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

Die meisten Produkte enthalten Verunreinigungen. Eine Möglichkeit, diese Produkte zu reinigen, ist die Rekristallisation. Die Rekristallisation beginnt mit dem Auflösen des unreinen Produkts in gerade so viel heißem Lösungsmittel, dass eine gesättigte Lösung entsteht, in der so viel gelöster Stoff wie möglich im Lösungsmittel gelöst ist. Jeder zusätzliche gelöste Stoff scheint sich nicht aufzulösen.
Es wird ein heißes Lösungsmittel verwendet, da die Löslichkeit typischerweise mit der Temperatur zunimmt. Mit steigender Temperatur nimmt die Menge des gelösten Stoffes, der im Lösungsmittel gelöst werden kann, zu. Wenn die Lösung abkühlt, nimmt die Löslichkeit des Produkts ab, und gelöste Moleküle schließen sich zu kleinen stabilen Kristallen zusammen, die als Kerne bezeichnet werden.
Dies ist der erste Schritt der Kristallisation, der als Keimbildung bezeichnet wird. Zusätzliches Kristallwachstum findet an den Kernen statt, da gelöste Moleküle eine größere Affinität zum Verbinden vorhandener gelöster Kristalle haben als zur Bildung neuer Kristalle. Lösliche Verunreinigungen bleiben in Lösung.
Die Kristallisation kann spontan erfolgen oder durch Kratzen im Inneren des Kolbens, Rühren der Lösung oder Zugabe eines Impfkristalls der Verbindung gefördert werden, was alles eine Oberfläche für weiteres Wachstum bietet.
Die Rekristallisation in große, reine, regelmäßig geformte Kristalle funktioniert nur, wenn ein geeignetes Lösungsmittel verwendet wird. Die Verbindung sollte bei Raumtemperatur im Lösungsmittel unlöslich und bei hohen Temperaturen löslich sein. Im Idealfall sollten die Verunreinigungen entweder im Lösungsmittel bei hoher Temperatur oder im Lösungsmittel bei Raumtemperatur löslich sein.
Sind die Verunreinigungen in heißem Lösungsmittel unlöslich, werden sie vor der Kristallisation herausgefiltert. Nach der Rekristallisation werden die Kristalle herausgefiltert und mit kaltem Lösungsmittel gewaschen, um Verunreinigungen von den Oberflächen zu entfernen. Nun kann die Reinheit der Kristalle analysiert werden.
Bei der Auswahl eines Lösungsmittels ist zu beachten, dass je größer der Unterschied in der Löslichkeit zwischen hohen und niedrigen Temperaturen ist, desto wahrscheinlicher ist es, dass der gelöste Stoff beim Abkühlen aus der Lösung austritt und Kristalle bildet. Auch die Abkühlgeschwindigkeit ist bei der Rekristallisation wichtig.
Schnelle Abkühlung begünstigt die Bildung vieler Keimbildungsstellen und das Wachstum kleinerer Kristalle, während die langsame Abkühlung die Bildung von weniger Keimbildungsstellen und das Wachstum von weniger, größeren, aber reineren Kristallen induziert. Daher wird eine langsamere Kühlung bevorzugt.
In diesem Labor rekristallisieren Sie zwei unreine organische Verbindungen, Acetanilid und Transzimtsäure, und beurteilen dann die Reinheit der zurückgewonnenen Verbindungen, indem Sie ihren Schmelzpunktbereich mit Werten in der Literatur vergleichen.

