Background and Theory of the Experiment
Recrystallization is a standard method in organic chemistry for separating and purifying solid organic molecules, depending on how soluble they are in various solvents at various temperatures. The desired chemical can preferentially crystallize by dissolving an impure solid in a hot solvent, chilling the solution, and filtering out any impurities.
The procedure includes choosing a solvent, dissolving and maybe decolorizing it, forming crystals, isolating and drying them, and selecting a solvent that will dissolve the substance well at high temperatures but poorly at room temperature or below. Additionally, it must not be poisonous, simple to evaporate, and incompatible with the substance being cleansed (Gilbert, 2015). A small amount of hot solvent is used to dissolve the impure substance, and more solvent may be added as necessary to ensure complete dissolution. To ensure good yield and purity, excessive solvent use should be avoided.
Decolorizing agents, such as activated charcoal, can eliminate colored impurities from impure solids. Standard techniques for removing insoluble impurities or decolorizing carbon include heat filtering and stirring the liquid with the decolorizing agent. Interest compound starts to crystallize as the heated solution cools.
Placing the flask in an ice bath will help the cooling process faster ( Ronkay et al., 2020). Once the solution has cooled to room temperature, vacuum filtering removes the crystals from the liquid. The crystals are dried on a Büchner funnel after being washed with a tiny amount of cold solvent to help eliminate any contaminants that may still be present.
A typical physical attribute of a compound is its melting point, which may be used to determine the purity of the crystals. Impurities often cause the melting point range to decrease and widen. Determining the compound’s purity is possible by comparing the obtained melting point with values from the literature. Three chemical compounds—benzoic acid, acetanilide, and naphthalene—will undergo recrystallization in this experiment.
The melting temperature of benzoic acid (C7H6O2), a white crystalline solid, ranges between 122-123 °C. Another white crystalline solid with a melting point of 114–116 °C is acetanilide (C8H9NO). The melting temperature of naphthalene (C10H8), a colorless solid that crystallizes as white particles, ranges between 80 and 82 °C. This experiment intends to purify these chemicals and evaluate their purity by recrystallizing them and measuring their melting points.
Procedure
Benzoic Acid
Benzoic acid was recrystallized by dissolving 1 g of the substance in 10 mL of distilled water in a 125 mL Erlenmeyer flask. To achieve a thorough dissolution, the mixture was heated until it reached a boiling point. It was added gradually, up to a maximum of 15 mL of extra water was required. Decolorizing carbon was added if necessary, and a hot filtration process eliminated any insoluble contaminants. After the solution had cooled, crystals began to form when the flask was submerged in an ice bath.
A Büchner or Hirsh funnel was used in a vacuum filtering arrangement to capture the crystals. The crystals were washed with cold water and dried on the funnel afterward (Gilbert, 2015). The dried crystals were weighed, and their melting point was calculated.
Acetanilide
One gram of the chemical, acetanilide, was dissolved in ten milliliters of toluene in a 125-milliliter Erlenmeyer flask for recrystallization. If more toluene was required to achieve complete dissolution, it was added progressively, up to 15 mL, and the liquid was heated until it boiled. If required, decolorizing charcoal was added; any insoluble impurities were then removed using hot filtering.
After chilling the solution and stimulating crystal formation in an ice bath, the crystals were collected using vacuum filtration and a Büchner or Hirsh funnel (Ronkay et al., 2020). The crystals were dried on the funnel after being cleaned with toluene. The dried crystals’ weight was calculated, and their melting point was noted.
Naphthalene
Naphthalene was recrystallized by dissolving 1 g of impure naphthalene in 10 mL of 95% ethanol in an Erlenmeyer flask. A maximum volume of 15 mL of extra ethanol was added progressively, if necessary, to dissolve the mixture after it reached the boiling point. Decolorizing charcoal was added if necessary, and insoluble impurities were removed using a hot filter. The flask was submerged in an ice bath once the crystals had formed and the solution had cooled.
A Büchner or a Hirsch funnel was used to collect the crystals using vacuum filtration. They were then rinsed with cold ethanol and dried on the funnel. The weight of the dried crystals was recorded, and their melting point was determined.
Results
Table 1: Melting Point Ranges of the Compounds
Table 2: Masses of Purified Crystals
Each chemical produced pure crystals using the recrystallization procedure. The following are the total volumes of solvents utilized during recrystallization:
- 10 mL of distilled water:
- Benzoic acid: 10 mL of distilled water
- Acetanilide: 10 mL of toluene
- Naphthalene: 10 mL of ethanol
Observations:
- Benzoic acid: The crystals made were white and looked like needles.
- Acetanilide: The crystals that were produced were white and granular in appearance.
- Naphthalene: The crystals were obtained in the form of white flakes.
Discussion
The benzoic acid demonstrated a significant degree of purification and separation success, with a recovery percentage of 96.5%. Acetanilide had a recovery percentage of 93.1%, indicating a reasonably effective purification method. On the other hand, naphthalene showed a lower recovery percentage of 26.8%, indicating less successful substance isolation.
Calculations
- Benzoic acid: Percent Recovery = (0.975 g / 1.009 g) * 100 = 96.5%.
- Acetanilide: Percent Recovery = (0.942 g / 1.011 g) * 100 = 93.1%.
- Naphthalene: Percent Recovery = (0.283 g / 1.058 g) * 100 = 26.8%.
The melting point range of benzoic acid came within the predicted range, suggesting a tight agreement when the melting points of the compounds were compared to the known literature values. However, acetanilide showed a somewhat lower melting point range than reported in the literature (Ronkay et al., 2020). Naphthalene’s measured melting point range was discovered to be within the range reported in the literature, indicating a reasonable level of agreement.
Potential Errors
Several potential error causes were found that might result in impure crystals and poor product recovery. Impurities could still be present in the crystal structure if the molecule is not completely dissolved in the selected solvent. Inadequate cleaning of the crystals may also leave behind solvent or lingering contaminants, affecting purity and recovery. Additionally, irresponsible handling that results in product loss during filtering may be a factor in poor recovery (Gilbert, 2015). It is also necessary to consider the possibility of contamination from tools, air, or incorrect storage, which might impure materials into the crystals.
Impurities can significantly impact the melting point of a substance. They can decrease melting points and increase melting ranges. The crystal lattice structure is disturbed by impurities, which makes it more difficult for the chemical to change from a solid to a liquid (Gilbert, 2015). A lower temperature is required to dissolve the extra intermolecular connections generated by the impurities, which leads to a lower melting point.
This experiment’s potential points of error might result in a wrong melting point estimate. The reported melting point might be considerably impacted by the sample being contaminated with foreign chemicals. The melting point can be determined inaccurately for various reasons, including inaccurate heating rates, thermometer readings, and improper sample sizes (Ronkay et al., 2020). Additionally, erroneous temperature readings or mistakenly selecting the melting point range might lead to inaccurate findings.
References
Gilbert, J. C. (2015). Experimental Organic Chemistry: A Miniscale & Microscale Approach. Cengage Learning.
Ronkay, F., Molnár, B., Nagy, D., Szarka, G., Iván, B., Kristály, F., Mertinger, V., & Bocz, K. (2020). Melting temperature versus crystallinity: New way for identification and analysis of multiple endotherms of poly(ethylene terephthalate). Journal of Polymer Research, 27(12). Web.