Kinetics of Carbaryl Hydrolysis: An Undergraduate Environmental

Laboratory Experiment pubs.acs.org/jchemeduc

Kinetics of Carbaryl Hydrolysis: An Undergraduate Environmental Chemistry Laboratory Darryl Hawker* School of Environment, Griffith University, Nathan, Queensland 4111, Australia S Supporting Information *

ABSTRACT: Kinetics is an important part of undergraduate environmental chemistry curricula and relevant laboratory exercises are helpful in assisting students to grasp concepts. Such exercises are also useful in general chemistry courses because students can see relevance to real-world issues. The laboratory exercise described here involves determination of the second-order rate constant for base-catalyzed hydrolysis of the carbamate pesticide, carbaryl (1-naphthyl methylcarbamate), still widely used in the US and other countries. The exercise employs relatively simple and readily available equipment and exploits a bathochromic (red) absorbance shift of a product (1-naphthoxide) at pH ≥ 10 that minimizes absorbance overlap between reactant and product. Using a pseudofirst-order approach, carbaryl half-life at pH 10 to 11 ranges from approximately 1.5 h to less than 10 min, enabling completion within a 3 h laboratory period. The exercise can also be adapted for higher-level courses by using transition state theory to discriminate between hydrolysis mechanisms. To emphasize the practical utility of this exercise to students, at its conclusion, they can prepare waste carbaryl-containing solutions for disposal by this same hydrolysis process. KEYWORDS: First-Year Undergraduate/General, Environmental Chemistry, Hands-On Learning/Manipulatives, Kinetics

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mong the current challenges facing chemical education is a lack of relevance of material to students. This results in failure to engage with the learning process and failure to appreciate the wider value of chemistry.1 A strategy to address this is contextualizing activities including laboratory exercises. In context-based learning (CBL), such exercises are seen as an essential ingredient to assist in developing a more thorough understanding of concepts rather than just the acquisition of specific laboratory skills.2 Experiments that include environmental topics for example enable learners to see the interconnection between chemistry and the real world and facilitate the linkage of seemingly abstract chemical concepts to useful applications.3 Reaction kinetics is an important part of most secondary school and university general chemistry curricula.4 It is also important in environmental chemistry, where chemical persistence in environmental media is a major theme. However, kinetics is an area where students often have difficulty grasping important concepts such as rates of reaction, the effect of catalysts, and rate laws.5 Investigation of kinetics using realworld environmental examples is an ideal way for underlying concepts to be better understood in the context of studentgenerated data.6 Despite this, there are relatively few laboratory exercises published in this Journal or elsewhere that deal with kinetics but are also applicable to the CBL pedagogy. The laboratory exercise described here involves the hydrolysis of carbaryl (1-naphthyl methylcarbamate) (Figure 1), the world’s first commercially successful carbamate pesticide, introduced in 1956. More carbaryl has been used throughout the world than all other carbamates combined.7 © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. General structure for carbamate pesticides. For carbaryl, R1 = naphthyl, R2 = H and R3 = CH3.

Although now banned in the European Union, it is still available for use in the U.S. and other countries such as Australia through hardware and home improvement outlets. Many students will be familiar with carbaryl, albeit usually via trade names such as Sevin. In 2005, approximately 85,000 kg was applied in California alone.8 Carbamate pesticides target the insect nervous system because their typical mode of action is inhibition of the enzyme acetylcholinesterase, responsible for hydrolyzing the neurotransmitter acetylcholine. Accumulation of acetylcholine in insects can result in muscle tremors, paralysis, and death.9 Hydrolysis is a common abiotic transformation pathway for organic contaminants in the aquatic environment.10 In this process, water (or hydroxide anion) displace another atom or group of atoms present in the molecule undergoing hydrolysis. Carbamates such as carbaryl are susceptible to hydrolysis because they contain the carbamate functional group, comprising both ester- and amide-type functionalities.11 Abiotic carbamate hydrolysis can potentially proceed via separate specific acid- and base-catalyzed mechanisms as well as reaction

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DOI: 10.1021/acs.jchemed.5b00097 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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with water itself (the neutral mechanism) eq 1. For carbaryl, then −

= {kA[H3O ] + kN + kB[OH ]}[Crl]

