Aim: To look at the solubility of impure acetanilide in thethree solvents. Then to select an appropriate recrystallizing solvent. After, separateand purify acetanilide from a mixture of a recrystallization. The final aim isto weight the purified solid product of acetanilide and calculate thepercentage recovery. IntroductionImpure substances have some impurities that usuallycontaminate organic compounds that have been synthesized in the laboratory orisolated from natural sources.
Recrystallization is a purification processwhich Is used to remove impurities from organic compounds that are solid atroom temperature. This procedure is based on the evidence that the solubilityof a compound in a solvent increase with temperature. On the other hand, thesolubility of the compound decreases as the solution cools down and thecrystals is formed. Selecting an appropriate recrystallizing solvent to use is themain step of purification and recrystallisation. When choosing arecrystallizing solvent, the primary consideration is the degree to which thecompound and impurities are soluble in the solvent at high and lowtemperatures. The compound that is recrystallized has to be very soluble in thechosen solvent at high temperatures, but just about insoluble in the coldsolvent. Impurities should be soluble inthe chosen solvent at all temperatures so that impurities stay in solution butit should be insoluble at all temperatures so they can be filtered from the hotsolutions. The purification of solids by recrystallisation depends onthe differences in their solubility in a given solvent or mixture of solvents.
The method is to dissolve the solid in some suitable solvent at or near theboiling point, then to filter the hot solution to remove insoluble impurities(when necessary). This will allow the hot solution to cool which causes thedissolved substances to crystallise out. Then crystals can be separated fromthe supernatants solution. Finally, crystals can be dried out.A suitable solvent is one which will dissolve the solidreadily when hot, but only to a relatively small amount when cold. It should yieldwith well-formed crystals of the purified compound and it must be able toeasily from the crystals of the purified compound.
The solution has to be filtered by gravity filtrationthrough a funnel containing a fluted filter paper to remove any insolublecompound. Using a fluted filter paper, it increases the surface area inside thefunnel and speeds the filtering process. Vacuum filtration is the besttechnique for separating the crystals from the mother liquor, or a solvent. Invacuum filtration, a receiver flask with a sidearm, called a filter flask, isconnected by heavy-walled vacuum tubing to a vacuum source.
A Buchner funnel isfitted to the filter flask with a rubber stopper or filter adapter. A typicalgravity filtration apparatus is shown in Figure 2. figure 2 Percentrecovery is calculated by: dividing the mass of the recrystallizedcompound by the mass of the crude compound beforerecrystallization.
AssessingPurity of a recrystallized compound is assessed by observing its colour: If the compound has an off-white colour, the compoundshould again be recrystallized using activated carbon. If the compound is purewhite it is purified. Apparatus:Ø Conicalflask Ø Watch-glassØ Suctionfiltration apparatus Method1. Selection of a solvent· The solubility of acetanilide in methanol, water, and lightpetroleum (80-100°C boiling fraction) should be examined by carrying out thetest tube experiments (The results of these experiments are reported in thetable). By examining the result that’s obtained and by reading the experimentalprocedure, select an appropriate solvent for the recrystallisation ofacetanilide.
· Place acetanilide (approximately 0.1g) in three test tubes andeach of the possible solvents should be added drop by drop, with continuousshaking, to one of these test-tubes. · After, 1 cm3 of each solvent had been added, the solid-liquidmixtures are then heated to boiling using a water bath for the organicsolvents.· If the acetanilide dissolvedin the hot solvents, the tubes has to be cooled to determine whethercrystallisation occurred.
The results obtained are shown below for the threesolvents. Recrystallisationof acetanilide· First, weigh out commercial acetanilide (4g) into a 250 cm3conical flask. Add enough amount of solvent (water) to wet the crystals.
Boilthe mixture for a few minutes for acetanilide to dissolve. If the acetanilidedoes not dissolve, slowly add a few more cm3 of solvent and boil again for afew minutes. Repeat this procedure until the solid has dissolved.· Allow the solution to cool slowly by covering the conical flaskcontaining the hot filtrate with a watch glass. When the solution has cooled substantiallyplace the flask in an ice-bath to aid the crystallisation process.
· When this is complete, separate the solid from the mother liquorby vacuum (suction) filtration. Wash the crystals twice with portions (5 cm3)of cold solvent. Maintain a vacuum on the system until the solid is dry.· For solid to dry rapidly,place the crystals on a watch glass in a vacuum oven, providing the temperatureof the latter is below the melting point of the solid.
· Finally, Weigh the product and calculate the percentage recoveryand place it in a properly labelled sample bag and hand in to the laboratorysupervisor.The melting point is not required. EvaluationIn this experiment the aim was to use a technique calledpurification by recrystallization to carry out purification of acetanilide.Then the percentage recovery of acetanilide was to be calculated. Our resultwas 88% of recovered acetanilide as shown in the report form. In this experiment we have successfully obtainedthe amount of dried crystal which was further used to calculate the percentagerecovery of acetanilide. The experiment showed that the suitable solvent has tobe used to dissolve the solute so recrystallisation can be carry out.
Acetanilide issoluble in methanol and less soluble in light petroleum. We observe thetemperature has influence in its solubility because acetanilide is more solublein hot water. Even after a solid has been recrystallized, it was still not pureas the result we achieved was 88%. This could have been influenced due to somehuman errors and because of acetanilide containing some insoluble impurities. Thechoice of solvent was the most important step in the process.
