Name: separating mixtures with solvent extraction
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 2 min 55 s
Channel: JoVE

A extração é uma técnica para separar componentes de uma mistura. Você já está familiarizado com a extração. Sempre que você infunde o chá, está extraindo compostos solúveis em água, como cafeína e sabores, das folhas de chá para a água quente. Este é um exemplo de extração sólido-líquido.
A extração líquido-líquido é outro tipo de extração em que a mistura é dissolvida em duas fases líquidas imiscíveis. Os solventes devem ser imiscíveis, o que significa que não se misturam e são fases separadas. As extrações líquido-líquido são realizadas em um funil de separação, de modo que o solvente mais denso se deposite no fundo e o solvente menos denso fique no topo. Em seguida, cada composto soluto será transferido para a fase em que é mais solúvel. Portanto, é importante escolher solventes com polaridades diferentes.
Em geral, os solutos apolares se dividem na fase orgânica e os solutos polares se dividem na fase aquosa. O coeficiente de partição, K, é a razão entre a concentração de soluto na fase orgânica e a concentração na fase aquosa. Uma vez que os solutos tenham se separado, as duas fases diferentes são coletadas. Lembre-se de que é provável que haja resíduos de cada componente em ambas as fases.
Uma extração ácido-base é um tipo especial de extração líquido-líquido que separa compostos ácidos e básicos com base nas diferenças de solubilidade. Lembre-se de que um ácido é um composto que doa um próton quando dissolvido em água, e uma base é um composto que aceita um próton.
Para separar os compostos ácidos, uma base é adicionada à mistura. A base aceitará um próton do composto ácido, tornando-o iônico. Por outro lado, adicionar um ácido transferiria um próton para um composto básico. Em ambos os casos, o composto iônico será transferido para a fase aquosa e os compostos neutros serão particionados para a fase orgânica. O composto iônico é coletado e, em seguida, desprotonado ou reprotonado para convertê-lo de volta ao seu composto original.
Neste laboratório, você separará uma mistura de celulose, ácido benzóico e cafeína. Primeiro, você usará extração e filtração para isolar a celulose. Em seguida, você realizará uma extração ácido-base para separar o ácido benzóico e a cafeína.

