Name: purifying crystals by solubility
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 3 min 12 s
Channel: JoVE

La mayoría de los productos contienen impurezas. Una forma de purificar estos productos es mediante la recristalización. La recristalización comienza con la disolución del producto impuro en el disolvente caliente suficiente para formar una solución saturada, en la que se disuelve la mayor cantidad posible de soluto en el disolvente. Cualquier soluto adicional no parecerá disolverse.
Se utiliza un disolvente caliente porque la solubilidad suele aumentar con la temperatura. A medida que aumenta la temperatura, aumenta la cantidad de soluto que se puede disolver en el solvente. A medida que la solución se enfría, la solubilidad del producto disminuye y las moléculas de soluto se unen para formar pequeños cristales estables llamados núcleos.
Este es el primer paso de la cristalización, llamado nucleación. El crecimiento adicional de cristales ocurre en los núcleos porque las moléculas de soluto tienen una mayor afinidad para unirse a los cristales de soluto existentes que para formar nuevos cristales. Las impurezas solubles se dejan en solución.
La cristalización puede ocurrir espontáneamente o puede fomentarse rascando el interior del matraz, agitando la solución o agregando un cristal semilla del compuesto, todo lo cual proporciona una superficie para un mayor crecimiento.
La recristalización en cristales grandes, puros y de forma regular solo funciona cuando se utiliza un disolvente adecuado. El compuesto debe ser insoluble en el disolvente a temperatura ambiente y soluble a altas temperaturas. Idealmente, las impurezas deben ser insolubles en el solvente a alta temperatura o solubles en el solvente a temperatura ambiente.
Si las impurezas son insolubles en disolvente caliente, se filtran antes de la cristalización. Después de la recristalización, los cristales se filtran y se lavan con disolvente frío para eliminar las impurezas de las superficies. Ahora, se puede analizar la pureza de los cristales.
Al elegir un solvente, tenga en cuenta que cuanto mayor sea la diferencia de solubilidad entre altas y bajas temperaturas, más probable será que el soluto se salga de la solución a medida que se enfría para formar cristales. La velocidad de enfriamiento también es importante en la recristalización.
El enfriamiento rápido favorece la formación de muchos sitios de nucleación y el crecimiento de cristales más pequeños, mientras que el enfriamiento lento induce la formación de menos sitios de nucleación y el crecimiento de menos cristales, más grandes pero más puros. Por lo tanto, se prefiere un enfriamiento más lento.
En este laboratorio, recristalizará dos compuestos orgánicos impuros, acetanilida y ácido transcinámico, y luego evaluará la pureza de los compuestos recuperados comparando su rango de punto de fusión con los valores de la literatura.