The right solvent had to be selected to forma product of high purity and good recovery. Water was chosen as the bestsolvent because when cooling, out of the three solvents water dissolvedcompletely and the most crystals were formed. Furthermore, water is the mostsuitable solvent due to it having the best polarity hence giving it theproperty of best solvent. Polar or ionic compounds are surrounded by watermolecule. The positive dipoles of hydrogen in water are attracted to negativelycharged components of the solute and vice versa for the negative dipoles. Thehydrogen bonds in water are formed and then broken therefore liquid water containspartly ordered structure. Furthermore, water was chosen because at roomtemperature the acetanilide appeared to have low solubility when it was inwater, and when the acetanilide was heated to boiling in water it seemed tohave high solubility. It is also nontoxic and not flammable.
Accuracyand precision of the measurements taken in the experiment can influence thedata obtained. There are a number of ways in which the sample can be lostthroughout the experiment. The sample could have been lost in a transfer stage.
For example, while transferring the sample to the conical flask from the weighingboat. Some of the sample may have remained in the weighing boat as it isunlikely you can get a 100% transfer. Furthermore, the sample was not 100% assome of it could have been lost when it was transferred from the conical flaskto the Buchner funnel when washing the conical flask using distilled water. As wellas this, a part of the sample may have been trapped to the pot and the vacuum whenthe crystals were transported into the glass plate. We also used the filterpaper to filter the acetanilide crystals. This means some of the residue maybehave remained on the filter paper resulting in less sample. Experimental errorin the results may have been due to the use of inaccurate glassware,calibration of measuring equipment which all a have % error.
There is asystematic error for each of the measuring devices such as the scale had asystematic error of ±0.0002g. it contained a Percentage error (scale) =(0.0002/3.52) x 100 = 0.0057 %. This will influence the percentage of recovery.In addition, the data could be influenced due to human errors such asmisjudging the colour of the crystal.
Infuture if the whole experiment was to be repeated, the errors that can beconnected with volumetric glass accuracy can be adjusted for by carefulcalibration of the glassware. we can minimalize errors using A class volumetricglass. We can also minimalize errors by weighing the sample at the end morethan three times and working out the average. This will make sure togive an accurate percentage recovery. ConclusionInconclusion, water was found to be the most suitable solvent to dissolveacetanilide out of the three solvents in investigation. The yield of crystalsformed was 3.
52 grams. The percentage recovery was not 100% but 88%. Thissuggests that in acetanilide there are there insolubleimpurities and due to some human errors and we might have lost 12% from theoriginal substance. Reportform EXPERIMENT 2 Purificationof Acetanilide by Recrystallisation 1.
Solvent selected Solvent selected for recrystallisation ofacetanilide was: Distilled water 2. Details of Recovered Acetanilide Amount obtained: 3.52 grams % Recovery: 88 % Calculation: Product appearance: whitecrystals Questions 1. Why should the use of diethylether as a solvent for recrystallisation be avoided wherever possible?Ø The use of diethyl ether as a solvent forrecrystallisation should be avoided because the boiling point of diethyl etheris very low (34.5 0 ) hence it evaporates quickly.
The cooling thattakes place during evaporation causes water from the atmosphere to condense on itssurface which would lead to crystal being wet. Furthermore, when filtering thepurified organic crystals “ice crystals” would be formed on thefilter paper when the diethyl ether evaporates. 2. Would you expect a polar substance to be more soluble inpolar solvents or non-polar solvents?Ø Thepolar substance is expected to be more soluble on a polar solvent because moleculesmix readily as both types of molecules engage in hydrogen bonding. Since theintermolecular attractions are roughly equal, the molecules can break away fromeach other and form new solute. 3. When would you use mixed solvents for recrystallisationof a compound?Ø Wewould use mixed solvent for recrystallisation If no solvent can be found inwhich the solute is relatively insoluble in the solvent at room temperature butis lot more soluble in the solvent at higher temperature.
Mixture of solventcan be used If the compound that is being purified is freely soluble in onesolvent and practically insoluble in another solvent. If two solvents in whichthe solute has different solubility characteristics are mixed, an appropriatesolvent can be found. For instant, if you dissolve a compound in warm solvent A,in which it dissolves very easily. Then you begin to add warm solvent B to thesolution, a little at a time, until the mixture starts to cloud.
This is thepoint where the compound is at the limit of its solubility in the mixture. If thesolution is now set aside and allowed to be cooled, pure crystals of yourcompound will form and can be filtered off. The idea is that the impuritieswill remain in solution. Using a single solvent may need excessive solvent todissolve a sample and if there are impurities, they can be difficult todistinguish from an insoluble product. 4.
Name one other way of purifying compounds. Give a briefexplanation of the principles behind it.Ø Distillationis another way of purifying compound. It is used to separate two or moreliquids that are miscible. Distillation is s method which is designed toseparate a mixture containing two or more liquids which has different boilingpoints.
The process includes heating themixture in a flask. One compound (compound A) will have different boiling pointto another compound (compound B). The compound with the lower boiling pointevaporates first, e.g. compound A. This vapour is then cooled and condensedinside the condenser to form a pure liquid.
The thermometer shows the boilingpoint of the pure liquid of compound A. When all the compound has evaporated from thesolution, the temperature rises and the compound B evaporates. Apparatus andthe steps are shown below: