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Comparative apparent kinetics of menthol and thymol O-vinylation with acetylene in KOH-DMSO

This study demonstrates that the O-vinylation of menthol and thymol with acetylene in a KOH–DMSO medium follows an apparent zero-order kinetic model with similar activation energies, identifying 130°C as the optimal temperature for maximizing vinyl ether yield before thermal degradation occurs.

Original authors: Mahammadjon I. Soliev, Shukhrat B. Nuraliev, Suvonkul E. Nurmonov, Akmaljon S. Arislanov

Published 2026-09-15

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CC BY 4.0

Original authors: Mahammadjon I. Soliev, Shukhrat B. Nuraliev, Suvonkul E. Nurmonov, Akmaljon S. Arislanov

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

Chemistry often feels like a study of invisible forces, but at its heart, it is simply the art of rearranging atoms to build something new. One particularly useful family of molecules is called vinyl ethers. These are flexible building blocks used to make plastics, medicines, and other complex materials. To create them, chemists often try to attach a vinyl group—a small, reactive carbon chain—to an alcohol or a phenol, which is a type of organic compound with a specific hydroxyl group. A classic way to do this is by mixing the starting material with acetylene gas. Acetylene is a simple, two-carbon gas that acts like a eager partner, ready to snap onto the alcohol. However, this reaction is tricky. It requires a strong base to wake up the alcohol and make it reactive, and it needs heat and pressure to push the gas into the liquid mixture. If the conditions are too mild, nothing happens; if they are too harsh, the mixture can turn into a useless, sticky mess. Finding the perfect balance is the key to making these valuable chemicals efficiently.

In a recent study, researchers set out to understand exactly how this process behaves over time when using two specific starting materials: menthol, the cooling agent found in mint, and thymol, the aromatic compound found in thyme. They worked in a specialized liquid environment made of potassium hydroxide and dimethyl sulfoxide, a solvent known for helping reactions happen quickly. The team placed their mixtures into sealed metal reactors, pressurized them with acetylene gas to fourteen atmospheres, and heated them to temperatures ranging from one hundred to one hundred and forty degrees Celsius. They did not just look at the final result; they took samples every fifteen minutes for two hours to watch the reaction unfold in real time. By measuring how much of the starting material disappeared and how much of the new vinyl ether product appeared, they could map the speed of the reaction with high precision.

The most surprising discovery was how the reaction moved. In many chemical processes, the speed slows down as the ingredients get used up, much like a car running out of fuel. The researchers expected this might happen, or that the speed might change in a complex way. Instead, they found that for the first two hours, the reaction proceeded at a perfectly steady pace. The amount of product created every hour remained constant, regardless of how much starting material was left in the tank. This behavior, known as zero-order kinetics, suggests that the reaction is not limited by how much alcohol is available, but rather by something else, perhaps how fast the gas can dissolve into the liquid or how quickly the active chemical species can form. This steady rhythm held true for both menthol and thymol across most of the temperatures they tested.

When they compared the two substances, a clear difference emerged. Thymol reacted faster than menthol at every temperature between one hundred and one hundred and thirty degrees Celsius. At the peak temperature of one hundred and thirty degrees, thymol produced a higher concentration of the desired product and did so at a faster rate. The researchers calculated the energy required to get these reactions started, known as activation energy, and found the values were very similar for both compounds. However, because the data showed a wide range of possible values, they could not say with statistical certainty that one required more energy than the other. The main takeaway was simply that thymol was the more eager partner in this specific chemical dance.

The story took a turn when the temperature was pushed to one hundred and forty degrees. At this higher heat, the steady, predictable rhythm broke down. The production of the desired product slowed down, and the amount of starting material that disappeared also decreased. Instead of making more of the target chemical, the mixture began to generate more byproducts and a dry, resinous residue that coated the bottom of the reactor. This indicated that the high temperature was triggering unwanted side reactions, effectively clogging the system and preventing the formation of the useful vinyl ether. The researchers concluded that one hundred and thirty degrees was the sweet spot for this process, offering the best balance between speed and purity.

This work provides a clear, operational map of how these reactions behave under specific conditions, distinguishing between what works and what fails. It confirms that while the reaction looks simple on the surface, the underlying rules are governed by factors like gas availability and temperature limits rather than just the amount of ingredients present. By identifying the exact temperature where the process starts to degrade, the study offers a practical guide for anyone looking to manufacture these chemicals efficiently. The findings do not claim to reveal the deepest, microscopic secrets of how every single atom moves, but they do provide a reliable, real-world description of how the reaction performs, ensuring that future efforts can focus on the conditions that truly matter.

