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

La maggior parte dei prodotti contiene impurità. Un modo per purificare questi prodotti è la ricristallizzazione. La ricristallizzazione inizia con la dissoluzione del prodotto impuro in un solvente caldo appena sufficiente a formare una soluzione satura, in cui nel solvente viene disciolto quanto più soluto possibile. Qualsiasi soluto aggiuntivo sembrerà non dissolversi.
Viene utilizzato un solvente caldo perché la solubilità aumenta tipicamente con la temperatura. All'aumentare della temperatura, aumenta la quantità di soluto che può essere disciolta nel solvente. Quando la soluzione si raffredda, la solubilità del prodotto diminuisce e le molecole di soluto si uniscono per formare piccoli cristalli stabili chiamati nuclei.
Questo è il primo passo della cristallizzazione, chiamato nucleazione. Un'ulteriore crescita dei cristalli avviene sui nuclei perché le molecole di soluto hanno una maggiore affinità per unire i cristalli di soluto esistenti rispetto alla formazione di nuovi cristalli. Le impurità solubili vengono lasciate in soluzione.
La cristallizzazione può avvenire spontaneamente o può essere incoraggiata grattando l'interno del pallone, agitando la soluzione o aggiungendo un cristallo seme del composto, che forniscono una superficie per un'ulteriore crescita.
La ricristallizzazione in cristalli grandi, puri e di forma regolare funziona solo quando viene utilizzato un solvente appropriato. Il composto deve essere insolubile nel solvente a temperatura ambiente e solubile ad alte temperature. Idealmente, le impurità dovrebbero essere insolubili nel solvente ad alta temperatura o solubili nel solvente a temperatura ambiente.
Se le impurità sono insolubili in solvente caldo, vengono filtrate prima della cristallizzazione. Dopo la ricristallizzazione, i cristalli vengono filtrati e lavati con solvente freddo per rimuovere le impurità dalle superfici. Ora è possibile analizzare la purezza dei cristalli.
Quando si sceglie un solvente, tenere presente che maggiore è la differenza di solubilità tra alte e basse temperature, più è probabile che il soluto esca dalla soluzione mentre si raffredda per formare cristalli. Anche la velocità di raffreddamento è importante nella ricristallizzazione.
Il raffreddamento rapido favorisce la formazione di molti siti di nucleazione e la crescita di cristalli più piccoli, mentre il raffreddamento induce lentamente la formazione di un minor numero di siti di nucleazione e la crescita di un minor numero di cristalli, più grandi ma più puri. Quindi, è preferibile un raffreddamento più lento.
In questo laboratorio, ricristallizzerai due composti organici impuri, acetanilide e acido trans-cinnamico, e quindi valuterai la purezza dei composti recuperati confrontando il loro intervallo del punto di fusione con i valori in letteratura.

