Doreen Foy Organic Chemistry 1 Laboratory The Recrystallization of Benzoic Acid and Determination of its Melting Point Hannah Loch 10/01/2012
Abstract The goal of the experiment was to obtain pure benzoic acid crystals from crude benzoic acid by using the laboratory techniques of recrystallization. Water was used as the suitable solvent which was heated using a heating mantle to dissolve the crude benzoic acid crystals. Charcoal was used to remove any colored impurities and it was filtered out together with any insoluble impurities to obtain a clear benzoic acid filtrate. The clear filtrate was cooled on an ice bath for pure benzoic acid crystals to precipitate out of the solution. The percent yield of the crystals was calculated to be 70.8%. The melting point of the benzoic acid crystals was measured (120.2 C) and compared to the literature value (122.4 C) 1 to determine the purity of the crystals. Since the melting point temperature of the benzoic acid crystals was close to the literature melting point temperature, it was concluded that the crystals were indeed pure, taking into account any experimental errors. Introduction Chemical reactions often result in a mixture of products, some of which are not of interest to the chemists; such mixtures are not pure. Chemists therefore need a way to isolate the product of interest from the original mixture of products, thus, purifying that product of interest from impurities. For an impure solid mixture, the solid of interest is more concentrated that the impurities. The impurities may include some combination of insoluble, soluble, and colored compounds. There are many techniques used by chemist to purify compounds, such as fractionation, filtration, extraction, and recrystallization. Fractionation refers to a purification strategy in which some relatively inefficient purification method is repeatedly applied to isolate the desired substance in progressively greater purity 2. Filtration is a mechanical method used to separate solids from liquids or gases by passing the feed stream through a porous sheet such as a
cloth or filter paper, which retains the solids and allows the liquid to pass through 2. Extraction removes an impurity from a solid, or recovers a desired product, by dissolving it in a solvent in which other components of that solid are insoluble in that solvent 2. Recrystallization separates a product from an impure liquid, often in extremely pure form, by cooling the liquid or adding precipitants which lower the solubility of the desired product so that it forms crystals. The pure solid crystals are then separated from the remaining liquid by filtration 2. Recrystallization is the primary method for the purification of non-volatile solids which are solids that do not vaporize readily and it is the purification method used in this laboratory. Recrystallization is possible because of the thermodynamics of the processes that take place which can be explained using Gibbs free energy ( G), which is the energy associated with a chemical reaction that can be used to do work (Equation 1). For a reaction to occur, the value of G has to be negative. G is negative for recrystallization due to enthalpic stabilization in crystals because the process is going from a less ordered system to a more ordered system. The enthalpy is negative and larger in magnitude than the positive entropy of the system which makes G negative, thus the process of recrystallization is favorable. ΔG = ΔH T ΔS (Equation 1) Solubility of solid compounds in different solvents plays a very crucial role in the process of recrystallization. It largely depends on the intermolecular interactions between the solid compounds and the solvents. It is often stated that like dissolves like, meaning that compounds that have similar structures will be soluble in one another. That is polar compounds dissolve in one another while non-polar compounds dissolve in one another. There are three solubility stages: collision, dissociation and solvation. Collision is when the solvent molecules collide with
the surface of the solid compound. The solid molecules become the solute component of the solid-solvent solution. Dissociation is when the solute molecules disassociate from each other and leave their crystal lattice by breaking the intermolecular forces that hold them together. Temperature affects the dissociation of the molecules because at a higher temperature the solvent has a higher kinetic energy and is more likely to collide with the solute causing the solute molecules to separate. Solvation is when the solvent molecules surround and interact with the solute molecules. At this point, the solute is said to have dissolved in the solvent because the solute is present in the solution. If a solid compound is less soluble in a given solvent than an impurity, at higher temperatures, the solid compound will be more saturated in the solution than the impurity. The general technique for recrystallization involves dissolving the impure solid in a minimum amount of an appropriate hot solvent (hot solvation) and cooling the solution slowly to allow the solid crystals to re-form. The solid compound being purified will recrystallize out because it is less soluble at a lower temperature and the impurity will still be dissolved in the solution because it is more soluble and less saturated at a lower temperature, thus the solid compound can be filtered out. The difference in the solubility of the compounds present in the impure solid is very crucial for recrystallization. If the desired compound and the impurities dissolved in a particular solvent have different solubility s, it becomes possible to isolate the desired compound from the impurities. Choosing a solvent determines the outcome of recrystallization and so it is important to choose the right solvent. When choosing a solvent, it is necessary to choose one in which the desired compound would be highly soluble at high temperatures but not at low temperatures. This is important because if the compound is highly soluble in the solvent at low temperatures, it