rekristallisation

Rekristallisation
Oft ist das gewünschte Produkt einer chemischen Reaktion Teil eines komplexeren Reaktionsgemisches, das aus dem Lösungsmittel, den Ausgangsstoffen und Verunreinigungen bestehen kann. Zu lernen, wie man organische Verbindungen richtig reinigt, ist eine wertvolle Technik in der organischen Chemie. Bei der Rekristallisation werden die Unterschiede in der Löslichkeit der gewünschten Verbindung und der Verunreinigung im Lösungsmittel genutzt, um das gewünschte Produkt als Feststoff zu reinigen. Es gibt drei Standardmethoden der Reinigung: Destillation, Extraktion und Rekristallisation.
Löslichkeit
Die Löslichkeit eines Stoffes ist die maximale Menge, die sich in einem festen Volumen eines bestimmten Lösungsmittels bei einer bestimmten Temperatur auflöst. Unterschiedliche gelöste Stoffe haben unterschiedliche Löslichkeiten und lösen sich in unterschiedlichen Lösungsmitteln. Gelöste Stoffe können definierende Eigenschaften aufweisen, die sich für die Rekristallisation eignen. Verbindungen zeigen in einem Lösungsmittel eines der folgenden Verhaltensweisen. Erstens kann die Verbindung bei allen Temperaturen unlöslich sein oder eine sehr geringe Löslichkeit im Lösungsmittel aufweisen. Zweitens kann die Verbindung bei höheren Temperaturen im Lösungsmittel löslich sein. Drittens kann die Verbindung bei allen Temperaturen im Lösungsmittel löslich sein.
Ein wichtiger Faktor bei der Bestimmung, ob sich ein gelöster Stoff in einem Lösungsmittel löst und eine Lösung bildet, ist die Stärke und Art der intermolekularen Kräfte zwischen dem gelösten Stoff und dem Lösungsmittel. Die allgemeine Faustregel lautet "Gleiches löst Gleiches auf", was bedeutet, dass sich Substanzen mit ähnlichen intermolekularen Kräften ineinander auflösen. Zum Beispiel lösen sich polare Substanzen wie Kochsalz (NaCl) gut in polarem Wasser.
Ein weiterer wichtiger Faktor, der die Löslichkeit verbessert, ist die Temperatur. Bei vielen Substanzen steigt die Löslichkeit bei höheren Temperaturen stark an. Dies liegt daran, dass die erhöhte kinetische Energie bei höheren Temperaturen die gelösten intermolekularen Kräfte bricht, die die Moleküle zusammenhalten. Das zeigt sich im Alltag. Wir wissen zum Beispiel, dass sich Speisesalz (NaCl) gut in Wasser auflöst; Bei höheren Temperaturen löst sich jedoch mehr auf als bei niedrigeren Temperaturen.
Qualitativ gilt eine Lösung als ungesättigt, wenn die maximale Menge an gelöstem gelösten Stoff noch nicht erreicht ist. Wenn sich der maximal mögliche gelöste Stoff aufgelöst hat, ist die Lösung gesättigt. Eine übersättigte Lösung enthält mehr gelösten gelösten Stoff als die unter typischen Bedingungen maximal mögliche Menge.
<h4>Rekristallisation </h4>
Bei der Rekristallisation werden die Unterschiede in der Löslichkeit zwischen dem gewünschten Produkt und den Verunreinigungen bei hohen Temperaturen ausgenutzt. Der erste Schritt der Rekristallisation besteht darin, das Produktgemisch in einem minimalen Volumen erhitzten Lösungsmittels aufzulösen, das immer noch zu einer gesättigten, aber nicht übersättigten Lösung führt. Dann wird die Lösung auf Raumtemperatur abgekühlt, wodurch die Löslichkeit sowohl der gewünschten Verbindung als auch der Verunreinigung verringert wird.
Wenn die Lösung abkühlt, beginnt die Kristallisation der reinen Komponente, während dies bei den noch löslichen Verunreinigungen nicht der Fall ist. Dies tritt auf, wenn die interessierende Komponente in einer signifikant höheren Konzentration als die Verunreinigung vorliegt. Zunächst initiiert das Lösungsmittel in der Keimbildungsphase die zufällige Agglomeration der gelösten Moleküle und bildet den ersten Kristall, der als Seed oder Kern bezeichnet wird. Als nächstes werden in der Partikelwachstums- oder Kristallisationsphase weitere Moleküle zum Samen hinzugefügt, wodurch ein Kristall entsteht. Der Kristall enthält die reine Verbindung, während die Verunreinigung im Lösungsmittel verbleibt.