EXPERIMENT SUMMARY

Overview

Rate = kHYD[Crl] +

Laboratory Experiment

The aim of this exercise is to determine the second-order rate constant for base-catalyzed hydrolysis of carbaryl. It may be done individually or by students working in pairs using a UV− vis spectrophotometer. The spectrophotometer simply requires a scan mode and a kinetic mode where absorbance at an analytical wavelength is recorded with time. With the wavelengths involved, either single or double beam operation is suitable. From eq 2, the rate of base-catalyzed hydrolysis is sensitive to pH and to determine the rate constant kB, a pseudofirst-order rate constant approach may be employed where solution pH is controlled by buffers. Buffers can be prepared from NaHCO3 and NaOH using standard procedures. Most commercial buffers are also acceptable. Hydrolyses can be followed by monitoring the ultraviolet absorbance of either carbaryl or 1-naphthoxide with time. Absorbance maxima occur at 279 and 333 nm, respectively, at pH ≥ 10 (Figure 2) due to a bathochromic (red) absorbance

(1)

Here, the overall (pseudo-first-order) hydrolysis rate constant is kHYD (e.g., s−1), kA and kB second-order rate constants (e.g., M−1 s−1) for the specific acid- and base-catalyzed mechanisms respectively, kN = kH2O.[H2O] a pseudo-first-order rate constant for neutral hydrolysis and [Crl] carbaryl concentration.12 Regardless of the pathway involved, carbamates are ultimately hydrolyzed to an alcohol, amine, and carbon dioxide. It has been established that for carbaryl, both the specific acid-catalyzed and neutral hydrolysis mechanisms are too slow to be of environmental importance unless conditions are very acidic (pH ≪ 5).13 Because carbaryl is a N-monosubstituted or primary carbamate (Figure 1; R1 = naphthyl, R2 = H, and R3 = CH3), it undergoes base-catalyzed hydrolysis by an E1cB (elimination, unimolecular, conjugate base) pathway. This refers to the rate-determining step being unimolecular elimination of the 1-naphthoxide anion from the conjugate base of carbaryl (Scheme 1). The conjugate base is itself formed from a previous reversible equilibrium involving attack of hydroxide on carbaryl.11 Scheme 1. E1cB Mechanism for Base-Catalyzed Carbaryl Hydrolysis

Figure 2. Ultraviolet absorption spectra of carbaryl and 1-naphthoxide (pH ≥ 10).

shift of 1-naphthol on ionization (pKa = 9.34).14,15 At lower pH values, there is considerable overlap of the absorbances of carbaryl and 1-naphthol (λMAX = 279 and 292 nm, respectively). Although monitoring the appearance of 1-naphthoxide is subject to slightly less spectral interference than monitoring the loss of carbaryl, the kinetic expressions for determining pseudo-first-order constants using products are somewhat more complex. Therefore, the accompanying student information is written in terms of carbaryl disappearance. These pH conditions (≥10) also result in pseudo-first-order half-lives of carbaryl suitable for laboratory classes. For example, at pH = 10 and 293 K, the half-life is approximately 90 min, whereas at pH = 11, it is 9.0 min.

The observed second-order rate constant for base-catalyzed hydrolysis (eq 2) is thus a composite of rate constants from the first two processes in the reaction mechanism (Scheme 1) rate = k 3[Crl−] but ⇒ rate =

k [Crl−] = 1 − [OH ][Crl] k2

k1k 3 [OH−][Crl] = kB[OH−][Crl] k2

Organization

The exercise can be accomplished in a 3 h laboratory period. Students first prepare an aqueous dilution (1:50) of a 1.5 × 10−2 M stock solution of carbaryl (Chem Service, West Chester, PA; 99%) in methanol and then record an ultraviolet spectrum. Because of the pH of the solution, the rate of hydrolysis is negligible. Some 2 M NaOH is added to the

(2)

Here, [Crl−] represents the concentration of the conjugate base of carbaryl. B




cuvette and mixed, resulting in instantaneous hydrolysis. A spectrum of 1-naphthoxide is then recorded and an analytical wavelength selected from these spectra for subsequent kinetic runs. With the spectrophotometer in kinetic mode, an aliquot of the methanol stock solution is added to a pH = 10 buffer (1:50), and absorbance recorded with time. The procedure is then repeated for pH = 10.5 and 11. The Beer−Lambert Law is obeyed for the concentrations employed16 and carbaryl concentration can be replaced by a corrected absorbance (A − A∞) in a standard pseudo-first-order kinetic treatment eq 3. Here, A∞ is absorbance of product at the analytical wavelength when t = ∞ −d(A − A∞) dt = kB[OH−](A − A∞) = k′(A − A∞)

rate =

⇒

∫0

t

d(A − A∞) = − k′ (A − A∞)

∫0

t

dt

⇒ ln(A t − A∞) = ln(A 0 − A∞) − k′t

(3)

This affords pseudo-first-order rate constants (k′ = kB[OH−]) for each pH, and from a plot of these against [OH−], the slope is kB (Figure 3). Detailed instructions are provided in Supporting Information. At the conclusion of the exercise, students can prepare solutions for disposal by using exactly the same hydrolysis process whose kinetics they were investigating. Some NaOH is added to exhaust the buffer and raise pH. At pH > 12 this takes less than 1 min. Acid can then be added to neutralize the waste.