extração

extração
A extração é uma técnica comum usada em química orgânica para isolar um composto alvo. No processo de extração, um soluto é transferido de uma fase para outra para separá-lo de materiais de partida ou impurezas que não reagiram. A extração também é usada para facilitar o isolamento de um soluto de um solvente de reação que é difícil de remover por evaporação, como um solvente com alto ponto de ebulição.<span style="color: rgb(0, 0, 0); família de fontes: Roboto, RobotoDraft, Helvetica, Arial, sans-serif; tamanho da fonte: 13px; estilo de fonte: normal; ligaduras variantes de fonte: normal; caps-variante-de-fonte: normal; peso da fonte: 400; espaçamento entre letras: normal; órfãos: 2; alinhamento de texto: início; recuo de texto: 0px; transformação de texto: nenhum; espaço em branco: pré-embrulho; viúvas: 2; espaçamento entre palavras: 0px; -webkit-text-stroke-width: 0px; cor de fundo: rgb (255, 255, 255); estilo de decoração de texto: inicial; text-decoration-color: inicial; exibição: inline !important; flutuar: nenhum;" > Geralmente, existem três tipos de extrações. Primeiro, na extração sólido-líquido, o soluto é transferido de uma fase sólida para uma fase líquida. Na extração líquido-líquido, um soluto é transferido de um líquido para outro. Na extração ácido-base, um soluto é transformado em um composto iônico e transferido de uma fase orgânica para uma fase aquosa. Um exemplo comum de extração é a fabricação de café ou chá. A fase sólida, que contém cafeína, sabores e odores vegetais, é extraída pela água quente para a fase líquida.
Extração Líquido-Líquido
Uma extração líquido-líquido transfere um composto orgânico que é dissolvido em uma fase aquosa para um solvente orgânico ou é usado para transferir reagentes não reagidos, sais e outras impurezas solúveis em água para a fase aquosa, deixando o composto orgânico de interesse na fase orgânica. Líquidos imiscíveis são líquidos que nunca formam uma solução homogênea, mesmo quando bem misturados. Em vez disso, os líquidos imiscíveis se separam em diferentes fases, como óleo e água.
Uma extração líquido-líquido transfere um composto orgânico que é dissolvido em uma fase aquosa para um solvente orgânico. Para realizar uma extração líquido-líquido, primeiro, a solução aquosa contendo o soluto é adicionada a um funil de separação. Em seguida, um solvente orgânico não solúvel em água é adicionado ao funil de separação. Quando o conteúdo do funil é bem misturado, o composto orgânico se divide na fase orgânica com base em sua maior solubilidade na fase orgânica do que na fase aquosa.
Como os dois solventes são imiscíveis, os dois líquidos formam camadas discretas, com o líquido denso na parte inferior e o líquido menos denso na parte superior. Uma vez que as duas fases se acomodam em duas camadas, elas são separadas abrindo a torneira no fundo do funil separatório e permitindo que uma camada flua para fora. O líquido que teve o soluto removido é chamado de refinado, enquanto o líquido que ganhou o soluto é chamado de extrato.
O composto orgânico é dividido entre as duas camadas com base em sua solubilidade em cada fase. O equilíbrio é alcançado quando o potencial químico do soluto é o mesmo nas duas fases. O coeficiente de partição de um soluto, K, é a razão entre a concentração da amostra na camada orgânica dividida pela concentração na fase aquosa. O coeficiente de partição é uma constante dependente do soluto e do par de solventes usados na extração. O coeficiente de partição é uma expressão da preferência do soluto por cada um dos dois solventes. Solutos com grande coeficiente de partição têm maior tendência a serem extraídos para a camada de solvente orgânico. Solutos com pequenos coeficientes de partição preferem fazer a transição para a fase aquosa.
Determinar qual par de solventes usar para uma extração líquido-líquido é uma etapa vital. Considere o seguinte ao escolher quais solventes usar: Primeiro, o soluto deve ser mais solúvel no solvente do que na água. Portanto, é necessário conhecer o coeficiente de partição do soluto em um par de solventes em potencial. Em segundo lugar, o par solvente deve ser imiscível em água e não formar uma solução homogênea quando misturado. Terceiro, os solventes devem ser inertes e não reagir com o soluto. O solvente também deve ser volátil para que possa ser removido facilmente do soluto. O solvente orgânico imiscível em água geralmente possui uma polaridade apolar ou de baixa polaridade.
Também é fundamental conhecer as densidades dos solventes para determinar as identidades das camadas superior e inferior. A maioria dos líquidos orgânicos tem uma densidade menor que a água, com exceção dos solventes orgânicos clorados, e se deposita no fundo do funil de separação.
Extração ácido-base
A extração ácido-base é um tipo de extração líquido-líquido que separa compostos orgânicos com base em suas propriedades ácido-base. Se um soluto é um ácido ou base, sua carga muda à medida que o pH é alterado. Geralmente, a maioria dos compostos orgânicos são neutros e, portanto, mais solúveis em solventes orgânicos do que em água. No entanto, se o composto orgânico se tornar iônico, ele se tornará mais solúvel em água. Isso é útil na extração de um ácido orgânico ou composto básico de uma fase orgânica para uma fase aquosa.
A extração ácido-base aproveita essa propriedade transformando o soluto em sua forma de sal solúvel em água, alterando assim sua solubilidade. A solubilidade do composto orgânico e seu sal deve ser dramaticamente diferente para que a técnica seja eficaz.
Por exemplo, considere uma mistura contendo um ácido carboxílico orgânico, uma amina e um composto neutro. Os ácidos carboxílicos constituídos por seis carbonos ou mais são insolúveis em água e totalmente solúveis em solventes orgânicos. No entanto, suas bases conjugadas (um composto iônico) são solúveis em água e insolúveis em solventes orgânicos. Uma amina constituída por pelo menos sete carbonos é insolúvel em água, mas solúvel em solventes orgânicos. O ácido conjugado dessa amina (um composto iônico) é solúvel em água e insolúvel em solventes orgânicos.
Quando reagido com uma base, o ácido carboxílico é neutralizado em sua forma de sal. Os outros compostos da mistura permanecem neutros. Uma vez que o ácido carboxílico é transformado em sal, ele se particiona para a fase aquosa, enquanto os compostos neutros permanecem na fase orgânica.
A extração ácido-base também é usada para separar dois ácidos fracos ou duas bases fracas com uma diferença significativa em seu pKa. No caso dos ácidos, o ácido relativamente mais forte, com um pequeno valor de pKa, é neutralizado a um sal usando uma base fraca. A base fraca não reage eficientemente com o ácido mais fraco, e apenas o ácido mais forte é transformado em sal. Em seguida, o sal é extraído para a fase aquosa durante a extração. O processo segue de forma semelhante para bases fracas, onde a base relativamente mais forte é neutralizada em um sal usando um ácido fraco.
Referências
<ol>
	<li style="font-size:17px">Harris, D.C. (2015). Análise Química Quantitativa. Nova York, NY: W.H. Freeman and Company.</li>
</ol>