Die Keimbildung verläuft schneller als das Partikelwachstum in einer übersättigten Lösung. Mit mehr Samen ist jeder Kristall kleiner. Wenn die Lösung also gesättigt und nicht übersättigt ist, bilden sich weniger Keime, was zu größeren Kristallen führt. Das Erhitzen der Lösung auf eine höhere Temperatur vor dem Abkühlen auf Raumtemperatur ermöglicht die Auflösung einer höheren Konzentration des gelösten Stoffes, wodurch die Übersättigung verringert wird. Darüber hinaus führt die schnelle Abkühlung zu einer schnellen Keimbildung, bei der viele kleine Kristalle gebildet und die Verunreinigung im Inneren der Kristalle eingeschlossen wird. Langsames Abkühlen wird bevorzugt, um weniger, dafür größere Kristalle zu erzielen.
Sobald die Lösung auf Raumtemperatur abgekühlt ist und sich die Kristalle gebildet haben, wird die Lösung durch Vakuumfiltration filtriert. Anschließend werden die Kristalle trocknen gelassen. Die prozentuale Rückgewinnung wird berechnet, indem die Masse des zurückgewonnenen Produkts durch die Masse des Rohprodukts dividiert wird.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
Die Rückgewinnung beträgt selten 100 %, da die Löslichkeit der Verbindung bei niedrigen Temperaturen bestimmt, wie viel der Verbindung kristallisiert wird.
<h4>Auswahl eines Lösungsmittels</h4>
Damit die Kristallisation effektiv ist, muss das optimale Lösungsmittel verwendet werden. Das gewünschte Produkt sollte eine geringe Löslichkeit in dem ausgewählten Lösungsmittel bei Raumtemperatur, aber eine hohe Löslichkeit im Lösungsmittel bei einer höheren Temperatur aufweisen. Im Idealfall sollten die Verunreinigungen bei allen Temperaturen im Lösungsmittel löslich sein. Wenn die Mischung dem Lösungsmittel bei hoher Temperatur zugesetzt wird, lösen sich das gewünschte Produkt und die Verunreinigungen leicht auf.
Wenn die Lösung abgekühlt wird, nimmt die Löslichkeit des gewünschten Produkts ab und es beginnt eine Kristallisation zu beginnen, bei der ein gereinigtes Produkt gebildet wird. Gelegentlich können Verunreinigungen auch bei hohen Temperaturen im Lösungsmittel der Wahl unlöslich bleiben. Durch die Heißschwerkraftfiltration der Lösung, die das gelöste Produkt enthält, können die festen Verunreinigungen entfernt werden. Das Produkt kann dann durch Abkühlen der Probe rekristallisiert werden.
Im Idealfall sollte das zu verwendende Lösungsmittel in der Lage sein, bei einer Temperatur deutlich unter dem Schmelzpunkt des gewünschten Produkts zu sieden. Das Lösungsmittel sollte außerdem inert sein und nicht mit dem gewünschten gereinigten Produkt reagieren.
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr., P.M., Townsend, J.R. (2012). Chemie und chemische Reaktivität. Belmont, Kalifornien: Brooks/Cole, Cengage Lernen.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) Chemie: Die molekulare Natur von Materie und Veränderung. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Quantitative chemische Analyse. New York, NY: W.H. Freeman und Company.</li>
</ol>

Rekristallisation

Rekristallisation
Oft ist das gewünschte Produkt einer chemischen Reaktion Teil eines komplexeren Reaktionsgemisches, das aus dem Lösungsmittel, ...

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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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Learn how recrystallization uses solubility, temperature, and slow cooling to purify organic solids and separate impurities from crystals.

Purifying Crystals by Solubility | JoVE K12

Bereiten Sie den Unterricht zu Rekristallisation mit diesem 3-minütigen Leitfaden für Lehrkräfte vor. Behandelt die wichtigsten Vorbereitungsschritte für das Chemistry-Labor auf JoVE Lab Manual.

Rekristallisation Lehrervorbereitung in Chemistry - 5.5K+ Aufrufe | JoVE Lab Manual

Folgen Sie den Schritt-für-Schritt-Verfahren von Rekristallisation in diesem 13-minütigen Studentenprotokoll-Video. Teil der Chemistry-Sammlung auf JoVE Lab Manual.

Rekristallisation Studentenprotokoll in Chemistry - 54K+ Aufrufe | JoVE Lab Manual

Purifying Crystals by Solubility

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