What do you know about recrystallisation experience and methods?

Crystallisation process control and product property regulation

Recrystallisation is a learning process. The choice of solvent (single or combined), crystallisation temperature, stirring speed, stirring method, choice of supersaturation, time for raising the crystals, the way and rate of solvent addition, etc. will all affect the purity and crystal shape of the crystals.

Principles of Recrystallisation Purification

The method of recrystallisation takes advantage of the fact that the components of a solid mixture are separated from each other by their different solubilities in a certain solvent. The simple procedure for recrystallisation is to dissolve the impure solid substance in a suitable hot solvent to make a nearly saturated solution, filter it while it is still hot to remove insoluble impurities, cool the filtrate so that the crystals precipitate out of the supersaturated solution and the soluble impurities remain in the mother liquor, pump and filter the crystals out of the mother liquor, dry it and then determine the melting point, such as the purity of the product is still not in accordance with the requirements, can be recrystallised again until the purity of the product meets the requirements. If the purity still does not meet the requirements, recrystallisation can be carried out again until it meets the requirements.

 

Crystallisation and recrystallisation detailed operating procedures

1. Dissolve the chemical reagent to be purified in a suitable solvent that is boiling or will come to a boil;
2. Filter the hot solution while it is still hot to remove insoluble impurities;
3. cooling the filtrate so that the crystals precipitate;
4. Filter out the crystals, if necessary, with a suitable solvent to wash the crystals

 

1, screening solvent: add a small amount in a test tube (the size of wheat grains) to be crystallised, add 0.5 mL of solvent selected according to the above rules, heating and boiling for a few minutes to see whether the solute is dissolved. If dissolved, use tap water to flush the test tube outside the test to see if there are crystals precipitated. Beginners often take the insoluble impurities as to be crystallised! If there are still insoluble substances even after prolonged heating, you can leave the test tube for a few moments and cool the test tube with cold water (do not shake). If the substance precipitates in the upper layer of the clear liquid, it means that some more can be dissolved. If slightly turbid, that the solvent solubility is too small; if there is no change, that insoluble solids are a thing, has been soluble substances and very soluble, not easy to precipitate.

2, routine operation: in a conical flask or round bottom flask 52A0 into the solute and a certain solvent, mounted on the ball cold, heated for 10 minutes, if there is still insoluble material, continue to make up from the condenser tube on the mouth of the solvent to complete dissolution and then add an excess of 30% solvent. Folded filter paper (folded filter paper and triangular funnel to be preheated in advance) while hot filtered into a conical flask. The filtrate cools naturally and is drawn through a Büchner funnel (rinse the conical flask with the filtrate in reverse!). . If the substance has little solubility at room temperature, the filter cake can be washed with a small amount of cold solvent (first withdraw the reduced pressure, add a small amount of solvent to wet the filter cake, and then pump it dry under reduced pressure. (Note: compacting the filter cake with a glass stopper helps to get rid of more solvent!) . If the solvent used is not volatile, the cake can be drenched with a small amount of volatile solvent at atmospheric pressure, e.g. DMF can be washed with ethanol, dichlorobenzene, chlorobenzene, xylene, cyclohexanone can be washed with toluene. Beginners often encounter problems: a large number of crystals precipitated on the filter paper, the reason is that the funnel and filter paper preheating is not good, too little solvent overdose, filtration time is too long. If the product is valuable, the triangular funnel and filter paper can be placed in a conical flask with steam preheating, while filtering with the filtrate has been filtered steam insulation, but the above operation is more dangerous, toluene, ether, petroleum ether, cyclohexane and other flammable solvents should be careful to use this method. Note: hot recrystallisation mother liquor to wash the filter paper and all the adherent solute apparatus and cooling can reduce crystalline loss.

3, the paradoxical operation of hot filtration: suction filter bottle can not be preheated, Brinell’s funnel and filter paper in the solute on the top of the conical flask to use the rising vapour wetting, put on the suction filter bottle and immediately while hot filtration. Note that the pumping pressure can not be too large to prevent the mother liquor in the suction filter bottle boiling! Common mistakes made by beginners: the filter paper is not tightly laminated (double-layered paper can be used), slow movements cause crystals to precipitate in the Büchner funnel, too high a pumping pressure causes the filtrate to be sucked into the pump, and a large amount of solvent is pumped into the pump because the pressure is not removed immediately after the filtration is completed.

In short, contrary to “similar solubility”, it is right to use medium-polar solvents to crystallise things of large polarity, and large-polar solvents to crystallise things of small polarity. In this way, more than half of the cases are suitable.

