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

A maioria dos produtos contém impurezas. Uma maneira de purificar esses produtos é por recristalização. A recristalização começa com a dissolução do produto impuro em solvente quente apenas o suficiente para formar uma solução saturada, onde o máximo de soluto é dissolvido no solvente possível. Qualquer soluto adicional não parecerá se dissolver.
Um solvente quente é usado porque a solubilidade normalmente aumenta com a temperatura. À medida que a temperatura aumenta, a quantidade de soluto que pode ser dissolvida no solvente aumenta. À medida que a solução esfria, a solubilidade do produto diminui e as moléculas de soluto se juntam para formar pequenos cristais estáveis chamados núcleos.
Esta é a primeira etapa da cristalização, chamada nucleação. O crescimento adicional de cristais ocorre nos núcleos porque as moléculas de soluto têm uma afinidade maior para unir cristais de soluto existentes do que formar novos cristais. As impurezas solúveis são deixadas na solução.
A cristalização pode acontecer espontaneamente ou pode ser estimulada arranhando o interior do frasco, agitando a solução ou adicionando um cristal de semente do composto, todos os quais fornecem uma superfície para crescimento adicional.
A recristalização em cristais grandes, puros e de formato regular só funciona quando um solvente apropriado é usado. O composto deve ser insolúvel no solvente à temperatura ambiente e solúvel em altas temperaturas. Idealmente, as impurezas devem ser insolúveis no solvente em alta temperatura ou solúveis no solvente à temperatura ambiente.
Se as impurezas forem insolúveis em solvente quente, elas são filtradas antes da cristalização. Após a recristalização, os cristais são filtrados e lavados com solvente frio para remover as impurezas das superfícies. Agora, a pureza dos cristais pode ser analisada.
Ao escolher um solvente, lembre-se de que quanto maior a diferença de solubilidade entre altas e baixas temperaturas, maior a probabilidade de o soluto sair da solução à medida que esfria para formar cristais. A taxa de resfriamento também é importante na recristalização.
O resfriamento rápido favorece a formação de muitos locais de nucleação e o crescimento de cristais menores, enquanto o resfriamento induz lentamente a formação de menos locais de nucleação e o crescimento de menos cristais maiores, mas mais puros. Portanto, é preferível um resfriamento mais lento.
Neste laboratório, você recristalizará dois compostos orgânicos impuros, acetanilida e ácido transcinâmico, e avaliará a pureza dos compostos recuperados comparando sua faixa de ponto de fusão com os valores da literatura.

