Recrystallization and Melting Point Determination

Con berthr what lapses when a clobber squ be is placed in a firmness in which it has a low solvability. Not much A elfin section of the trueheartedity manoeuvre oning discount, simply the rest get out that taunt there. (Actually, it doesnt just sit there since an equilibrium is in effect, with solid molecules going into termination, and an equivalent weight number of crashd molecules re building solid, but we see the same arrive of undissolved sensible). How keister we get the entire solid to dissolve? One elan is to arouse the ascendent close textiles atomic number 18 to a greater extent fat-soluble in cale featureory resultant role than in stale.Suppose we heat to the boiling temperature of the final result (the highest temperature which is practical), and the entire solid goes into solution. If we now let the solution cool, what will happen? Obviously, at close to temperature, the concentration of solute will exceed its solubility, and the solid will start to set out backrest out of solution it will crystallize. By the time we return to the initial temperature, nearly all of the buffer material has create crystals, which empennage be removed from the termination by filtration. Now, be resideve that the reliable solid contained small totalitys of impurities.These would go into solution as well, but since the impurities atomic number 18 present only in small amounts, they remain soluble when the temperature is razeed again. Thus, the crystals, which come out of solution, be more small than the original sample. This is the essence of purification by re lechatelierite (Fig. 1). pic Figure 1 An impure solid in (A) cold dissolvent in which it has low solubility, (B) solvent that has been change up to dissolve all of the solid, and (C) solvent that has cooled d possess to the original temperature.The crystalized solid is now pure, while the impurities remain in solution. Re lechatelierite Recrystallization is th e preferred order for purification of organic solids. The technique female genital organ be apply for gravid or small quantities of materials, and is unremarkably very effective and efficient. The roughly st prisegic aspect of the recrystallization technique is the selection of the solvent. A cock-a-hoop number of solvents argon familiarly apply for recrystallization of organic compounds. Among these ar water, ethanol, ethyl ether, and hexane to name just a few.A solvent, which works beautifully for one recrystallization, may be totally unsuitable for purification of a different compound the survival of the fittest for recrystallization solvent is made on a berth-by- character reference basis. This is be have got of the variation of solubility of different organic compounds in these solvents. We can identify a unbending of characteristics which are delectable for a recrystallization solvent 1. The substance to be purified must(prenominal)(prenominal) have a high solubility in the solvent at its boiling consign, and a significantly diminished solubility at lower temperatures (limited only by the freezing temperature of the solvent).Obviously, the first part is necessary so that you can get the material into solution, and the entropy part is required so that the purified compound will come back out of solution. 2. The solvent should dissolve the impurities at all temperatures. Actually, its OK if the impureness dissolves readily or doesnt dissolve at all, as well see. The important point here is that purification will be accomplished only if the impurities remain in solution as the mix is cooled.This isnt too hard to accomplish if the amount of impurity is small. Obviously, its a bit more difficult if a large amount of an impurity is present, but if the solubility characteristics of the desired substance are sufficiently different from the impurity, as is a lot the case, a suitable solvent can usually be found. 3. The solvent must not re act with the substance to be purified. Its hard to get your compound to come back out of solution when it is consumed by a reaction. 4. A reasonably volatile solvent is preferred.This devotion is dictated by the learn to ultimately remove traces of solvent from the crystals, and this is most easily accomplished by evaporation, either at room temperature or in an oven. Even with an oven to aid in the drying, only moderate temperatures can be utilisationd, since you should stay down the stairs the break up temperature of the solid, and definitely below temperatures where the material will decompose. Generally speaking, solvents with boiling temperatures below 150C are OK. 5. Ideally, the solvent should be inexpensive, non-toxic, and non-flammable.These are not absolutely required, but desirable characteristics, which can help to determine the choice of a solvent. Obviously, water is an sample choice based on these criteria, and is the preferred solvent if it satisfies conditions 1-4. How does one go about choosing the proper solvent? If you are dealing with a cognise compound, it makes sense to use solvents that have been used before. You can go to the CRC enchiridion of Chemistry and Physics (printed or online), and frequently a solvent suitable for recrystallization will be indicated.This may show up in devil places First, in the column headed color, crystalline form is oft listed the solvent used to obtain the crystals in question. Otherwise, you can move to the columns near the right-hand side of the page, which deal with solubility. The CRC has changed its presentation of solubility data over the years. For this purpose, the older editions (before 1980) are significantly recrudesce than the recent ones. Whereas the new editions simply list solvents in which the compound is soluble, the older ones provide more information, which can be profitable in selecting a crystallization solvent.Qualitative indicators for solubility in variant solvents rang e from i ( indissoluble), d (slightly), s (soluble), to v (very). Especially effective is the fact that some solvents are given two listings, one for hot solvent, the other for cold. What if you are working with a brand-new compound, which is ofttimes the case? Here, the only thing to do is try different solvents. The choice of solvents is oft more art than science everybody has their personal