Isomería cis-trans
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En química orgánica, a isomería cis/trans (tamén chamada isomería xeométrica, isomería de configuración, ou isomeríaE/Z) é u tipo de estereoisomería que describe a orientación dos grupos funcionais nunha molécula. En xeral, ditos isómeros presentan dobres enlaces, os cales non poden rotar, pero poden tamén orixinarse en estruturas cíclicas, nas que a rotación dos enlaces está tamén moi restrinxida. Os isómeros cis e trans danse en moléculas orgánicas, pero tamén en complexos de coordinación inorgánicos.
Os termos cis e trans veñen do latín, e cis significa "no mesmo lado" e trans "no lado contrario" ou "a través". O termo "isomería xeométrica" considérase pola IUPAC un sinónimo obsoleto de "isomería cis/trans".[1] Porén, utilízase ás veces como sinónimo de estereoisomería en xeral (por exemplo, a isomería óptica chámase ás veces isomería xeométrica); o termo correcto para a estereoisomería non óptica é diastereoisomería.
En química orgánica
Cando os grupos substituíntes están orientados na mesma dirección, o isómero denomínase cis, e cando os substituíntes están orientados en direccións opostas, o isómero denomínase trans. Un exemplo de isomería cis/trans dun pequeno composto hidrocarbonado dáse no 2-buteno (nas figuras de arriba).
Os compostos alicíclicos poden tamén presentar isomería cis/trans. Como exemplo de isómero xeométrico debido á estrutura do seu anel consideraremos o 1,2-diclorociclohexano:
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| trans-1,2-diclorociclohexano | cis-1,2-diclorociclohexano |
Comparación das propiedades físicas
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 dipole moment.
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| cis-2-pentene | trans-2-pentene |
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| cis-1,2-dichloroethene | trans-1,2-dichloroethene |
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| cis-butenedioic acid (maleic acid) |
trans-butenedioic acid (fumaric acid) |
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| Oleic acid | Elaidic acid |
These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such as 2-pentene, which is 37°C in the cis isomer and 36°C in the trans isomer.[2] 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.[3] 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).
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.
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.
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 in a cis isomer will add up to give an overall molecular dipole. In a trans isomer, the dipoles of the substituents will cancel out [Cómpre referencia] due to their being on opposite site of the molecule. Trans isomers also tend to have lower densities than their cis counterparts.[Cómpre referencia]
As a general trend, trans alkenes tend to have higher melting points and lower solubility in inert solvents, as trans alkenes, in general, are more symmetrical than cis alkenes.[4]
Vicinal coupling constants (3JHH), measured by NMR spectroscopy, are larger for trans– (range: 12–18 Hz; typical: 15 Hz) than for cis– (range: 0–12 Hz; typical: 8 Hz) isomers.[5]
Estabilidade
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 [Cómpre referencia], and trans isomers also have a lower heat of combustion, indicating higher thermochemical stability.[4] 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.[6] This phenomenon is called the cis effect.[7]
Notación E/Z
-1-Bromo-1,2-dichloroethene.svg)
- Artigo principal: E-Z notation.
The cis/trans system for naming isomers is not effective when there are more than two different substituents on a double bond. The E/Z notation should then be used. Z (from the German zusammen) means "together" and corresponds to the term cis; E (from the German entgegen) means "opposite" and corresponds to 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, it is necessary to determine the priority of each substituent. If both the higher-priority substituents are on the same side, the arrangement is Z; if on opposite sides, the arrangement is E.
En química inorgánica
A isomería cis-trans pode tamén presentarse en compostos inorgánicos, principalmente en diazenos e compostos de coordinación.
Diazenos
Os diazenos (e os relacionados difosfenos) poden mostrar isomería cis-trans. Igual ca nos compostos orgánicos, o isómero cis é xeralmente o máis reactivo dos dous, e é o único que pode reducir alquenos e alquinos a alcanos, pero por diferente razón: o isómero trans non pode aliñar os seus hidróxenos de xeito adecuado para reducir o alqueno, pero o isómero cis, pola súa forma diferente, si pode.
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| trans-diazeno | cis-diazeno |
Complexos de coordinación
Nos complexos de coordinación inorgánicos con xeometrías octaédricas ou planas cadradas, hai tamén isómeros cis, nos que os ligandos similares están máis xuntos, e trans, nos que están máis afastados.
Por exemplo, hai dous isómeros planos cadrados de Pt(NH3)2Cl2, como xa explicou Alfred Werner en 1893. O isómero cis, cuxo nome completo é cis-diaminadicloroplatino(II), como demostrou en 1969 Barnett Rosenberg, ten actividade antitumoral, e é agora unha droga quimioterápica coñecida como cisplatino. Polo contrario, o isómero trans (transplatino) non ten actividade anticanceríxena útil. Cada un dos isómeros pode sintetizarse utilizando o efecto trans para controlar cal dos isómeros se produce.
![cis-[Co(NH3)4 Cl2]+.](/https/gl.wikipedia.org/wiki/Ficheiro:Cis-dichlorotetraamminecobalt(III).png) |
![trans-[Co(NH3)4 Cl2]+.](/https/gl.wikipedia.org/wiki/Ficheiro:Trans-dichlorotetraamminecobalt(III).png)
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| cis-[Co(NH3)4 Cl2]+ | trans-[Co(NH3)4 Cl2]+ |
Para os complexos octaédricos de fórmula MX4Y2, existen tamén dous isómeros. (Aquí M é un átomo de metal, e X e Y son dous tipos diferentes de ligandos.) No isómero cis, os dous ligandos Y son adxacentes formando 90°, como se ve nos dous átomos de cloro que se mostran en verde no cis-[Co(NH3)4Cl2]+, á esquerda. No isómero trans da dereita, os dous átomos de Cl están en lados opostos do átomo central de Co.
Un tipo de isomería relacionada nos complexos octaédricos MX3Y3 é a isomería facial-meridional (ou fac/mer), na cal os diferentes tipos de ligandos son cis ou trans.
Pode caracterizarse se un composto carbonilo metálico é fac ou mer utilizando espectroscopía infravermella.
Notas
- ↑ "IUPAC Gold Book - geometric isomerism". Goldbook.iupac.org. 2009-09-07. Consultado o 2010-06-22.
- ↑ "Chemicalland values". Chemicalland21.com. Consultado o 2010-06-22.
- ↑ CRC Handbook of Chemistry and Physics, 60th Edition (1979-80), p.C-298
- ↑ 4,0 4,1 Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. page 111 Jerry March ISBN 0-471-85472-7
- ↑ "Spectroscopic Methods in Organic Chemistry," Dudley H. Williams and Ian Fleming, 4th ed. revised, McGraw-Hill Book Company (UK) Limited, 1989.Table 3.27
- ↑ 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 Abstract
- ↑ Use {{Cita publicación periódica}} no canto deste marcador. Pode indicar a referencia DOI no parámetro
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