ricristallizzazione

ricristallizzazione
Spesso, il prodotto desiderato di una reazione chimica fa parte di una miscela di reazione più complessa, che può essere composta dal solvente, dai materiali di partenza e dalle impurità. Imparare a purificare correttamente i composti organici è una tecnica preziosa in chimica organica. La ricristallizzazione sfrutta le differenze di solubilità del composto desiderato e l'impurità nel solvente per purificare il prodotto desiderato come solido. Esistono tre metodi standard di purificazione: distillazione, estrazione e ricristallizzazione.
solubilità
La solubilità di una sostanza è la quantità massima che si dissolve in un volume fisso di un dato solvente a una data temperatura. Soluti diversi hanno solubilità diverse e si dissolvono in solventi diversi. I soluti possono avere caratteristiche distintive che si prestano ad essere sfruttate per la ricristallizzazione. I composti mostrano uno dei seguenti comportamenti in un solvente. Innanzitutto, il composto può essere insolubile o avere una solubilità molto bassa nel solvente a tutte le temperature. In secondo luogo, il composto può essere solubile nel solvente a temperature più elevate. In terzo luogo, il composto può essere solubile nel solvente a tutte le temperature.
Un fattore importante nel determinare se un soluto si dissolve in un solvente e forma una soluzione è la forza e il tipo di forze intermolecolari tra il soluto e il solvente. La regola generale è "il simile dissolve il simile", il che significa che le sostanze con tipi simili di forze intermolecolari si dissolvono l'una nell'altra. Ad esempio, le sostanze polari come il sale da cucina (NaCl), si dissolvono bene nell'acqua polare.
Un altro fattore chiave che migliora la solubilità è la temperatura. Per molte sostanze, la solubilità aumenta notevolmente a temperature più elevate. Ciò è dovuto al fatto che l'aumento dell'energia cinetica a temperature più elevate rompe le forze intermolecolari del soluto che tengono insieme le molecole. Questo si vede nella vita di tutti i giorni. Ad esempio, sappiamo che il sale da cucina (NaCl) si scioglie bene in acqua; tuttavia, si dissolve di più a temperature più elevate che a temperature più basse.
Qualitativamente, una soluzione è considerata insatura se la quantità massima di soluto disciolto non è stata ancora raggiunta. Quando il soluto massimo possibile si è dissolto, la soluzione è satura. Una soluzione supersatura contiene più soluto disciolto della quantità massima possibile in condizioni tipiche.
<h4>ricristallizzazione </h4>
La ricristallizzazione sfrutta le differenze di solubilità tra il prodotto desiderato e i contaminanti ad alte temperature. La prima fase della ricristallizzazione consiste nel sciogliere la miscela di prodotti in un volume minimo di solvente riscaldato che si traduce comunque in una soluzione satura, ma non sovrasatura. Quindi, la soluzione viene raffreddata a temperatura ambiente, diminuendo la solubilità sia del composto desiderato che dell'impurità.
Quando la soluzione si raffredda, inizia la cristallizzazione del componente puro, mentre le impurità ancora solubili non lo fanno. Ciò si verifica quando il componente di interesse si trova in una concentrazione significativamente superiore all'impurità. In primo luogo, nella fase di nucleazione, il solvente avvia l'agglomerazione casuale delle molecole di soluto, formando il primo cristallo chiamato seme o nucleo. Successivamente, nella fase di crescita delle particelle o di cristallizzazione, vengono aggiunte più molecole al seme, formando un cristallo. Il cristallo contiene il composto puro, mentre l'impurità rimane nel solvente.
La nucleazione procede più velocemente della crescita delle particelle in una soluzione supersatura. Con più semi, ogni cristallo è più piccolo. Pertanto, se la soluzione è satura, piuttosto che sovrasatura, si formano meno semi, risultando in cristalli più grandi. Il riscaldamento della soluzione a una temperatura più elevata prima di raffreddarla a temperatura ambiente consente la dissoluzione di una maggiore concentrazione di soluto, diminuendo la sovrasaturazione. Inoltre, il rapido raffreddamento provoca una rapida nucleazione, formando molti piccoli cristalli e intrappolando l'impurità all'interno dei cristalli. Il raffreddamento lento è preferito per ottenere un minor numero di cristalli più grandi.
Una volta che la soluzione si è raffreddata a temperatura ambiente e si sono formati i cristalli, la soluzione viene filtrata mediante filtrazione sotto vuoto. Quindi, i cristalli vengono lasciati asciugare. La percentuale di recupero viene calcolata dividendo la massa del prodotto recuperato per la massa del prodotto grezzo.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
Il recupero è raramente del 100%, poiché la solubilità del composto a basse temperature determina la quantità di composto cristallizzata.
<h4>Selezione di un solvente</h4>
Affinché la cristallizzazione sia efficace, è necessario utilizzare il solvente ottimale. Il prodotto desiderato deve avere una bassa solubilità nel solvente selezionato a temperatura ambiente ma un'elevata solubilità nel solvente a una temperatura più elevata. Idealmente, le impurità dovrebbero essere solubili nel solvente a tutte le temperature. Pertanto, quando la miscela viene aggiunta al solvente ad alta temperatura, il prodotto desiderato e le impurità si dissolvono prontamente.
Man mano che la soluzione viene raffreddata, la solubilità del prodotto desiderato diminuisce e inizia la cristallizzazione, formando un prodotto purificato. Occasionalmente, le impurità possono rimanere insolubili nel solvente scelto, anche ad alte temperature. La filtrazione a gravità calda della soluzione che contiene il prodotto disciolto può rimuovere le impurità solide. Il prodotto può quindi essere ricristallizzato raffreddando il campione.
Idealmente, il solvente da utilizzare dovrebbe essere in grado di bollire a una temperatura ben al di sotto del punto di fusione del prodotto desiderato. Il solvente deve inoltre essere inerte e non reagire con il prodotto purificato desiderato.
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) Chimica: la natura molecolare della materia e il cambiamento. Boston, MA: McGraw Hill.</li>
	<li style="dimensione del carattere:17px">Harris, D.C. (2015). Analisi chimica quantitativa. New York, NY: W.H. Freeman e compagnia.</li>
</ol>

Ricristallizzazione

Ricristallizzazione
Spesso, il prodotto desiderato di una reazione chimica fa parte di una miscela di reazione più complessa, che può essere composta dal solvente, ...

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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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