will be difficult for it to recrystallize when cooled, thus making the process of recrystallization useless. Also, the boiling point of the solvent should be lower than the melting point of the solid. The solvent should dissolve all the impurities easily or not at all; if the impurities do not dissolve at all, they can be removed by hot filtration before the solution is cooled. On the other hand, if the impurities fully dissolve in the solvent, they will remain in solution when the desired compound reforms. Also the solvent must be non-reactive with the solid to avoid changing the property of the solid, relatively volatile in order to evaporate off the formed crystal and cheap. Sometimes it is difficult to find a solvent that will dissolve the compound when it is hot, but will precipitate it out when it is cold. If no solvent can be found with the appropriate solubility parameters, sometimes it is possible to create an appropriate solvent by mixing two inappropriate miscible solvents. By using two solvents, it is possible to successfully complete the recrystallization. The solvent pair should be chosen such that one solvent dissolves the solute at elevated temperature and the other one does not dissolve the solute 3. Charcoal is used in some cases to remove colored impurities which are usually large and nonpolar and so they stick to the charcoal surface and can be filtered out by hot filtration because charcoal will be insoluble in the solvent. The filtrate solution is cooled slowly to prevent the trapping of the impurities in the pure crystals being formed. As the pure crystals begin to form they require a surface for the molecules to begin to adhere and the process of starting efficient crystal growth is called nucleation. Nucleation can be induced if the crystals take much time to recrystallize. Some methods of induced nucleation include; scratching the sides of the flask with a glass rod, seeding the solution with some pure compound, and forcing the solution to cool on ice. What these methods have in common is the fact that the presence of few crystals in solution
help anchor other crystals in the solution, thereby speeding the recrystallization process. The formed crystals are isolated from the solution by vacuum filtration and allowed to air dry. Since the main purpose is to obtain pure crystals, the purity of the compounds can be determined by measuring the melting point of the crystals formed using a Mel-Temp apparatus and comparing it to the literature melting point of that compound. The melting point of a solid is the temperature at which the solid and liquid phase is at equilibrium and so the rate of the liquid formed equals the rate of the solid formed. Experimentally, the melting point of a solid is a range. The lower temperature point is when the crystals just start to melt which is very noticeable because the crystals are distorted and the higher temperature point represents when all the crystals have melted into a liquid. If the experimental melting point of a solid is close to the literature melting point, then the compound can be said to be pure. The smaller the range (up to a one degree difference,) the more pure the crystals and the larger the range, the less pure the crystals. Reagent table 1, 4 Name and structure Benzoic Acid Molecular weight(g/mol) Amount used Density(g/mL) Melting point (Celsius) Boiling point (Celsius) 122.1g/mol 3.97g 1.32 g/ml 122.4 C 249 C Water 18.01g/mol 75mL 1.00g/mL 0 C 100 C
Experimental A 3.97g sample of wet crude benzoic acid was placed in a 125mL Erlenmeyer flask and a minimum amount ( 75mL) of boiling water that was boiled on a heating mantle (scheme1) was slowly added to it to dissolve the crystals. Charcoal was added to the flask after the crystals had fully dissolved to decolorize the solution. A hot gravity filtration technique was performed using
fluted filter paper (scheme 2). The Erlenmeyer flask was kept on the heating mantle during the hot filtration process. The filtrate was removed from the steam bath and allowed to cool at room temperature in the hood to form pure crystals as the benzoic acid recrystallizes. The filtrate was cooled with an ice bath to allow the crystals to form a little faster. The pure crystals were collected by vacuum filtration (scheme 3). The mass of the benzoic crystals collected was 2.81g (23.0mmol). The melting points of crude and pure benzoic acid were measured simultaneously using a Mel-Temp apparatus (scheme 5) and they were compared to the literature melting point of benzoic acid. Melting point range of crude benzoic acid: 118.3 C - 119.8 C. Melting point range of pure benzoic acid: 119.1 C - 120.2 C. Results The crude benzoic acid crystals were wet and had a light pink color. When hot water was added to dissolve the benzoic acid crystals, the solution was light pink. Adding charcoal to the solution changed the color to black and made the solution thick. The hot gravity filtration process was very slow and the filtrate was very clear. The filtrate looked a little oily. The crystals that started to form looked like small moth balls. When the ice bath was used to cool the solution, there was an increase in the rate of formation of crystals. The isolated pure crystals from vacuum filtration had a white color and as they air dried, they looked like snowflakes. In melting point procedure, when the capillary tubes where place in the Mel-Temp, the crystals were very visible and so when the crystals started to melt, it was very noticeable because they looked distorted. When the crystals had completely melted, the liquid in the capillaries was visible.