 

1. Try first: petroleum ether (hexane), ether, ethyl acetate, ethanol, water, and then: acetone, methanol, acetonitrile, benzene, chloroform, acetic acid, pyridine and so on. If that doesn’t work, you’ll have to mix them. Ether can be used for its (1) volatility and (2) property of precipitating solids by extending the glass upwards. Acetone, if not with water, should be dried.

2. Mixed solvent method: dissolve with excess hot good solvent, filter, heat, slowly add bad solvent until cloudy, heat until clarified. Let it stand and wait.

3. By graded crystallisation method. Accumulated mother liquor over column.
1) over the column pre-purification, crude separation and then crystallisation; 2) petroleum ether hot extraction – cold precipitation method; 3) choose a low-boiling point solvent such as ether; 4) the acquisition of crystalline species, with a glass rod to dip a drop of the solution, volatilised dry. 5) do not freeze easily, with the method of letting the solvent volatilise naturally.

On crystallisation with ether. When refluxing ether, add a condenser tube. Keep adding ether from the upper port until the turbidity disappears, sometimes it is because the dissolution is slower, not insoluble, so be patient. If a lot of ether is added and there is still a small amount of precipitate that is insoluble, it is filtered off and the filtrate is concentrated until a solid precipitates, then heated again and a small amount of ether is added to clarify it. Let cool naturally, and crystals of good crystalline form may be obtained. Filter. Wash the crystals with a small amount of ether. The washings are combined into the mother liquor, and the mouth of the bottle containing the mother liquor is covered with a layer of filter paper, or stuffed with a ball of tissue paper, to allow the ether to evaporate naturally without falling into dust. Look at it every morning until a satisfactory number of crystals come out, and don’t be too greedy, you’ll have to start all over again if it evaporates and dries up 🙂 I have used this method to successfully split both levo- and dextro-alkaloids. The “petroleum ether hot extraction – cooling method” is also the method I use to deal with the oil, add petroleum ether, boiling, pour out the supernatant, the bottom oil continue to add petroleum ether hot extraction, until the petroleum ether layer is colourless, then the extraction is basically complete. Crystals are usually precipitated upon cooling.
Another reason for becoming oil is cooling down too quickly.

 

 

A few problems need to pay attention to

1. When dissolving the pre-purified chemical reagents, we should strictly abide by the laboratory safety regulations, and when heating flammable and explosive solvents, we should operate in an environment without open flames, and we should avoid direct heating. Because the solubility curve rises steeply near the boiling point of the solvent under normal circumstances, the solvent should be heated to the boiling point during crystallisation and recrystallisation. In order to make the crystallisation and recrystallisation yield is high, the amount of solvent is as small as possible, so in the beginning the amount of solvent added is not enough to dissolve all the chemical reagents that want to be purified, in the process of heating can be carefully added solvent, until the boiling of the solid material is completely dissolved. Be careful when adding solvent, the solution is cooled to below its boiling point, explosion-proof zeolite is not effective, you need to add a new zeolite.

2. In order to quantitatively evaluate the operation of crystallisation and recrystallisation, as well as to facilitate repetition, solids and solvents should be weighed and measured.

3. In the use of mixed solvents for crystallisation and recrystallisation, it is best to dissolve the chemical to be purified in a small amount of a solvent of greater solubility, and then slowly add the second solvent of lesser solubility in small portions while still hot, until it touches the part of the solution where the precipitate is produced but dissolves immediately. If the total volume of the solution is too small, add more of the more soluble solvent and repeat the procedure. Sometimes the opposite procedure can also be used, the chemical reagent to be purified will be suspended in the solvent of small solubility, slowly add the solvent of large solubility, until dissolved, and then drop into a small solvent of small solubility to be cooled.

4. If necessary, you can add activated carbon for decolourisation after the chemical reagent to be purified is dissolved (the dosage is equivalent to about 1/50~1/20 of the weight of the substance to be purified), or add filter paper pulp, diatomaceous earth and so on to make the solution clarified. The solvent should be cooled slightly before adding the decolourising agent, as the addition of the decolourising agent may automatically trigger a previously inhibited boiling, resulting in a violent and explosive flash boil. Activated carbon contains a large amount of air and is therefore capable of foaming. The activated carbon may be boiled for 5-10 minutes after addition, and then the activated carbon may be removed by filtration while still hot. In non-polar solvents, such as benzene, petroleum ether in the activated carbon decolourisation effect is not good, you can try other methods, such as adsorption with alumina decolourisation.