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HAZARDS Carbaryl is moderately toxic to nervous and respiratory systems on acute exposure resulting in nausea, stomach cramps, diarrhea, and excessive salivation. Other symptoms at high doses include sweating, blurring of vision, lack of coordination, and convulsions. Direct contact with skin or eyes results in burns.17 The high-pH buffers also represent a hazard. Use of automatic dispensing equipment can reduce the possibility of accidents in handling these buffers. However, laboratory coat, gloves, and eye protection should always be worn. Both the 1-naphthoxide and methylamine formed on hydrolysis are degradable and very much less toxic than carbaryl. For example, 48 h LC50 values with Daphnia magna (waterfleas) are 12.3 mg/L and 6 μg/L for methylamine and carbaryl, respectively.18,19

Figure 3. (a) Plots of student generated data from pseudo-first-order kinetic runs at pH = 10, 10.5, and 11. Slope is the negative of the pseudo-first-order rate constant (−k′). (b) Plot of k′ at pH = 10, 10.5, and 11 versus [OH−] where slope is the true second-order basecatalyzed hydrolysis rate constant kB.

difference of 0.5 units provides sufficient discrimination in kinetic data obtained. The plots are linear (r2 > 0.997), and although runtimes were just over 3 min in the example shown, they can be extended to suit class needs providing sufficient carbaryl remains in solution. The data shown are typical of student output and afford a second-order rate constant (1.32 M−1 s−1) at a temperature of 293 K. Hydrolysis kinetics at other temperatures is an important environmental consideration. This can be done in a laboratory situation if facilities and time permit. Alternatively, as described below, there are literature data available applicable to either the Arrhenius Equation or transition state theory to enable students to discuss this in their laboratory report. In the former

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RESULTS AND DISCUSSION This particular laboratory exercise has been undertaken for a number of years by undergraduate (Environmental) Science and Engineering students. Student response to this exercise has been extremely positive. It utilizes relatively simple instrumentation, although spectrofluorometers can be used because a bathochromic shift on ionization occurs in excitation and emission wavelengths of 1-naphthoxide. Excitation wavelengths for carbaryl and 1-naphthoxide are 285 and 330 nm, respectively, and emissions wavelengths are 333 and 460 nm, respectively.15 Figure 3 shows typical plots of pseudo-first-order treatment for kinetic runs at pH = 10, 10.5, and 11 using an analytical wavelength of 279 nm (carbaryl). It can be seen that a pH

Ea 1 1 kB2 ( − ) = e R T1 T2 kB1

(4)

where kB1 and kB2 are the (second-order) rate constants for base-catalyzed hydrolysis at temperatures T 1 and T 2 , respectively. An activation energy (Ea) of 7.07 × 101 kJ mol−1 has been reported and a rate constant of 8.95 M−1 s−1 determined at 310 K.16 Using this data and eq 4, the predicted rate constant at 293 K is 1.82 M−1 s−1. This rate constant C




To emphasize the practical utility of the exercise, at its conclusion, students can prepare solutions for disposal using the same hydrolysis process that they were investigating. This demonstrates that kinetics in particular and chemistry in general can provide practical solutions to contemporary environmental issues.

compares favorably with the experimentally derived data from this exercise. Activation energy is approximately equal to the enthalpy of activation (Δ⧧H) from transition state theory.20 The temperature dependence of the rate constant in terms of transition state theory eq 5 is therefore similar to that from the Arrhenius equation kB2 T Δ⧧H ( 1 − 1 ) = 2 e R T1 T2 kB1 T1

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ASSOCIATED CONTENT

S Supporting Information *

(5)

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.5b00097.

