Kolbe-Schmitt Carboxylation of Resorcinol: A Benchtop NMR Spectroscopy Approach

Introduction

The Kolbe-Schmitt reaction is a fundamental carboxylation method widely used in organic chemistry for the synthesis of hydroxybenzoic acids. This process involves the reaction of phenolic compounds with carbon dioxide under basic conditions. Initially developed by Kolbe as a solid-gas reaction to synthesize salicylic acid, Schmitt later improved the methodology by employing elevated pressures to reduce phenol losses, increasing overall efficiency. Beyond industrial applications, the Kolbe-Schmitt reaction remains essential in laboratory-scale organic chemistry experiments and is a key reaction in organic chemistry training.

This experiment aims to perform the Kolbe-Schmitt reaction on resorcinol (1,3-dihydroxybenzene) to yield β-resorcylic acid (2,4-hydroxybenzoic acid) through carboxylation in an aqueous sodium bicarbonate solution. The product is subsequently analyzed using benchtop NMR spectroscopy, including HSQC NMR experiments, to elucidate molecular structure and couplings in aromatic compounds. This experiment provides students with hands-on experience in spectral analysis and dynamic insights into the behavior of organic compounds through spectroscopy.

Scheme 1. Carboxylation of resorcinol with sodium bicarbonate and hydrochloric acid to obtain β-resorcylic acid.

Experimental Procedure

Materials and Reagents

Instrumentation

Nanalysis -100 Benchtop NMR Spectrometer

Synthesis of β-Resorcylic Acid

1. Reaction Setup: In a 100 mL round-bottom flask, 25 g of sodium bicarbonate, 5.9 g of resorcinol, and 60 mL of distilled water were combined. A magnetic stir bar was added, and the flask was attached to a reflux apparatus with an oil bath, reflux condenser, and magnetic stir plate.
2. Heating and Reflux: The reaction mixture was heated to 100°C and refluxed for 2 hours. The temperature was then increased to 130°C for 15 minutes.
3. Acidification and Crystallization: The reaction mixture was cooled to room temperature and transferred to a 400 mL beaker. With continuous stirring, 29 mL of hydrochloric acid (37%) was added slowly to prevent excessive foaming. The beaker was placed in an ice bath to promote crystallization.
4. Filtration and Washing: The resulting precipitate was collected via vacuum filtration and washed with cold water.

Recrystallization of β-Resorcylic Acid

The crude product was dissolved in minimal hot water in a 50 mL Erlenmeyer flask. Upon cooling, recrystallization was induced by placing the flask in an ice bath. The purified crystals were collected by vacuum filtration, yielding 3.30 g (40%) of β-resorcylic acid as a white solid.

Figure 1. The recrystallization of β-Resorcylic Acid

Spectral Analysis

NMR Spectroscopy of β-Resorcylic Acid

Discussion

The experiment successfully demonstrated the Kolbe-Schmitt reaction for carboxylation of resorcinol, yielding β-resorcylic acid as confirmed through spectral analysis. The proton NMR spectra exhibited expected couplings for an aromatic system, with well-resolved 3JHH, 4JHH, and 5JHH interactions. The HSQC experiments further provided insight into proton-carbon connectivity.

Figure 2. ¹H(102.3 MHz) NMR spectrum of recrystallized β-resorcylic acid in dimethyl sulfoxide-d6. Residual solvent is denoted with a grey star and carboxylic group acid and hydroxy groups are denoted with black stars.

Figure 3. _gHSQC (¹H: 102.3 MHz, ¹³C: 25.7 MHz) NMR spectrum of recrystallized β-resorcylic acid in dimethyl sulfoxide-d_6.

Conclusion

This organic chemistry experiment effectively integrates synthesis, carboxylation, and advanced spectral analysis using benchtop NMR spectroscopy. The successful formation of β-resorcylic acid was confirmed through proton and HSQC NMR spectra, highlighting the relevance of coupling interactions in aromatic compounds. The Kolbe-Schmitt reaction remains a versatile and green approach, as it produces minimal waste and allows for scalable synthesis. Moreover, this study provides valuable hands-on experience in dynamic mechanical analysis and organic spectroscopy, making it a crucial addition to undergraduate organic chemistry training.

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