichlorocyclohexane-2D-skeletal.png|100px]] [[Image:Trans-1,2-dichlorocyclohexane-3D-balls.png|150px]] || [[Image:Cis-1,2-dichlorocyclohexane-2D-skeletal.png|100px]] [[Image:Cis-1,2-dichlorocyclohexane-3D-balls.png|150px]]
|- align="center"
| ''trans''-1,2-dichlorocyclohexane || ''cis''-1,2-dichlorocyclohexane
|}

==Cis vs trans physical properties== 
'''Cis isomers''' and '''trans isomers''' often have different physical properties. Differences between isomers generally arise from the differences in the shape of the molecule or the overall [[Bond dipole moment|dipole moment]]. These differences can be very small as in the case of the boiling point of straight chained alkenes, such as 2-pentene where the trans isomer has a boiling point of 36°C and cis isomer is 37°C <ref>[http://www.chemicalland21.com/info/Alkene%20Compound%20Boiling%20Points.htm Chemicalland values]</ref>. The differences between cis and trans isomers can also be quite large as in the case of [[cyclooctene]]. The cis isomer in this case has a boiling point of 
145°C <ref>[http://search.be.acros.com/_KFPNKUMTNBQNKQMVNMPOLWNM_?for=acros&from=3501&show_rn=false&show_den=true&show_mp=true&show_bp=true&bl=25&found=27028&border=yes&bgcolor=%23ffffff&bgcolor_td=%23ffeebb&bgcolor_th=%23ffcc66&bgcolor_th1=%23ffcc66&face_font=arial&nlink=%23000000&color_font=%23000000&alink=%23EB0000&vlink=%2366bbFF&STYLE=&submitUrlSearch=&bgcolor_th3=%23ffcc66&show_pict=&show_fw=&show_hr= Akros data]</ref>, while the trans isomer has a boiling point of 75°C.<ref>[[Organic Syntheses]], Coll. Vol. 5, p.315 (1973); Vol. 49, p.39 (1969)
[http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV5P0315 Link]</ref> The large difference between the two isomers of cyclooctene arise from the large ring strain for trans-cyclooctene, which also makes it significantly less stable than the cis isomer. In fact, the two isomers of 2-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 is named [[fumaric acid]]. 

In the case of geometric isomers that are a consequence of double bonds and in particular when both substituents are the same some general trends usually hold. These trends can be attributed to the fact that the dipoles of the substituents will add when together for a cis isomer to give an overall molecular dipole while for trans isomer the dipoles of the substituents will cancel out. Trans isomers also tend to have lower densities than their cis counterparts.

March <ref>''Advanced organic Chemistry, Reactions, mechanisms and structure'' 3ed. page 111 Jerry March ISBN 0-471-85472-7</ref> observes that as trans alkenes in general have more symmetry that cis alkenes, the trans alkenes also tend to have higher [[melting point]]s and lower [[solubility]] in inert solvents. 

[[Vicinal (chemistry)|Vicinal]] [[J-coupling|coupling constant]]s (<sup>3</sup>J<sub>HH</sub>), measured by [[NMR spectroscopy]], are larger for <i>trans</i>- (range: 12-18 Hz, typical: 15 Hz) than for <i>cis</i>- (range: 0-12 Hz, typical: 8 Hz) isomers.<ref>"Spectroscopic Methods in Organic Chemistry," Dudly H. WIlliams and Ian FLeming, 4th ed. revised,McGraw-Hill Book COmpany (UK) Limited, 1989.Table 3.27</ref>

===Stability===
According to March trans isomers also have a lower [[heat of combustion]], indicating higher [[thermochemistry|thermochemical]] stability. In the Benson [[Heat of formation group additivity]] dataset cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist. For instance for 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF) and several other halogen and oxygen substituted ethylenes 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>''Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane'' Norman C. Craig et al. J. Am. Chem. Soc.; '''1997'''; 119 p 4789 {{DOI|10.1021/ja963819e}}</ref>

==E/Z notation==
[[Image:(Z)-1-bromo-1,2-dicholroethene.gif|right|frame|Bromine has a higher [[Cahn Ingold Prelog priority rules|CIP priority]] than chlorine, so this alkene is the ''Z'' isomer.]]
The cis/trans system for naming isomers breaks down when there are more than two different substituents on a double bond. The E/Z notation should then be used. '''Z''' (from the German {{lang|de|''zusammen''}}) means ''together'' and corresponds to the term ''cis''; '''E''' (from the German {{lang|de|''entgegen''}}) means ''opposite'' and corresponds to the term ''trans''.

Whether a molecular configuration is designated E or Z is determined by the [[Cahn Ingold Prelog priority rules]] (higher atomic numbers are given higher priority). For each of the two atoms in the double bond, individually determine which of the two substituents is of a higher priority. If both of the substituents of higher priority are on the same side, the arrangement is ''Z''; if they are on opposite sides the arrangement is ''E''.

==External links==
*[http://www.iupac.org/goldbook/S05983.pdf The IUPAC definition of "stereoisomerism"]
*[http://www.iupac.org/goldbook/G02620.pdf The IUPAC definition of "geometric isomerism"]
*[http://www.iupac.org/goldbook/C01093.pdf The IUPAC definition of "cis-trans isomers"]

==See also==
*[[List of publications in chemistry#Organic chemistry|Important publications in organic chemistry]]

==References==
{{Reflist|2}}
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[[Category:Stereochemistry]]

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[[pl:Izomeria geometryczna]]
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