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

La plupart des produits contiennent des impuretés. Une façon de purifier ces produits est la recristallisation. La recristallisation commence par la dissolution du produit impur dans juste assez de solvant chaud pour former une solution saturée, où autant de soluté que possible est dissous dans le solvant. Tout soluté supplémentaire ne semblera pas se dissoudre.
Un solvant chaud est utilisé car la solubilité augmente généralement avec la température. À mesure que la température augmente, la quantité de soluté qui peut être dissoute dans le solvant augmente. Au fur et à mesure que la solution refroidit, la solubilité du produit diminue et les molécules de soluté se rassemblent pour former de petits cristaux stables appelés noyaux.
C’est la première étape de la cristallisation, appelée nucléation. Une croissance cristalline supplémentaire se produit sur les noyaux parce que les molécules de soluté ont une plus grande affinité pour joindre des cristaux de soluté existants que pour former de nouveaux cristaux. Les impuretés solubles sont laissées en solution.
La cristallisation peut se produire spontanément ou être encouragée en grattant l’intérieur du flacon, en agitant la solution ou en ajoutant un cristal de graine du composé, qui fournissent tous une surface pour une croissance ultérieure.
La recristallisation en gros cristaux purs et de forme régulière ne fonctionne que lorsqu’un solvant approprié est utilisé. Le composé doit être insoluble dans le solvant à température ambiante et soluble à haute température. Idéalement, les impuretés doivent être soit insolubles dans le solvant à haute température, soit solubles dans le solvant à température ambiante.
Si les impuretés sont insolubles dans le solvant chaud, elles sont filtrées avant la cristallisation. Après la recristallisation, les cristaux sont filtrés et lavés avec un solvant froid pour éliminer les impuretés des surfaces. Maintenant, la pureté des cristaux peut être analysée.
Lorsque vous choisissez un solvant, gardez à l’esprit que plus la différence de solubilité entre les températures élevées et les basses températures est grande, plus le soluté est susceptible de sortir de la solution lorsqu’il refroidit pour former des cristaux. La vitesse de refroidissement est également importante dans la recristallisation.
Le refroidissement rapide favorise la formation de nombreux sites de nucléation et la croissance de cristaux plus petits, tandis que le refroidissement induit lentement la formation de moins de sites de nucléation et la croissance de cristaux moins nombreux, plus grands mais plus purs. Ainsi, un refroidissement plus lent est préférable.
Dans ce laboratoire, vous recristalliserez deux composés organiques impurs, l’acétanilide et l’acide trans-cinnamique, puis évaluerez la pureté des composés récupérés en comparant leur plage de point de fusion aux valeurs de la littérature.

recristallisation

recristallisation
Souvent, le produit souhaité d’une réaction chimique fait partie d’un mélange réactionnel plus complexe, qui peut être composé du solvant, des matériaux de départ et des impuretés. Apprendre à purifier correctement les composés organiques est une technique précieuse en chimie organique. La recristallisation exploite les différences de solubilité du composé souhaité et l’impureté du solvant pour purifier le produit souhaité sous forme solide. Il existe trois méthodes standard de purification : la distillation, l’extraction et la recristallisation.
solubilité
La solubilité d’une substance est la quantité maximale qui se dissout dans un volume fixe d’un solvant donné à une température donnée. Différents solutés ont des solubilités différentes et se dissolvent dans différents solvants. Les solutés peuvent avoir des caractéristiques déterminantes qui se prêtent à être exploitées pour la recristallisation. Les composés présentent l’un des comportements suivants dans un solvant. Tout d’abord, le composé peut être insoluble ou avoir une très faible solubilité dans le solvant à toutes les températures. Deuxièmement, le composé peut être soluble dans le solvant à des températures plus élevées. Troisièmement, le composé peut être soluble dans le solvant à toutes les températures.
L’un des principaux facteurs permettant de déterminer si un soluté se dissout dans un solvant et forme une solution est l’intensité et le type de forces intermoléculaires entre le soluté et le solvant. La règle générale est « semblable se dissout semblable », ce qui signifie que les substances ayant des types similaires de forces intermoléculaires se dissolvent les unes dans les autres. Par exemple, les substances polaires telles que le sel de table (NaCl) se dissolvent bien dans l’eau polaire.
La température est un autre facteur clé qui améliore la solubilité. Pour de nombreuses substances, la solubilité augmente considérablement à des températures plus élevées. Cela est dû au fait que l’augmentation de l’énergie cinétique à des températures plus élevées brise les forces intermoléculaires du soluté qui maintiennent les molécules ensemble. Cela se voit dans la vie de tous les jours. Par exemple, nous savons que le sel de table (NaCl) se dissout bien dans l’eau ; Cependant, plus de dissolutions à des températures plus élevées qu’à des températures plus basses.
Qualitativement, une solution est considérée comme insaturée si la quantité maximale de soluté dissous n’a pas encore été atteinte. Lorsque le maximum de soluté possible est dissous, la solution est saturée. Une solution sursaturée contient plus de soluté dissous que la quantité maximale possible dans des conditions typiques.
<h4>recristallisation </h4>
La recristallisation tire parti des différences de solubilité entre le produit souhaité et les contaminants à haute température. La première étape de la recristallisation consiste à dissoudre le mélange de produit dans un volume minimal de solvant chauffé qui donne toujours une solution saturée, mais pas sursaturée. Ensuite, la solution est refroidie à température ambiante, ce qui diminue la solubilité du composé souhaité et l’impureté.
Au fur et à mesure que la solution refroidit, la cristallisation du composant pur commence, contrairement aux impuretés encore solubles. Cela se produit lorsque la concentration du composant d’intérêt est nettement supérieure à celle de l’impureté. Tout d’abord, dans la phase de nucléation, le solvant initie l’agglomération aléatoire des molécules de soluté, formant le premier cristal appelé graine ou noyau. Ensuite, dans la phase de croissance ou de cristallisation des particules, d’autres molécules sont ajoutées à la graine, formant un cristal. Le cristal contient le composé pur, tandis que l’impureté reste dans le solvant.
La nucléation se déroule plus rapidement que la croissance des particules dans une solution sursaturée. Avec plus de graines, chaque cristal est plus petit. Ainsi, si la solution est saturée, plutôt que sursaturée, moins de graines se forment, ce qui donne des cristaux plus gros. Le chauffage de la solution à une température plus élevée avant de la refroidir à température ambiante permet la dissolution d’une concentration plus élevée de soluté, diminuant ainsi la sursaturation. De plus, le refroidissement rapide entraîne une nucléation rapide, formant de nombreux petits cristaux et piégeant l’impureté à l’intérieur des cristaux. Le refroidissement lent est préférable pour obtenir des cristaux moins nombreux et plus gros.
Une fois que la solution a refroidi à température ambiante et que les cristaux se sont formés, la solution est filtrée par filtration sous vide. Ensuite, on laisse sécher les cristaux. Le pourcentage de récupération est calculé en divisant la masse du produit récupéré par la masse du produit brut.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg" font-size>Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.
	<li style="font-size :17px">Silberberg, M.S. (2009) Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
	<li style="font-size :17px">Harris, D.C. (2015). Analyse chimique quantitative. New York, NY : W.H. Freeman et compagnie.</li>

Recristallisation

Recristallisation
Souvent, le produit souhaité d’une réaction chimique fait partie d’un mélange réactionnel plus complexe, qui peut être composé du solvant, ...

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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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