📄 chemistry

Pressure-Controlled Activity and Selectivity in Superbase-Catalyzed O-Vinylation of Menthol and Thymol with Acetylene

This study demonstrates that the KOH–DMSO superbase-catalyzed O-vinylation of menthol and thymol with acetylene exhibits distinct pressure-dependent activity and selectivity profiles, where optimal yields are achieved within specific pressure windows (12 atm for menthol and 14 atm for thymol) before secondary reactions degrade performance at higher pressures.

Original authors: Makhammadjon Soliev, Suvonkul Nurmonov, Shukhrat Nuraliev, Vokhidkhuja Azizov, Akmaljon Arislanov, Gulmira Parpieva

Published 2026-09-01

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CC BY 4.0

Original authors: Makhammadjon Soliev, Suvonkul Nurmonov, Shukhrat Nuraliev, Vokhidkhuja Azizov, Akmaljon Arislanov, Gulmira Parpieva

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of chemical manufacturing, creating useful materials often starts with simple building blocks. One such block is acetylene, a gas that can be coaxed into reacting with liquids to form vinyl ethers. These vinyl ethers are valuable because they serve as intermediates for making plastics, resins, and other complex chemicals. The challenge lies in getting the reaction to happen efficiently. Chemists often use powerful mixtures called superbases to speed things up, but the process is tricky because it involves a gas trying to dissolve into a liquid to react. The speed of this reaction depends heavily on how much gas is pushed into the liquid, but simply adding more pressure does not always mean the reaction gets faster or produces more of the desired product. Understanding exactly how pressure influences this balance is crucial for designing factories that are both efficient and safe.

A team of researchers at universities in Uzbekistan set out to map out this relationship with precision. They focused on two specific starting materials: menthol, the compound that gives mint its cooling sensation, and thymol, found in thyme. They wanted to see how these molecules would react with acetylene gas when mixed with a potent catalyst made of potassium hydroxide and dimethyl sulfoxide. The team did not just guess at the outcome; they built a controlled environment using a set of parallel reactors. Inside these vessels, they heated the mixture to 130 degrees Celsius and subjected it to varying levels of pressure, ranging from 4 to 16 atmospheres. They ran the experiments multiple times, taking samples every fifteen minutes for two hours to track exactly how much of the starting material disappeared and how much of the new vinyl ether product appeared.

The results revealed a clear pattern, but one that defies the simple idea that "more pressure equals more product." For both menthol and thymol, increasing the pressure from 4 to 12 atmospheres did speed up the reaction. However, the improvement was not a straight line. When the pressure tripled, the speed of the reaction increased by less than double. This indicates that the system hits a limit where the gas cannot dissolve fast enough or the catalyst cannot keep up, regardless of how much more pressure is applied. The researchers found that the reaction followed a specific mathematical pattern where the speed remained steady regardless of how much starting material was left, a behavior known as zero-order kinetics. This suggests that the bottleneck is not the amount of liquid ingredients, but rather how quickly the gas can reach the active sites of the catalyst.

The story changes when the pressure goes even higher. For menthol, the amount of product made began to level off once the pressure reached 12 atmospheres. Pushing the pressure to 14 or 16 atmospheres did not yield significantly more product, but it did start to create unwanted side materials. The situation was even more dramatic with thymol. This molecule showed a very sharp peak in performance at 14 atmospheres, where the yield reached 70.26 percent with a high degree of purity. However, the moment the pressure was raised to 16 atmospheres, the yield dropped sharply to 52.23 percent, and the formation of unwanted byproducts and sticky resins increased. This demonstrates that for thymol, there is a narrow window of ideal pressure, and stepping outside of it causes the reaction to lose its efficiency.

These findings provide a practical guide for anyone looking to manufacture these specific chemicals. The study proves that simply cranking up the pressure is not a universal solution. Instead, there is a specific operating range where the reaction works best. For menthol, the process stabilizes after a certain point, while for thymol, there is a precise peak before the system starts to degrade. The researchers concluded that the pressure must be carefully tuned to match the specific needs of the molecule being processed. By identifying these limits, the team has shown that optimizing chemical production requires understanding the delicate balance between gas pressure, reaction speed, and the purity of the final result, ensuring that the drive for higher output does not compromise the quality of the product.