favorites based on their own experience, but those mentioned above are typical solvents to try. A small amount of the solid is placed in a test-tube and a few drops of cold solvent are added.If the solid dissolves, you can forget this solvent you need low solubility at low temperature. If the solid remains, so far, so good, but this is only the beginning. Now the sample is warmed to the boiling point. If the solid dissolves, things look good. The next test is to let this solution cool, and to see if small crystals actually form. If they do, youre in luck. promote tests with this solvent on larger samples of material can thence determine whether the desired degree if purification is achieved, and if the deviation of material is acceptable. Sometimes, though, no crystals form.This may be a problem with the rate of crystal nucleation that event where the first few molecules get together to form the growing crystal. A trick which often works in this case is to take a supply rod and rub vigorously the skirt of the test-tube just below the solvent surface. Often, this will stimulate crystal formation, presumably because some of the microscopic scratches serve as nucleation sites. Alternatively, the solution can be seeded with a few tiny crystals of the substance (if these are available), and then to chuck up the sponge the solution to stand undisturbed until crystal formation is complete.If no crystals form, you must consider the possibility that the substance is too soluble in this solvent, and the fact that it did not dissolve at first may simply be because it di ssolves very slowly. If this is the case, its back to the draftsmanship board with a new solvent. A not-uncommon result is for the solute to separate from solution as an oil which will not crystallize. This annoying situation arises particularly for low- thaw solids and frequently results when the solution becomes saturated at a temperature higher than the run point of the solute.A remedy which is often effective is to ensure that separation does not occur until the solution has cooled below this melt down temperature, either by using more solvent or by switching to a different solvent. Once this is settled, scratching and/or seeding are often helpful for obtaining crystalline harvest-festival. Quite often, a iodine solvent which gives suitable results cannot be identified. In such cases, the use of entangled solvents (solvent pair) is often successful. Two solvents are chosen, one which dissolves the substance readily, and another in which the solute is only sparingly soluble .Note that the two solvents must be tout ensemble miscible. The usual procedure is to dissolve the crude solid in the founder solvent at its boiling point, and to add the weaker solvent dropwise until a deliquium cloudiness persists, indicating that the solute is starting to come out of solution. A few drops more of the better solvent are then added to put the solute back into solution, and the commixture is allowed to cool to effect crystallization. Typical solvent pairs are water and ethanol, or hexane and benzene, where the weaker solvent is listed first.These mixed solvent systems are interesting it seems that the solubility characteristics of the better solvent dominate at higher temperatures, while the mixture behaves more like the weaker solvent at lower temperatures. Once the crystallization solvent is chosen, consideration can be given to other features of the crystallization process. The creative thinker is to maximize the recovery of purified compound. Enough solven t must be used to keep the impurities in solution, but if an excessive amount of solvent is used, a large part of the desired compound will remain in solution, and the recovery will be low.For this reason, the minimum amount of hot solvent which will dissolve the desired compound should be used. It is possible to get ahead improve the recovery of material by harvesting a second crop of crystals. After the purified crystals have been separated by filtration, the filtrate (called the engender liquor) is intemperate by evaporation of 1/2 2/3 of the solvent. On cooling, more of the compound will crystallize, and this can be isolated by filtration. The purity of these crystals is generally not as good as the original crop, since the concentration of the contaminants in the breed liquor is always increasing.The proper instrument for recrystallization is the Erlenmeyer flask. A beaker is not appropriate. The shape of the Erlenmeyer flask allows tumbleing of the mixture without slos hing, and the conical walls execute to condense and return refluxing solvent. The flask should not be overfilled about 60% of the listed capacity is the maximum, and so the flask volume should be chosen accordingly. Decolorizing a Solution and Hot Filtration Special problems are posed by insoluble impurities and highly colored, resinous contaminants.The best way to remove insoluble material is to interpenetrate the hot mixture, while the desired material is dissolved. Insoluble rubble will be trapped on the filter paper. The colored, resinous impurities are usually large, relatively polar organic molecules which have a tight tendency to be adsorbed on surfaces. Crystals grown from solutions containing such resins are often discolored by surface contamination by these materials, and it is consequently desirable to in like manner remove them from the solution before recrystallization.They can be taken do by of by addition of a small amount of powdered, activated wood coal (N orit or Darco, as well as decolorizing carbon) to the hot solution before filtering. Decolorizing carbon has a huge surface area, and readily absorbs the colored impurities. The hot filtration then removes the lignite grey the filtrate, now free of this contamination, is allowed to cool and deposit crystals. The decolorizing/hot filtration procedure requires some care. First, the addition of the powdered charcoal to hot solvent can cause a vigorous foaming which may result in personnel casualty of some of the solution.This can be obviateed if the hot solution is allowed to cool for 15-20 seconds, and the charcoal is added cautiously. Only a small amount of charcoal is required add too much and significant amounts of the desired compound will also be absorbed, and low recovery will result. After