5. want to purify the chemical reagents for organic reagents, the formation of supersaturated solution tendency is very large, to avoid this phenomenon, can be added to the same kind of reagent or homogeneous homocrystalline species of crystals. Rubbing the wall with a glass rod can also form a nucleus, after which the crystals will grow along this core.

6. The rate of crystallisation is sometimes very slow, and the crystallisation of cold solutions sometimes takes several hours to be complete. In some cases, crystals will continue to precipitate weeks or months later, so the mother liquor should not be discarded too soon.

7. In order to reduce the solubility of the reagent to be purified in solution, so as to precipitate more crystals and increase the yield, the solution is often frozen. Can be placed in a refrigerator or cooled with ice, mixed refrigerants.

8. The prepared hot solution must be filtered to remove insoluble impurities, and it must be avoided crystallising out on the filter during filtration. If all operations are regular, really because the reagent is too easy to precipitate crystals and hinder the filtration, the solution can be prepared slightly dilute, or the use of insulation or heated filtration device (such as insulated funnel) filtration.

9. In order to effectively separate the precipitated crystals from the mother liquor, filtration is generally carried out using a Brinell’s funnel. In order to better separate the crystals from the mother liquor, it is best to squeeze the crystals through the Brinell’s funnel with a clean glass stopper and remove as much of the mother liquor as possible by pumping. The mother liquor on the surface of the crystals may be washed with as little solvent as possible. In this case, the pumping should be stopped temporarily, and a glass rod or stainless steel knife should be used to loosen the pressed crystals, add a small amount of solvent to moisten them, and wait for a few moments, so that the crystals can be uniformly impregnated, and then pumped dry again, and this should be repeated for one or two times, so as to make the mother liquor attached to the impregnated surfaces all be removed until it is removed.

10. If the crystals do not decompose when exposed to heat, it can be dried by heating in the oven. If the crystals are easy to decompose with heat, the temperature of the oven should be careful not to be too high, or put in a vacuum drier to dry at room temperature. If recrystallisation is carried out with solvents having a high boiling point, the crystals should be washed with solvents having a low boiling point and a small solubility to facilitate drying. The crystals which are easy to be deliquescent should be heated to a certain temperature in the oven first, and then put the crystals into it; however, the crystals which are very easy to be deliquescent can not be baked in the oven, and they must be put into the vacuum desiccator for drying quickly. With flammable organic solvents recrystallised crystals in the oven before sending, should be dried in the air, otherwise it may cause solvent combustion or explosion.

11. small and trace amounts of substances recrystallised: small amounts of substances crystallised or recrystallised with the basic requirements of the aforementioned, but are used with the amount of the substance appropriate to the small container. Crystallisation and recrystallisation of trace substances may be carried out in small centrifuge tubes. The hot solution is centrifuged immediately after preparation, so that the inadmissible impurities settle to the bottom of the tube, and the upper layer is pipetted to another small centrifuge tube to crystallise. After crystallisation, the crystals are separated from the mother liquor by centrifugation. The crystals can also be washed with a small amount of solvent in the centrifuge tube, and the solvent can be separated from the crystals by centrifugation.

12. The mother liquor often contains a certain amount of the desired substance, which should be recovered with care. If the solvent is partially removed and then allowed to cool so that the crystals precipitate, they are usually not as pure as the crystals precipitated for the first time. If the purity check does not meet the requirements, can be used to fresh solvent crystals, until it meets the purity requirements.

 

Selection of solvent

Select the appropriate solvent for the success of the recrystallisation operation is of great significance, a good solvent must meet the following conditions:
1, no chemical reaction with the purified material
2, in the higher temperature can dissolve a large amount of the purified material and at room temperature or lower temperature can only dissolve a small amount;
3. The solubility of impurities is very large or very small, the first case of impurities remain in the mother liquor, the latter case when the impurities are filtered out while hot;
4. The boiling point of the solvent should not be too low, nor too high. The boiling point of the solvent is too low when made into a solution and cooling crystallisation of the two-step operation of the temperature difference is small, the solubility of the group material does not change much, affecting the yield, and low boiling point solvent operation is not convenient, such as ether should not be used. Solvent boiling point is too high, the solvent attached to the surface of the crystal is not easy to remove.

5. Can give better crystallisation.

In several solvents are applicable, should be based on the crystallisation of the recovery rate, the ease of operation, the size of the toxicity of the solvent and whether or not flammable, the price is high or low and so on the selection of the best.