Calculations Discussion Recrystallization purifies impure solids thus every step is crucial to getting a good percent yield of pure crystals. Picking the right solvent is crucial to dissolving the solid compound. Water was chosen as the solvent for the recrystallization of benzoic acid because benzoic acid is highly soluble in it at high temperatures but insoluble in it at room temperature. This makes water a good solvent for the recrystallization of benzoic acid, since it meets the criteria for a good solvent to be used for such a process. Benzoic acid dissolved in water at high temperatures because the energy applied to the system in the form of heat increased the temperature of the system which causes an increase in the kinetic energy of the solvent and solute molecules and thus increase the collision between them. The energy provided is also needed to break the intermolecular forces between the crystallized benzoic acid molecules and water molecules, thus dissociating them from their crystal lattice structure. The process for the recrystallization of benzoic acid is possible because the Gibbs free energy ( G) of the system is negative. Another reason why water was chosen was because its boiling point (100 C) 4 is lower than the melting point of benzoic acid (122.4 C) 1 so during the recrystallization process, benzoic acid did not melt.
It is important that the crystal solution be kept near boiling to maximize solubility which is why the solution is kept on the steam bath as boiling water is added to it. After the crude crystals had dissolved, the solution was light pink in color because it contained methyl orange. Charcoal was used to decolorize the solution because charcoal is finely divided carbon which absorbs organic molecules that are highly conjugated and thus colored. The insoluble impurities together with the charcoal and colored impurities were removed by the process of hot filtration. Hot filtration was performed instead of vacuum filtration because the benzoic solution needed to be kept hot. Had vacuum filtration been used, the drop in pressure would have caused a decrease in temperature of the benzoic acid solution which would in turn cause benzoic acid crystals to start to come out too early. Fluted filter paper was used in the hot gravitational process because it maximizes the filtering surface area thereby hastening filtration, it minimizes the contact between the paper and the funnel and it does not have multiple layers of paper that cone filters have thus minimizing precipitation. The solution had to be cooled slowly to prevent the insoluble impurities from being trapped in the pure crystals as they come out of the solution because if they do, the goal of the experiment will be of no use. The benzoic acid crystals formed allow only other benzoic acid crystals to be incorporated in the crystal lattice structure. Nucleation was induced by cooling the solution on an ice bath. The melting point of the recrystallized crystals (120.2 C) was very close to the literature melting point (122.4 C) 1 which indicates that the crystals formed are pure. Also the magnitude of the melting point range (1.1 C difference) indicates that the crystals formed are indeed pure. It makes sense that the range for the melting point of crude benzoic acid is higher than that of pure benzoic acid, because crude benzoic acid contains impurities which lowers the melting point of benzoic acid and thus increasing its melting point range. It equally makes sense that the melting
point of the recrystallized benzoic acid is closer to that of the literature value because it contains little or no impurities. Since the experimental value of the melting point is lower that the literature value it suggests that the recrystallized crystals might still contain a few impurities because the solution was rapidly cooled on an ice bath instead of being allowed to slowly cool at room temperature due to time constraints, thus trapping some impurities. The experiment yielded a 70.8% of pure benzoic acid which is due to some experimental errors. Maybe too much water was added to the crystals to dissolve, thus making fewer crystals to be formed. Other possible error sources might have been due to incomplete product transfer, for example, when the crude benzoic acid was transferred from the watch glass to the Erlenmeyer flask and also during the hot filtration process. Also, some crystals might have started to precipitate early in the filter during the hot filtration. The 70.8% yield can be considered high if it still contains a significant amount of impurities due to the rapid cooling on the ice bath.
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