addition of the charcoal, the mixture should be heated to a gentle boil for a few minutes, to ensure that any compound which might have precipitated during cooling is brought back into solution. Filtration of the hot mixture must be done quickly to avoid crystallization of material on the filter paper or in the move.The setup depicted in Figure 2 shows how the filter can be kept hot during filtration. The speed of filtration is enhanced by the use of a fluted filter paper and a stemmed funnel. Fluting a filter paper is organic chemistry origami and the process is best demonstrated in person. A fluted paper uses the entire surface area of the filter paper, and therefore filters more rapidly than the conventional conical fold. The stem of a funnel is susceptible to clogging by formation of crystals, so a stemless funnel is used.To minimize crystallization in the filter paper it is common to place a small amount of solvent in the receiving flask, and to heat this to boiling during the filtration. The hot vapors help to keep the filter warm. In addition, it is prudent to use about 10% more than the minimum amount of solvent, to account for evaporation during the he ating, and to allow for the small amount of cooling which will occur. Finally, the mixture should be poured onto the filter in small portions which will filter within about 30 s, so that excessive cooling does not occur. The pic remainder of the mixture is kept heated until it can be poured on the filter.When all of the solvent has been filtered, it is a good idea to rinse the flask and filter with a small portion of fresh, hot solvent (5 10% of the volume already used). This will help to dissolve and pass through the filter any material which has crystallized. The filtrate should be undisturbed in an appropriately sized Erlenmeyer flask. Some crystals may have formed in the flask by the end of the filtration, but these are often very small and not well formed. They should be dissolved by warming the filtrate. The best crystals are obtained by slow cooling without fermenting of the solution.Therefore, the filtrate should be placed in a spot where it can remain undisturbed as it c ools. The mouth of the flask may be loosely stoppered, but this is normally not necessary. Crystal growth is normally well along by the time the mixture reaches room temperature. Further cooling in an ice-water bath will cause additional crystallization. Crystals come in a variety of shapes and sizes long needles, blocky prisms, and flat plates are common. Slow crystal growth not only gives the most harming display the product is generally purer. Too-rapid crystallization generally leads to smaller, more badly formed (and less pure) crystals. pic Vacuum Filtration Once crystal formation is complete, the product is isolated by vacuum filtration.This technique permits fast and efficient removal of the mother liquor. A special flat-bottomed funnel (a Buchner funnel) is used in conjunction with a heavy-walled Erlenmeyer flask with a sidearm called a filtering flask. The setup is illustrated below (Figure 3). A circle of filter paper, small enough to lie completely flat on the perfor ated base of the Buchner funnel, but large enough to cover all of the holes, is placed in the funnel, and moistened with a small amount of solvent to hold it in place.With the vacuum on, the cold crystallization mixture is poured as rapidly as possible onto the filter paper. It helps to swirl the flask a few times to loosen the crystals, and with a belittled bit of skill the entire mixture can be deposited on the filter, with only a few stray crystals adhering to the walls of the flask. The mother liquors are rapidly drawn into the filter flask by suction, and the crystals are squeezed dry of solvent by pressing with a clean cork while drawing air through them. Now, this procedure does not remove all of the mother liquor. Invariably, there is a film of solvent coating the surface of the crystals.Since the mother liquor contains impurities, this must be washed away with fresh solvent. The crystals are washed as follows the suction is stopped and enough fresh, cold (why? ) solvent is added to just cover the crystals in the funnel (this solvent can be used to rinse the closing curtain few crystals out of the crystallization flask). The crystals should be stirred gently with a glass rod or a spatula so that all come in contact with the wash liquid, then the vacuum is restarted and the crystals sucked and pressed dry as before. In some cases, it may be necessary to repeat the wash a 2nd time. aviation is drawn through the crystals until they are as dry as possible, at which time they are spread on a watch glass for further drying in the air or in an oven. The steps in recrystallization can be summarized 1. Dissolve the crude material in a minimum amount of hot solvent. 2. Treat with decolorizing charcoal. 3. Filter hot solution to remove charcoal and other insoluble impurities. 4. Cool to effect crystallization 5. Filter crystals, wash to remove adhering mother liquor. 6. Dry crystals to remove last traces of solvent. Melting Point Determination Once we have pur ified a solid, how do we establish that the product is, in fact, pure?Undoubtedly, the simplest criterion for purity is purpose of the melting point, or more accurately, the melting range. Strictly speaking, the melting point is defined as the single temperature at which the vapor pressure of the liquid phase of a compound is equal to the vapor pressure of the solid phase, and so the liquid and solid are in equilibrium. These values are actually rather difficult to measure. Much simpler is the determination of the capillary melting range, which, as the name implies, is the temperature interval over which the compound is found to melt.Since, during the melting process, the system is not at equilibrium, this is not identical to the true melting point, but the values are normally very similar. In fact, most of the melting points in the literature are actually capillary melting point ranges. An important distinction is that the latter is always presented as a range of two temperature r eadings, from the temperature where melting is first observable to that where melting is complete. For pure compounds, complete melting normally occurs over a very nail down range, often