 

1、Commonly used solvents: DMF, chlorobenzene, xylene, toluene, acetonitrile, ethanol, THF, chloroform, ethyl acetate, cyclohexane, butanone, acetone, petroleum ether.

2, more commonly used solvents: DMSO, hexamethylphosphoramide, N-methylpyrrolidone, benzene, cyclohexanone, butanone, cyclohexanone, dichlorobenzene, pyridine, acetic acid, dioxane, ethylene glycol monomethyl ether, 1,2-dichloroethane, ethyl ether, n-octane.

3, a good solvent near the boiling point to treat the crystalline material solubility is high and at low temperatures the solubility is very small. DMF, benzene, dioxane, cyclohexane at low temperatures close to the freezing point, the solubility is very poor, is the ideal solvent. Acetonitrile, chlorobenzene, xylene, toluene, butanone, ethanol are also ideal solvents.

4, the boiling point of the solvent is better than the melting point of the crystalline material is lower than 50 ℃. Otherwise, it is easy to produce solute liquefaction stratification phenomenon.

5, the higher the boiling point of the solvent, the stronger the solvency when boiling, for high melting point substances, it is best to choose a high boiling point solvent.

6, containing hydroxyl, amino and melting point is not too high as far as possible not to choose oxygenated solvents. Because the solute and the solvent form intermolecular hydrogen bonding is difficult to precipitate.

7, containing oxygen, nitrogen substances try not to choose alcohol as a solvent, for the same reason.

8, the solute and solvent polarity should not be too different. Water>Formic acid>Methanol>Acetic acid>Ethanol>Isopropanol>Acetonitrile>DMSO>DMF>Acetone>HMPA>CH2Cl2>Pyridine>Chloroform>Chlorobenzene>THF>Dioxane>Ether>Benzene>Toluene>CCl4>Octane>Cyclohexane>Petroleum ether.

 

 

 

About the precipitation of crystals

Crystallisation conditions: mainly temperature, pressure, stirring or not. Temperature is very important, generally we are low-temperature refrigeration, in fact, sometimes need high temperature insulation! This mainly needs to find out the relationship between its solubility in determining the crystallisation temperature. Stirring is also an influential factor, he has an impact on the crystallisation of the crystal form, the speed of crystallisation.

 

Selection of gradient cooling conditions on the crystalline form and yield also has a greater impact.

There is also the timing of the addition of crystal seed: crystal seed added too early, the crystal seed dissolved or produced by the crystalline form is generally fine; added late, the solution may have produced the nucleus, resulting in crystallisation may be wrapped in impurities

 

When the filtrate obtained by filtration is cooled, the crystals will precipitate. Rapid cooling with cold water or ice water and stir the solution vigorously, you can get very small crystals, will be a hot solution in the air temperature conditions to make the slow cooling, you can get a uniform and larger body.

 

If the solution is cooled and the crystals are still not precipitated, can be used to glass wipe Mo control the container wall under the liquid level, can also be added to the species, or to further reduce the temperature of the solution (cooled with ice water or other frozen solution).

 

If the solution is not precipitated after cooling the product and get oil, can be re-heated, to the formation of a clarified hot solution, let it cool itself, and constantly stirring the solution with a glass rod, rubbing the walls of the vessel or casting varieties, in order to accelerate the precipitation of the product. If there is still an oily substance begins to tiki out, it should be immediately stirred vigorously to make the oil droplets dispersed.

 

 

Determine the purity of the crystalline method

Physical and chemical properties are uniform; solid compound melting distance ≤ 2 ℃; TLC or PC unfolding is a single spot; HPLC or GC analysis is a single peak, the NMR spectrum has no burrs.

 

Certain products made more, can be based on experience, such as the sample after repeated recrystallisation, see the kind of crystal type should appear, according to the previous test results, its purity should be about eight or nine.

 

 

Crystallisation tips

 

1. Preparation of crystals, pay attention to the selection of suitable solvents and the application of the right amount of solvent. Suitable solvents, preferably in the cold on the required components of the solubility is small, while the heat when the solubility is large. The boiling point of the solvent should not be too high. Generally used methanol, acetone, chloroform, ethanol, ethyl acetate vinegar and so on. However, some compounds are not easy to form crystals in general solvents, while in some solvents are easy to form crystals.