Technical Summary: Pressure-Controlled Activity and Selectivity in Superbase-Catalyzed O-Vinylation

Problem Statement
The direct nucleophilic addition of hydroxy compounds to acetylene is an atom-economical route for producing vinyl ethers, valuable intermediates and monomers. While superbase systems, specifically potassium hydroxide–dimethyl sulfoxide (KOH–DMSO), are effective for this O-vinylation, the process involves complex gas–liquid dynamics. The observed reaction rate is a composite of intrinsic chemical kinetics, acetylene dissolution, and interphase mass transfer. Because acetylene solubility increases with pressure but decreases with temperature, and because elevated pressure does not necessarily yield a proportional rate increase, there is a need to distinguish between empirical pressure responses and intrinsic elementary reaction orders. Previous studies have addressed mechanism and modeling for specific substrates like thymol, but a systematic quantification of pressure-dependent activity and selectivity trade-offs for both menthol and thymol within a superbase system remains necessary to define optimal operating windows.

Methodology
The study utilized a parallel-reactor system (GPXF-4×250 mL) to conduct acetylene O-vinylation of menthol and thymol under controlled conditions.

Reaction Conditions: Reactions were performed at 130°C with stirring at 240 rpm. The system employed approximately 1.00 mol L⁻¹ substrate (menthol or thymol) and 50 mol% KOH in dried DMSO.
Pressure Variables: Experiments were conducted across a pressure range of 4 to 16 atm, with specific set points at 4, 6, 8, 10, 12, 14, and 16 atm.
Data Collection: Three independent charges were run at each pressure level. Samples were withdrawn automatically at intervals from 0 to 120 minutes. Concentration profiles were generated using Gas Chromatography with Flame-Ionization Detection (GC–FID), utilizing biphenyl as an internal standard.
Kinetic Analysis: Integrated kinetic models (zero-, first-, and second-order) were compared using the Akaike Information Criterion (AIC) to determine the best fit. For the selected model, apparent zero-order rate constants ( $k_0$ ) were calculated. The dependence of $k_0$ on pressure (4–12 atm) was analyzed via logarithmic regression to determine the apparent acetylene order ( $n_{app}$ ). High-pressure data points (14 and 16 atm) were treated as diagnostics to identify activity–selectivity trade-offs rather than being included in the initial power-law fit.

Key Results

Kinetic Regime: The zero-order model provided the best statistical fit (lowest AIC) for all 14 substrate–pressure combinations, indicating that pressure alters the rate constant without changing the apparent zero-order regime over the 120-minute observation period.
Apparent Reaction Orders: Between 4 and 12 atm, the apparent acetylene orders were sublinear: 0.553 for menthol and 0.474 for thymol. This indicates that while increased pressure improves catalytic activity, the response is constrained by factors such as interphase mass transfer, active species saturation, or solvation effects, rather than representing fractional elementary stoichiometry.
Yield and Selectivity Trends:
- Menthol: Yield increased from 30.20% at 4 atm to 48.99% at 12 atm, subsequently approaching a plateau. Selectivity remained high (95.00%) across the range, though resin formation increased slightly at higher pressures.
- Thymol: Exhibited a sharper optimum. Yield peaked at 14 atm with 70.26% yield and 95.4% selectivity. However, increasing pressure to 16 atm caused a significant decline in performance: yield dropped to 52.23%, selectivity fell to 90.2%, and secondary product formation intensified.
Pressure Limits: For both substrates, increasing pressure beyond a specific threshold (12 atm for menthol, 14 atm for thymol) did not monotonically improve target-product formation. Instead, it led to increased byproduct and resin formation, compromising selectivity.

Significance and Claims
The paper claims to define a practical pressure window for the superbase-catalyzed O-vinylation of menthol and thymol. The primary contribution is the demonstration that higher acetylene pressure cannot be assumed to increase vinyl-ether yield monotonically. The study establishes that increased pressure improves apparent catalytic activity only within a restricted operating window; beyond this window, secondary transformations and resin formation compromise target-product formation.

The authors explicitly state that the reported apparent orders are strictly empirical pressure-response exponents, not evidence of fractional elementary stoichiometry, due to the lack of independent measurements for dissolved acetylene concentrations and volumetric mass-transfer coefficients. The work provides a defensible framework for separating empirical kinetic regimes using reaction-progress analysis and information-criterion-based model discrimination, highlighting that substrate-specific responses (smooth plateau for menthol vs. narrow optimum for thymol) are likely driven by differences in ion pairing and solvation of the respective alkoxide and phenolate species.

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