recristalização

recristalização
Freqüentemente, o produto desejado de uma reação química faz parte de uma mistura de reação mais complexa, que pode ser composta pelo solvente, materiais de partida e impurezas. Aprender a purificar compostos orgânicos adequadamente é uma técnica valiosa em química orgânica. A recristalização aproveita as diferenças de solubilidade do composto desejado e a impureza no solvente para purificar o produto desejado como um sólido. Existem três métodos padrão de purificação: destilação, extração e recristalização.
solubilidade
A solubilidade de uma substância é a quantidade máxima que se dissolve em um volume fixo de um determinado solvente a uma determinada temperatura. Soluto diferente Os solutos podem ter características definidoras que se prestam a serem explorados para recristalização. Os compostos exibem um dos seguintes comportamentos em um solvente. Primeiro, o composto pode ser insolúvel ou ter solubilidade muito baixa no solvente em todas as temperaturas. Em segundo lugar, o composto pode ser solúvel no solvente em temperaturas mais altas. Terceiro, o composto pode ser solúvel no solvente em todas as temperaturas.
Um fator importante para determinar se um soluto se dissolve em um solvente e forma uma solução é a força e o tipo de forças intermoleculares entre o soluto e o solvente. A regra geral é "semelhante dissolve semelhante", o que significa que substâncias com tipos semelhantes de forças intermoleculares se dissolvem umas nas outras. Por exemplo, substâncias polares, como sal de cozinha (NaCl), dissolvem-se bem em água polar.
Outro fator chave que melhora a solubilidade é a temperatura. Para muitas substâncias, a solubilidade aumenta muito em temperaturas mais altas. Isso se deve ao fato de que o aumento da energia cinética em temperaturas mais altas quebra as forças intermoleculares do soluto que mantêm as moléculas juntas. Isso é visto na vida cotidiana. Por exemplo, sabemos que o sal de cozinha (NaCl) se dissolve bem na água; no entanto, mais se dissolve em temperaturas mais altas do que em temperaturas mais baixas.
Qualitativamente, uma solução é considerada insaturada se a quantidade máxima de soluto dissolvido ainda não tiver sido atingida. Quando o soluto máximo possível se dissolver, a solução está saturada. Uma solução supersaturada contém mais soluto dissolvido do que a quantidade máxima possível em condições normais.
<h4>recristalização </h4>
A recristalização aproveita as diferenças de solubilidade entre o produto desejado e os contaminantes em altas temperaturas. O primeiro passo da recristalização é dissolver a mistura do produto em um volume mínimo de solvente aquecido que ainda resulta em uma solução saturada - mas não supersaturada. Em seguida, a solução é resfriada à temperatura ambiente, diminuindo a solubilidade do composto desejado e da impureza.
À medida que a solução esfria, a cristalização do componente puro começa, enquanto as impurezas ainda solúveis não. Isso ocorre quando o componente de interesse está em uma concentração significativamente maior do que a impureza. Primeiro, na fase de nucleação, o solvente inicia a aglomeração aleatória das moléculas de soluto, formando o primeiro cristal chamado semente ou núcleo. Em seguida, na fase de crescimento ou cristalização das partículas, mais moléculas são adicionadas à semente, formando um cristal. O cristal contém o composto puro, enquanto a impureza permanece no solvente.
A nucleação ocorre mais rapidamente do que o crescimento de partículas em uma solução supersaturada. Com mais sementes, cada cristal é menor. Assim, se a solução estiver saturada, em vez de supersaturada, menos sementes se formam, resultando em cristais maiores. Aquecer a solução a uma temperatura mais alta antes de resfriar à temperatura ambiente permite a dissolução de uma concentração mais alta de soluto, diminuindo a supersaturação. Além disso, o resfriamento rápido resulta em nucleação rápida, formando muitos cristais pequenos e prendendo a impureza dentro dos cristais. O resfriamento lento é preferido para obter menos cristais maiores.
Uma vez que a solução tenha esfriado à temperatura ambiente e os cristais tenham se formado, a solução é filtrada usando filtração a vácuo. Em seguida, os cristais são deixados secar. A porcentagem de recuperação é calculada dividindo a massa do produto recuperado pela massa do produto bruto.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
A recuperação raramente é de 100%, pois a solubilidade do composto em baixas temperaturas determina quanto do composto é cristalizado.
<h4>Selecionando um solvente</h4>
Para que a cristalização seja eficaz, o solvente ideal deve ser usado. O produto desejado deve ter baixa solubilidade no solvente selecionado à temperatura ambiente, mas alta solubilidade no solvente a uma temperatura mais alta. Idealmente, as impurezas devem ser solúveis no solvente em todas as temperaturas. Assim, quando a mistura é adicionada ao solvente em alta temperatura, o produto desejado e as impurezas se dissolvem prontamente.
À medida que a solução é resfriada, a solubilidade do produto desejado diminui e a cristalização começa a ocorrer, formando produto purificado. Ocasionalmente, as impurezas podem permanecer insolúveis no solvente de escolha, mesmo em altas temperaturas. A filtração por gravidade a quente da solução que contém o produto dissolvido pode remover as impurezas sólidas. O produto pode então ser recristalizado resfriando a amostra.
Idealmente, o solvente a ser usado deve ser capaz de ferver a uma temperatura bem abaixo do ponto de fusão do produto desejado. O solvente também deve ser inerte e não reagir com o produto purificado desejado.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análise Química Quantitativa. Nova York, NY: W.H. Freeman and Company.</li>
</ol>

Recristalização

Recristalização
Muitas vezes, o produto desejado de uma reação química faz parte de uma mistura de reação mais complexa, que pode ser composta pelo 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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