2. Preparation of crystalline solution, need to become supersaturated solution. Generally, the appropriate amount of solvent should be applied to dissolve the compound under the condition of heating and then placed in a cold place. If the crystals can be precipitated at room temperature, it is not necessary to place them in a refrigerator to avoid precipitating more impurities along with the crystals. The “new ecological” substances, i.e. the newly free substances or amorphous powdery substances, are much more soluble than the crystalline substances and can easily form supersaturated solutions. Generally refined compounds, in the evaporation of solvent extraction for the amorphous powder is so, sometimes as long as the addition of a small amount of solvent, often immediately can be dissolved, a little time to precipitate crystals.

3. Preparation of crystalline solution, in addition to the choice of a single solvent, but also often use a mixture of solvents. Generally is the first compound dissolved in soluble solvents, and then add the appropriate amount of insoluble solvents dropwise at room temperature, until the solution is slightly turbid, and this solution is slightly warmed, so that the solution is completely clarified and placed.

4. Crystallisation process, generally a high concentration of solution, cooling knack, precipitation of crystals faster. But the crystalline particles are smaller, impurities may also be more. Sometimes from the solution precipitation rate is too fast, more than the formation of the compound nuclei persuade the speed of molecular orientation, often only get amorphous powder. Sometimes the solution is too thick, viscosity is not easy to crystallise. If the concentration of the solution is appropriate, the temperature is slowly reduced, it is possible to precipitate larger crystals and higher purity crystals. Some compounds require a longer time for the formation of their crystals.

5. In addition to the preparation of crystals should pay attention to the above points, in the process of placing, it is best to plug the bottle tightly, to avoid the liquid surface first appeared crystals, resulting in lower purity of crystals. If no crystals precipitated after a period of time, you can add a very small amount of seed crystals, i.e., the same compound crystallised tiny particles. The addition of seed crystals is a common and effective means of inducing the formation of crystal nuclei. Generally speaking, the crystallisation process is highly selective, and when the same molecule or ion is added, the crystals tend to grow immediately. Moreover, if the solution is a mixture of optical isomers, the same optical isomer can be precipitated preferentially according to the nature of the seed crystal. There is no seed crystal, can be dipped in a glass rod over a drop of saturated solution, in the air and let the solvent evaporate, and then used to rub the inner wall of the container at the edge of the solution, in order to induce the formation of crystals. If there is still no precipitation of crystals, you can open the bottle stopper to allow the solution to gradually dissipate, slowly precipitate crystals. Or choose another appropriate solvent treatment, or refining once more, as far as possible to remove all impurities after crystallisation.

6. In the preparation of crystals, it is best to form a batch of crystals, immediately after the formation of the upper layer of solution, and then left to get a second batch of crystals. The crystalline material can be crystallised and refined again by dissolving it in a solvent. This method is called recrystallisation. Crystallisation after recrystallisation of the mother liquor obtained from the various parts, and then processed to obtain the second and third batch of crystals, respectively. This method is known as step-by-step crystallisation or graded crystallisation. Crystalline substances in the repeated crystallisation process, the precipitation of crystals is always faster and faster, the purity is also higher and higher. Step-by-step crystallisation method of the crystallisation of the parts obtained, the purity of which there are often large differences, but often more than one crystalline component can be obtained, in the absence of checking do not hastily mixed together.

7. The crystallization of compounds have a certain crystalline shape, colour, melting point and melting distance, one can be used as a preliminary basis for identification. This is a non-crystalline substance does not have the physical properties. The shape and melting point of compound crystals often vary depending on the solvent used. Protopine base in chloroform to form prismatic crystals, melting point of 207 ℃; in acetone, the formation of hemispherical crystals, melting point of 203 ℃; in chloroform and acetone mixed solvent is the formation of the above two crystal shape of the crystals. Therefore, the literature often in the crystal shape of the compound, melting point, followed by the indication of the solvent used. General monomer pure compound crystallisation of the melting distance is narrower, sometimes required in the 0.5 ℃ or so, if the melting distance is longer, it means that the compound is not pure!

 

 

The main branches of modern crystallography

 

(1) crystal generation (crystallogeny): the study of natural and artificial crystals, growth and change of the process and mechanism, as well as the factors that control and influence them.
(2) Geometrical crystallography: the study of the shape of geometrical polyhedra on the exterior of crystals and the regularity between them.
(3) Crystallography: the study of the regularity of the arrangement of plasmas in the internal structure of crystals and the imperfections of the crystal structure.
(4) crystallochemistry: the study of the chemical composition of crystals and their structure, as well as the regularity of the relationship between the physical and chemical properties of crystals.
(5) Crystallophysics: The study of the physical properties of crystals and the mechanisms that produce them.