Technical Summary: Comparative Apparent Kinetics of Menthol and Thymol O-Vinylation

Problem Statement
The direct O-vinylation of hydroxy compounds with acetylene is an atom-economical route to vinyl ethers, which serve as versatile intermediates and monomers. While previous studies have established technological yield trends and identified a maximum product yield near 130°C for menthol and thymol in potassium hydroxide–dimethyl sulfoxide (KOH–DMSO) media, these earlier works did not discriminate between integrated kinetic orders using replicated concentration-time profiles. Furthermore, prior research lacked uncertainty-qualified Arrhenius parameters derived under a single, common protocol. There is a need to distinguish whether observed rate laws represent elementary mechanisms or operational regimes influenced by mass transfer, solvation, or secondary pathways.

Methodology
The study investigated the apparent kinetics of menthol and thymol O-vinylation with acetylene over a 0–120 minute window at temperatures ranging from 100°C to 140°C.

Experimental Setup: Reactions were conducted in independently operated 250 mL reactor cells at 14 atm acetylene pressure, a KOH/substrate molar ratio of 0.50, and 240 rpm agitation.
Conditions: Experiments were performed at 100, 110, 120, 130, and 140°C. "Time zero" was defined only after temperature and pressure stabilization.
Replication: Each condition involved three independently prepared and operated reactor runs (R1–R3), yielding 105 total concentration observations per substrate.
Analysis: Substrate and vinyl ether concentrations were quantified via GC-FID using biphenyl as an internal standard.
Kinetic Modeling: Integrated zero-, first-, and second-order models were fitted to the original concentration data using nonlinear least squares. Model selection was performed using the Akaike Information Criterion (AIC). Arrhenius parameters were estimated for the 100–130°C interval, with 95% confidence intervals calculated for activation energies ( $E_a$ ).

Key Results

Kinetic Order Discrimination: The zero-order model provided the best fit for all substrate-temperature combinations, as indicated by the lowest AIC values. The advantage of the zero-order model over first-order was moderate at 100–110°C ( $\Delta$ AIC 2.6–3.7) and strong at 120–140°C ( $\Delta$ AIC 4.9–28.3). The authors emphasize that this apparent zero-order behavior does not imply a zero-molecularity elementary event but likely reflects an operational regime governed by gas–liquid transfer, acetylene availability, or basic phase saturation.
Temperature Dependence (100–130°C):
- Menthol: The apparent zero-order rate coefficient ( $k_0$ ) increased from 0.1079 to 0.2399 mol L⁻¹ h⁻¹. The apparent activation energy was 34.9 kJ mol⁻¹ (95% CI: 8.9–60.8).
- Thymol: Thymol reacted faster than menthol across this range, with $k_0$ increasing from 0.1341 to 0.2958 mol L⁻¹ h⁻¹. The apparent activation energy was 37.5 kJ mol⁻¹ (95% CI: −4.0–79.1).
- Statistical Comparison: Due to wide, overlapping 95% confidence intervals and a two-sided slope test p-value of 0.060 for thymol, the study does not claim a statistically established difference in activation barriers between the two substrates.
Deviation at 140°C: At 140°C, both substrates exhibited a decline in target-product accumulation and substrate-depletion rates compared to 130°C. Concurrently, byproduct signals and dry resinous residue increased significantly (e.g., thymol residue increased from 0.032 g to 0.045 g). This indicates a shift in the operational kinetic regime, likely due to secondary pathways such as polymerization, catalyst deactivation, or altered mass transfer, rather than a simple continuation of the Arrhenius trend.

Significance and Claims
The paper's primary contribution is the rigorous discrimination of integrated rate expressions using AIC and the provision of uncertainty-qualified kinetic parameters for menthol and thymol vinylation under a unified protocol.

Operational vs. Elementary: The authors explicitly state that the measured zero-order rate law is an "operational, rather than elementary, kinetic description." The study refrains from assigning a specific elementary mechanism, noting that apparent constants in base-catalyzed systems often combine equilibrium, reaction, and decomposition contributions.
Optimization: The data identifies 130°C as the most favorable temperature for target vinyl ether formation, balancing high conversion with minimal byproduct formation.
Limitations: The authors acknowledge that the apparent zero-order law cannot distinguish intrinsic chemical kinetics from gas-liquid mass transfer limitations. They conclude that establishing intrinsic kinetics would require further variation of acetylene pressure and mixing rates, which were not performed in this study. The Arrhenius parameters are presented as apparent values with wide confidence intervals, precluding definitive claims about differences in activation energy between the two substrates.

Technical Summary: Comparative Apparent Kinetics of Menthol and Thymol O-Vinylation

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