Extração

Extração
A extração é uma técnica comum usada em química orgânica para isolar um composto alvo. No processo de extração, um soluto é transferido ...

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Extraction

separating mixtures with solvent extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point.

There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase into a liquid phase. In liquid-liquid extraction, the solute moves from one liquid into another. In acid-base extraction, the solute is changed into an ionic compound and moved from an organic phase into an aqueous phase. A familiar example is brewing coffee or tea, where hot water extracts caffeine, plant flavors, and odors from the solid material.

Liquid-liquid extraction is used when an organic compound is dissolved in water or when water-soluble impurities need to be removed from an organic product. It works with immiscible liquids, which are liquids that do not form one uniform solution, even after mixing. Oil and water are a common example. To carry out this extraction, the aqueous solution is placed in a separatory funnel, and a non-water-soluble organic solvent is added. After mixing, the compound partitions into the phase where it is more soluble.

Because the two liquids are immiscible, they separate into layers after settling. The denser liquid forms the bottom layer, and the less dense liquid stays on top. The layers are then separated by opening the stopcock at the bottom of the separatory funnel and letting one layer drain out. The layer that loses the solute is called the raffinate, and the layer that gains the solute is called the extract. At equilibrium, the solute has the same chemical potential in both phases.

The partition coefficient, K, describes how a solute divides between the two layers. It is the concentration in the organic layer divided by the concentration in the aqueous layer. This value depends on the solute and the solvent pair used. A large partition coefficient means the solute prefers the organic layer. A small partition coefficient means the solute prefers the aqueous layer. Choosing the right solvent pair is important, so the solute should be more soluble in that solvent than in water, the two solvents should be immiscible, and the solvents should be inert and volatile. The water-immiscible organic solvent is usually non-polar or low in polarity.

Knowing density is also important because it tells which layer is on top and which is on the bottom. Most organic liquids are less dense than water and rise to the top, although chlorinated organic solvents are an exception and can settle to the bottom. Acid-base extraction is a special type of liquid-liquid extraction that separates organic compounds based on acid-base behavior. When pH changes, the charge on an acid or base can change too. Neutral organic compounds are usually more soluble in organic solvents, but ionic forms are more soluble in water.

This property lets chemists turn an organic acid or base into a water-soluble salt. The salt and the neutral compound must have very different solubilities for the method to work well. For example, a mixture may contain an organic carboxylic acid, an amine, and a neutral compound. Carboxylic acids with six carbons or more are insoluble in water and soluble in organic solvents, but their conjugate bases are water-soluble and insoluble in organic solvents. Amines with at least seven carbons are insoluble in water and soluble in organic solvents, while their conjugate acids are water-soluble and insoluble in organic solvents.

When a base is added, the carboxylic acid is neutralized to its salt form, while the other compounds remain neutral. The salt then moves into the aqueous phase, and the neutral compounds stay in the organic phase. Acid-base extraction can also separate two weak acids or two weak bases when their pKa values are far enough apart. For weak acids, the stronger acid with the smaller pKa is converted to a salt using a weak base, while the weaker acid is not reacted much. The same idea applies to weak bases, but a weak acid is used to neutralize the stronger base.

Separating Mixtures with Solvent Extraction

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Separating Mixtures with Solvent Extraction

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