Recrystallization Paired with Benchtop NMR

Recrystallization is an important purification technique in chemistry used to obtain pure solid compounds. The most common method exploits the temperature-dependent solubility of compounds in specific solvents. In this process, an impure solid (crude product) is dissolved in a solvent in which its solubility is low at room temperature but high at elevated temperatures. A minimum amount of hot solvent is added to the crude compound until all of it dissolves. During the process, the impurities present within the solid crude remain solution. As the solution cools down, the solubility of the desired compound decreases and pure crystals begin to aggregate and precipitate out, leaving the soluble impurities in solution.¹ Variations of this technique exist, including methods where insoluble impurities are filtered out while the pure compound remains in solution before crashing out, and approaches that utilize solvent mixtures to optimize solubility and separation.¹ These diverse recrystallization strategies provide chemists with flexible options for purifying a wide range of solid compounds.

Nuclear Magnetic Resonance (NMR) spectroscopy can assist the recrystallization process. As, the NMR spectra of both the crude sample and the purified crystals can be compared. The spectrum of a successfully purified sample will typically show sharper peaks and the absence of signals from impurities that were present in the crude product. This method is particularly effective for organic compounds, as NMR can detect even trace impurities and offer detailed structural information. Below is a simple organic reaction in which the desired product, acetanilide, is isolated and recrystallized (Figure 1). Benchtop NMR was used to assess the effectiveness of the recrystallization process.

Figure 1. Reaction scheme of the synthesis of acetanilide.²

Figure 2. Acetylation of aniline with acetic anhydride at room temperature in water.²

For the experiment, aniline (2 mL) and water (15 mL) were added to a 125 mL Erlenmeyer flask. With stirring, acetic anhydride (2.5 mL) was added, and the formation of crude product was observed, as seen in Figure 2. The crude was gravity filtered and wash with water (5 mL). When the crude was dry, a small sample was taken for ¹H NMR analysis and the rest of the crude was placed in a clean 125 mL Erlenmeyer flask. The flask was placed on a hot plate and water (50 mL) was added. The mixture was heated until the solid fully dissolved, then the flask was removed from the hot plate and solution was allowed to cool to room temperature (Figure 3).

Figure 3. Recrystallized acetanilide precipitating out of solution after removing the solution from the hot plate.

The flask was then placed in an ice bath to maximize the formation of crystals. Afterwards the mixture was filtered, isolating the recrystallized compound. A comparison between the isolated crude and recrystallized products (Figure 4) and their respective ¹H NMR spectrums (Figure 5) are shown below.

Figure 4. Comparison of the synthesized crude acetanilide (left) and the recrystallized acetanilide (right).

Figure 5. ¹H (102.3 MHz) NMR spectra (left – crude, right - recrystallized) of acetanilide in a 5:1 mixture of CDCl₃ and DMSO-d₆.

With HPLC, a knowledgeable user might be able to confirm whether there are impurities present in their sample, however, it is possible that they will not be able to identify these without knowledge of the various chromatographic retention times. With NMR analysis, chemists can verify the success of their recrystallization process and depending on the NMR solvent used, the sample can also be recovered. Recrystallizations in tandem with NMR, ensures high-quality products for further studies such as multi-step organic syntheses, which often require pure products before proceeding to avoid undesired side reactions.

References

[1] LibreTexts Chemistry, 3: Crystallization: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Techniques_(Nichols)/03%3A_Crystallization (Accessed July 9, 2024)

[2] Preparation of acetanilide, Synthesis and purification of an organic solid: https://www.cs.gordon.edu/courses/organic/_acetanilide.html (Accessed July 10, 2024)