recristalización

recristalización
A menudo, el producto deseado de una reacción química es parte de una mezcla de reacción más compleja, que puede estar compuesta por el solvente, los materiales de partida y las impurezas. Aprender a purificar correctamente los compuestos orgánicos es una técnica valiosa en química orgánica. La recristalización aprovecha las diferencias en la solubilidad del compuesto deseado y la impureza en el solvente para purificar el producto deseado como un sólido. Existen tres métodos estándar de purificación: destilación, extracción y recristalización.
solubilidad
La solubilidad de una sustancia es la cantidad máxima que se disuelve en un volumen fijo de un disolvente determinado a una temperatura determinada. Los diferentes solutos tienen diferentes solubilidades y se disuelven en diferentes solventes. Los solutos pueden tener características definitorias que se prestan para ser explotadas para la recristalización. Los compuestos exhiben uno de los siguientes comportamientos en un solvente. En primer lugar, el compuesto puede ser insoluble o tener una solubilidad muy baja en el disolvente a todas las temperaturas. En segundo lugar, el compuesto puede ser soluble en el disolvente a temperaturas más altas. En tercer lugar, el compuesto puede ser soluble en el disolvente a todas las temperaturas.
Un factor importante para determinar si un soluto se disuelve en un solvente y forma una solución es la fuerza y el tipo de fuerzas intermoleculares entre el soluto y el solvente. La regla general es "lo similar se disuelve lo similar", lo que significa que las sustancias con tipos similares de fuerzas intermoleculares se disuelven entre sí. Por ejemplo, las sustancias polares, como la sal de mesa (NaCl), se disuelven bien en el agua polar.
Otro factor clave que mejora la solubilidad es la temperatura. Para muchas sustancias, la solubilidad aumenta considerablemente a temperaturas más altas. Esto se debe al hecho de que el aumento de la energía cinética a temperaturas más altas rompe las fuerzas intermoleculares de soluto que mantienen unidas a las moléculas. Esto se ve en la vida cotidiana. Por ejemplo, sabemos que la sal de mesa (NaCl) se disuelve bien en agua; sin embargo, se disuelve más a temperaturas más altas que a temperaturas más bajas.
Cualitativamente, una solución se considera insaturada si aún no se ha alcanzado la cantidad máxima de soluto disuelto. Cuando el máximo soluto posible se ha disuelto, la solución se satura. Una solución sobresaturada contiene más soluto disuelto que la cantidad máxima posible en condiciones típicas.
<h4>recristalización </h4>
La recristalización aprovecha las diferencias de solubilidad entre el producto deseado y los contaminantes a altas temperaturas. El primer paso de la recristalización es disolver la mezcla del producto en un volumen mínimo de solvente calentado que aún da como resultado una solución saturada, pero no sobresaturada. Luego, la solución se enfría a temperatura ambiente, disminuyendo la solubilidad tanto del compuesto deseado como de la impureza.
A medida que la solución se enfría, comienza la cristalización del componente puro, mientras que las impurezas aún solubles no lo hacen. Esto ocurre cuando el componente de interés se encuentra en una concentración significativamente mayor que la impureza. Primero, en la fase de nucleación, el solvente inicia la aglomeración aleatoria de las moléculas de soluto, formando el primer cristal llamado semilla o núcleo. A continuación, en la fase de crecimiento o cristalización de las partículas, se añaden más moléculas a la semilla, formando un cristal. El cristal contiene el compuesto puro, mientras que la impureza permanece en el disolvente.
La nucleación procede más rápido que el crecimiento de partículas en una solución sobresaturada. Con más semillas, cada cristal es más pequeño. Por lo tanto, si la solución está saturada, en lugar de supersaturada, se forman menos semillas, lo que da como resultado cristales más grandes. Calentar la solución a una temperatura más alta antes de enfriarla a temperatura ambiente permite la disolución de una mayor concentración de soluto, disminuyendo la sobresaturación. Además, el enfriamiento rápido da como resultado una nucleación rápida, formando muchos cristales pequeños y atrapando la impureza dentro de los cristales. Se prefiere el enfriamiento lento para lograr menos cristales más grandes.
Una vez que la solución se ha enfriado a temperatura ambiente y se han formado los cristales, la solución se filtra mediante filtración al vacío. Luego, los cristales se dejan secar. El porcentaje de recuperación se calcula dividiendo la masa del producto recuperado por la masa del producto bruto.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
La recuperación rara vez es del 100%, ya que la solubilidad del compuesto a bajas temperaturas determina la cantidad de compuesto que se cristaliza.
<h4>Selección de un disolvente</h4>
Para que la cristalización sea eficaz, se debe utilizar el disolvente óptimo. El producto deseado debe tener una baja solubilidad en el disolvente seleccionado a temperatura ambiente, pero una alta solubilidad en el disolvente a una temperatura más alta. Lo ideal es que las impurezas sean solubles en el disolvente a todas las temperaturas. Por lo tanto, cuando la mezcla se agrega al solvente a alta temperatura, el producto deseado y las impurezas se disuelven fácilmente.
A medida que la solución se enfría, la solubilidad del producto deseado disminuye y comienza a producirse la cristalización, formando un producto purificado. Ocasionalmente, las impurezas pueden permanecer insolubles en el solvente de elección, incluso a altas temperaturas. La filtración por gravedad caliente de la solución que contiene el producto disuelto puede eliminar las impurezas sólidas. A continuación, el producto puede recristalizarse enfriando la muestra.
Lo ideal es que el disolvente que se vaya a utilizar pueda hervir a una temperatura muy por debajo del punto de fusión del producto deseado. El disolvente también debe ser inerte y no reaccionar con el producto purificado deseado.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análisis químico cuantitativo. Nueva York, NY: W.H. Freeman and Company.</li>
</ol>

Recristalización

Recristalización
A menudo, el producto deseado de una reacción química forma parte de una mezcla de reacción más compleja, que puede estar compuesta por el disolvente, ...

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Recrystallization

purifying crystals by solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction.

Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary widely. A compound may stay insoluble or only slightly soluble at all temperatures, it may dissolve better at high temperature, or it may remain soluble at all temperatures. The main reason is the strength and type of intermolecular forces between the solute and the solvent. The rule “like dissolves like” applies here, so substances with similar intermolecular forces tend to dissolve in each other. For example, polar table salt (NaCl) dissolves well in polar water. Temperature also matters. In many cases, solubility rises as temperature rises because added kinetic energy helps break the forces holding solute molecules together. A solution is unsaturated when more solute can still dissolve. It is saturated when it has reached the maximum amount. A supersaturated solution contains more dissolved solute than expected under normal conditions.

Recrystallization uses those solubility differences at high temperature and low temperature. The crude product is first dissolved in a minimal volume of heated solvent. The goal is a saturated solution, but not a supersaturated one. The solution is then cooled to room temperature. As it cools, the solubility of both the desired compound and the impurity drops. The pure compound begins to crystallize first, while impurities that are still soluble remain in the liquid. This works best when the compound of interest is present at a much higher concentration than the impurity.

Crystal formation happens in two stages. In nucleation, the solvent helps solute molecules gather into the first tiny crystal, called a seed or nucleus. In particle growth, more molecules attach to that seed and the crystal gets larger. The crystal contains the pure compound, while the impurity stays in the solvent. In a supersaturated solution, nucleation happens faster than particle growth, so more seeds form and the crystals stay small. If the solution is only saturated, fewer seeds form and the crystals become larger. Heating the solution more before cooling can reduce supersaturation by dissolving a higher concentration of solute. Slow cooling is also preferred because rapid cooling makes many small crystals and can trap impurity inside them.

After the crystals form, the solution is filtered by vacuum filtration and the crystals are allowed to dry. The percent recovery is found by dividing the mass of the recovered product by the mass of the crude product. Recovery is rarely 100% because the compound’s low-temperature solubility limits how much can crystallize. Choosing the right solvent is important for successful crystallization. The desired product should be poorly soluble at room temperature but highly soluble when hot. Ideally, impurities stay soluble at all temperatures, so they remain in solution as the product crystallizes. If some impurities do not dissolve even when hot, hot gravity filtration can remove them before the sample is cooled again. The solvent should also boil well below the melting point of the desired product and should not react with it.

Purifying Crystals by Solubility

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Purifying Crystals by Solubility

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