La isomería geométrica es un caso particular de la estereoisomería y es común en alquenos y cicloalcanos; la cual estudia la manera en que los átomos se encuentran orientados en el espacio. Generalmente a los pares de isómeros geométricos se les da el nombre de isómeros cis y trans. Si se adicionan a un doble enlace carbono-carbono cuatro grupos diferentes, el isómero cis es aquel en que los dos grupos de cadena más larga están en el mismo lado del doble enlace. Por su parte, los isómeros trans son aquellos que lados diferentes del doble enlace.
Por ejemplo tenemos el hept-3-eno
CH3-CH2–CH=CH-CH2-CH2-CH3
Los isómeros cis y trans serían los siguientes:
Estos isoméros poseen la misma fórmula, sin embargo tienen diferentes propiedades físicas (puntos de fusión, puntos de ebullición, solubilidad, densidad, índice de refracción, polaridad, entre otros.) y algunas de sus propiedades químicas también cambian.
Nomenclatura E-Z
Este tipo de nomenclatura surge debido a que existe compuestos que poseen 3 o más sustituyentes diferentes y por tal razón la nomenclatura cis-trans resulta ineficiente. Este sistema fue creado para la IUPAC ya que con él se pueden nombrar todos los alquenos que poseen isómeros geométricos.
Las letras E-Z vienen del alemán, donde E es entgegen que en español es «separados» y la letra Z viene de zusammen que significa «juntos».
Este tipo de sistema se rige bajo las reglas de prioridad de Cahn, Ingold y Prelog, las cuales son las siguientes:
Primero se debe observar cada carbono; el número 1 será el sustituyente que tenga mayor número atómico y el restante será el 2. En caso de que ambos tengan la misma masa molar, se realiza el mismo procedimiento con los otros átomos unidos a ellos.
Entonces, el isómero E es aquel que tendrá los dos sustituyentes con mayor categoría a diferentes lados o separados y el isómero Z es aquel que se le asignó el número uno y por lo tanto posee los sustituyentes juntos o del mismo lado.
Por ejemplo:
El 4-metil-5etil-non-4-eno
Si observamos los 4 sustituyentes que posee los carbonos 1 y 2 (C=C) son diferentes. Tenemos el propil, el metil, etil y el butil. Los que poseen prioridad en este caso, son aquellos que tienen mayor masa molar, o sea el propil y el butil. Entonces estos son los que me determinan el isómero Z (juntos) y el isómero E (opuestos).
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