Recrystallization is a technique used come purify heavy compounds.1 Solids often tend to be much more soluble in hot liquids than in cold liquids. During recrystallization, an impure solid link is dissolved in a hot liquid until the equipment is saturated, and then the liquid is enabled to cool.2 The compound have to then kind relatively pure crystals. Ideally, any type of impurities the are current will remain in the solution and will no be integrated into the growing crystals (Figure 1). The crystals have the right to then be removed from the solution by filtration. Not all of the link is recoverable — some will remain in the solution and also will be lost.

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Recrystallization is not usually thought of as a separation technique; rather, that is a purification method in i m sorry a tiny amount of one impurity is eliminated from a compound. However, if the solubility properties of two compounds room sufficiently different, recrystallization have the right to be used to different them, even if they are present in almost equal amounts. Recrystallization works ideal when many impurities have already been removed by another method, such together extraction or tower chromatography.

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Figure 1. The basic scheme for recrystallization.


Principles


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A effective recrystallization relies on the proper choice of solvent. The compound must be dissolve in the warm solvent and insoluble in the same solvent once it is cold. Because that the function of recrystallization, consider 3% w/v the dividing line between soluble and also insoluble: if 3 g the a compound dissolves in 100 mL the a solvent, that is thought about soluble. In selecting a solvent, the enlarge the difference in between hot solubility and cold solubility, the an ext product recoverable from recrystallization.

The rate of cooling determines the size and quality of the crystals: rapid cooling favors small crystals, and also slow cooling favors the expansion of large and generally purer crystals. The price of recrystallization is usually biggest at about 50 °C listed below the melting point of the substance; the maximum formation of crystals occurs at around 100 °C listed below the melt point.

Although the terms "crystallization" and "recrystallization" are occasionally used interchangeably, they technically refer to different processes. Crystallization refers to the development of a new, insoluble product by a chemistry reaction; this product then precipitates out of the reaction systems as an amorphous hard containing numerous trapped impurities. Recrystallization does no involve a chemistry reaction; the crude product is simply dissolved into solution, and then the problems are adjusted to permit crystals to re-form. Recrystallization produce a an ext pure final product. Because that this reason, speculative procedures that develop a hard product by crystallization normally include a last recrystallization action to provide the pure compound.


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Procedure


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Perform all actions in a fume hood to stop exposure come solvent fumes.

1. Selecting a Solvent

Place 50 mg that the sample (N-bromosuccinimide) in an Erlenmeyer flask.Add 0.5 mL of boiling solvent (water). If the sample disappear completely, the solubility in the cold solvent is too high to be a good recrystallization solvent.If the sample does no dissolve in the cold solvent, heat the test tube until the solvent boils.If the sample has actually not fully dissolved in ~ this point, add more boiling solvent drop-wise, until all of the solid dissolves. If that takes an ext than 3 mL to dissolve the sample in the hot solvent, the solubility in this solvent is most likely too low to do it a good recrystallization solvent.If the first choice of solvent is not a good recrystallization solvent, try others. If a single solvent that works cannot it is in found, shot a 2 solvent system.If girlfriend cannot uncover a suitable solitary solvent system, then a solvent pair might be necessary. When identifying a solvent pair, there room several an essential considerations 1) The first solvent should readily dissolve the solid. 2) The 2nd solvent must be miscible v the first solvent, yet have a much lower solubility for the solute.As a general preeminence "likes dissolve likes" meaning that polar compounds tend to be dissolve in polar solvents and non-polar compounds room often more soluble non-polar compounds.Common solvent pairs (Table 1)Make certain the solvent has actually a boiling point of at the very least 40 °C, so over there is a reasonable temperature difference in between boiling solvent and also room-temperature solvent.Ensure that the solvent has actually a boiling suggest below about 120 °C, for this reason it's less complicated to remove the critical traces that solvent indigenous the crystals.Also make certain the boiling allude of the solvent is reduced than the melting point of the compound, so the compound develops as heavy crystals quite than as an insoluble oil.Confirm that the impurities are either insoluble in the hot solvent (so they can be hot-filtered out, once the link is dissolved) or dissolve in the cold solvent (so lock stay dissolved during the entire process).

2. Dissolving the Sample in warm Solvent

Place the compound to be recrystallized in an Erlenmeyer flask. This is a better choice than a beaker, since the sloping sides help trap solvent vapors and slow the price of evaporation.Place the solvent (water) in a separate Erlenmeyer flask, and include boiling chips or a stir bar to keep it boiling smoothly. Heat it come boiling top top a hotplate.Add warm solvent come a flask at room temperature containing the link in tiny portions, swirling after every addition, until the compound is totally dissolved.During the dissolution process, store the solution hot at all times by resting it ~ above the hotplate, too. Execute not add an ext hot solvent than crucial - just sufficient to dissolve the sample.If a section of the solid does no seem come dissolve, also after much more hot solvent has been added, the is likely as result of the presence of an extremely insoluble impurities. If this happens, stop including solvent and also do a hot filtration prior to proceeding.To perform a warm filtration, fold a item of filter record into a fluted cone shape and also place it right into a glass stemless funnel.Add a 10-20% overabundance of warm solvent to the hot solution to permit for evaporation in the procedure.Pour the equipment through the paper. If crystals begin to form at any type of time throughout the process, include a small section of warmth solvent come dissolve them.

3. Cooling the Solution

Set the flask include the dissolved compound on a surface that does no conduct the warmth away too quickly, such as a paper towel collection on a benchtop.Lightly covering the flask as it cools to prevent evaporation and also to prevent dust indigenous falling right into the solution.Leave the flask undisturbed till it cools come room temperature.Once the crystals have formed, place the solution in an ice cream bath come ensure that the maximum lot of crystals is obtained. The solutions should be left undisturbed in the ice cream bath for 30 min come 1 h, or it rotates the compound shows up to have completely crystalized the end of solution.If no crystal development is evident, it can be induced by scratching the inside wall surfaces of the flask through a glass pole or by including a tiny seed crystal of the exact same compound.If this still stops working to work, climate too lot solvent was probably used. Reheat the solution, enable some the the solvent to cook off, then cool it.

4. Isolating and also Drying the Crystals

Set the cold flask include the newly created crystals on a benchtop.Lightly sheathe the flask to prevent evaporation and also to protect against dust native falling into the solution.Isolate the crystals by vacuum filtration, using either a Büchner or Hirsch funnel (clamp the flask to a ring stand first).Rinse the crystals ~ above the Büchner funnel through a tiny amount of fresh, cold solvent (the exact same solvent provided for recrystallization) to remove any impurities that may be sticking to the crystals.To dried the crystals, leaving them in the filter funnel and also draw air v them for several minutes. Crystals can additionally be air-dried by allowing them to stand uncovered because that several hours or days. More efficient methods include vacuum dry or put in a desiccator.

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Polar SolventLess Polar Solvent
Ethyl acetateHexane
MethanolMethylene chloride
WaterEthanol
TolueneHexane

Table 1. Common solvent pairs.