Extraction is a method used to isolate a chemical from a mixture based on its relative solubility in two immiscible solvents.
When two immiscible liquids are mixed (like oil and water) they will form two layers when put into a separating funnel due to their different densities. A solvent with a higher density will be on the bottom and the lower density will be on the top. These form into two phases, the organic phase, and the aqueous phase. The aqueous phase contains water and other charged ions and species, whereas the organic phase contains neutral compounds in the organic solvent. A good example of separable, immiscible liquids is water and oil. The aqueous phase forms at the bottom because water is denser, and the oil layer forms on top since it has a lower density.
In practical organic chemistry, this extraction method of separation can be performed using a separating funnel, which holds the liquids in a way that they can separate based on their different densities. The stopcock at the bottom of the separating funnel is opened to remove the bottom layer into another flask and the stopcock is close again to achieve complete separation of the layers. The more dense, top layer (often the organic layer) can then be put into a separate beaker. This technique is useful to separate reaction product mixtures where the desired product and the side products have different solubilities. When the immiscible solvents are mixed, some compounds can be drawn into the other layer if they have better solubility in that solvent. In this way, one can separate desired organic products to then further purify if needed.
In most cases, extraction is not an effective method to separate two organic layers because most organic solvents are miscible with each other. To form an “aqueous layer” an acid or base can be used to form a layer of ions that would separate from the other organic layer.
Practice Questions
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Key Points
• Two immiscible liquids will partition based on their different densities, forming two layers.
• The aqueous phase contains water and other charged ions and species.
• The organic phase contains uncharged species and neutral compounds.
• A separating funnel can be used to separate the two different liquids based on their densities. The more dense solution can be drained off first and then the less dense layer into a separate beaker.
Key Terms
immiscible: two liquids are said to be immiscible if they do not mix to form a homogeneous mixture. Example: oil and water are immiscible
separating funnel: a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture into two immiscible solvent phases of different densities
organic layer: consists of uncharged and non-polar organic molecules including the organic solvent
aqueous layer: consists of water, charged particles, highly ionic, or polar molecules
miscible: two liquids are said to be miscible if they form a homogeneous mixture when combined. Most organic solvents are miscible with each other
solvent: able to dissolve other substances
stopcock: the tap on separating funnels to control the flow of chemicals