Using the measured rate constant data at 310 K and assuming Δ⧧H = 7.07 × 101 kJ,16 eq 5 predicts a value for kB of 1.73 M−1 s−1 at 290 K, compared to the 1.32 M−1 s−1 determined here. Student-generated kinetic data can therefore be used to infer carbaryl persistence with respect to specific base-catalyzed hydrolysis at other temperatures and pH values. The kinetic exercise described is suitable for an introductory general or environmental chemistry course. It may be adapted for more advanced learners by further consideration of transition state theory, enabling identification of the reaction mechanism. Both E1cB (Scheme 1) and BAc2 (base-catalyzed, acyl cleavage, bimolecular) mechanisms have been invoked to explain the hydrolysis of carbamates.11 However, the E1cB and BAc2 mechanisms are kinetically equivalent. The obtained value of Δ⧧H is consistent with either of the proposed mechanisms whereas consideration of Δ⧧S allows for mechanism discrimination. The positive value of the entropy of activation (Δ⧧S = 3.77 × 101 J K−1 mol−1)13,21 is consistent only with the E1cB mechanism (Scheme 1). In the BAc2 mechanism, the ratedetermining step involves nucleophilic attack of hydroxide anion on the acyl carbon of the carbamate moiety to form a tetrahedral intermediate. The transition state is more ordered than reactants in this mechanism and hence Δ⧧S typically negative,13,21 in contrast to the positive value actually found for carbaryl. In fact, it is only secondary or N,N-disubstituted carbamates (Figure 1; R2, R3 ≠ H) that undergo base-catalyzed hydrolysis by a BAc2 mechanism. This is because the initial deprotonation characteristic of the E1cB pathway cannot occur as there are no hydrogens on the carbamate nitrogen.

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Student Instructions; Instructors Notes; Laboratory Resource Statement. (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: d.hawker@griffith.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Technical assistance from Scott Byrnes is gratefully acknowledged.

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SUMMARY The laboratory exercise described determines the persistence of a widely used carbamate pesticide, carbaryl, toward basecatalyzed hydrolysis. It is suitable for an introductory general or environmental chemistry course. The procedure employs relatively simple, readily available equipment and exploits a bathochromic absorbance shift of a product at pH ≥ 10. Using a pseudo-first-order approach, rates of hydrolysis are shown to be sensitive to pH values. Half-lives of 1.5 h to less than 10 min in the pH range of 10 to 11 mean that the exercise can be completed within 3 h. Temperature dependence of the hydrolysis is an important environmental consideration and can be investigated or discussed if desired using the empirical Arrhenius equation or the enthalpy of activation. The exercise can also be adapted for a more advanced physical organic course by consideration of entropy and transition state theory, enabling discrimination between hydrolysis mechanisms. Common carbamate hydrolysis mechanisms are kinetically equivalent meaning the basic exercise will not distinguish between mechanisms. D




(13) Wolfe, N. L.; Zepp, R. G.; Paris, D. F. Carbaryl, Propham and Chlorpropham: A Comparison of the Rates of Hydrolysis and Photolysis with the Rate of Biolysis. Water Res. 1978, 12, 565−571. (14) Guo, X.; Wang, X.; Zhou, X.; Ding, X.; Fu, B.; Tao, S.; Xing, B. Impact of the Simulated Diagenesis on Sorption of Naphthalene and 1-Naphthol by Soil Organic Matter and its Precursors. Environ. Sci. Technol. 2013, 47, 12148−12155. (15) Blanco, C. C.; Sanchez, F. G. A Spectrophotometric and Spectrofluorimetric Study of the Chemical Behaviour of the Insecticide Carbaryl: Ground and Excited State pKa Values of its Metabolite 1Naphthol. J. Photochem. Photobiol., A 1988, 42, 357−373. (16) Aly, A. M.; El-Dib, M. A. Studies on the Persistence of Some Carbamate Insecticides in the Aquatic Environment−1. Hydrolysis of Sevin, Baygon, Pyrolan and Dimetilan in Waters. Water Res. 1971, 5, 1191−1205. (17) EXTOXNET (Extension Toxicology Network). Pesticide Information Profile: Carbaryl; http://extoxnet.orst.edu/pips/carbaryl. htm (accessed Jul 2015). (18) National Pesticide Information Center (NPIC). Oregon State University and U.S. Environmental Protection Agency. Corvallis, OR. Carbaryl: General Fact Sheet; http://npic.orst.edu/factsheets/carbtech. pdf (accessed Jul 2015). (19) U.S. EPA. Pesticides: Endangered Species Protection Program. Effects Determination for Metam Sodium and the California Tiger Salamander. (Appendix B. Evaluation of toxic degradates of concern); http://www.epa.gov/espp/litstatus/effects/redleg-frog/2010/metamsodium/analysis.pdf (accessed Jul 2015). (20) Atkins, P.; de Paula, J. Atkins’ Physical Chemistry, 9th ed.; W. H. Freeman: New York, 2010; p 848. (21) Patel, J. M.; Wurster, D. E. Catalysis of Carbaryl Hydrolysis in Micellar Solutions of Cetyltrimethylammonium Bromide. Pharm. Res. 1991, 8, 1155−1158.

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Kinetics of Carbaryl Hydrolysis: An Undergraduate Environmental

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