insight into the kinetics and mechanism of the

American Journal of Chemistry and Applications 2015; 2(3): 55-62 Published online April 30, 2015 (http://www.openscienceonline.com/journal/ajca)

Insight into the Kinetics and Mechanism of the Tetrahydrobenzo[b]pyran Formation in the Presence of Caffeine as a Green Catalyst

Younes Ghalandarzehi, Sayyed Mostafa Habibi-Khorassani*, Mehdi Shahraki

Department of Chemistry, Faculty of Science, University of Sistan and Baluchestan, Zahedan, Iran

Email address

[email protected] (S. M. Habibi-Khorassani)

To cite this article Younes Ghalandarzehi, Sayyed Mostafa Habibi-Khorassani, Mehdi Shahraki. Insight into the Kinetics and Mechanism of the Tetrahydrobenzo[b]pyran Formation in the Presence of Caffeine as a Green Catalyst. American Journal of Chemistry and Applications. Vol. 2, No. 3, 2015, pp. 55-62.

Abstract

The kinetics and mechanism of the reaction between 4-methoxybenzaldehyde 1, malononitrile 2, and dimedone 3 in the presence of caffeine, 1, 3, 7-trimethylxanthine, as a green catalyst has been spectrally studied in a mixture of water and ethanol as green solvents. Based on the experimental data, the overall order of reaction for the formation of product followed the second order kinetics and under pseudo-order conditions the partial order with respect to 4-methoxybenzaldehyde 1, malononitrile 2 and dimedone 3 were one, one and zero, respectively. From the temperature, concentration and solvent studies, the activation energy (Ea = 84.35 kJmol-1) and the related activation parameters (∆G

‡ = 58.47 ± 1.31 kJmol-1, ∆S‡ = 75.45 ±

4.54 Jmol-1 and ∆H‡ = 81.72 ± 1.31 kJ mol-1) were calculated. The first step of the proposed mechanism was recognized as a

rate-determining step (k1) and this was confirmed based upon the steady-state approximation.

Keywords

Kinetics, Mechanism, 4-Methoxybenzaldehyde, Caffeine

1. Introduction

Multicomponent reaction (MCR) is a chemical reaction which three or more compounds react to form a single product. The major advantages of MCRs include lower costs, shorter reaction times, higher atom-economy, energy saving, and avoidance of time consuming expensive purification processes [1]. 4H-benzo[b]pyran have occupied an important place in drug research because of their various biological and pharmacological activities such as spasmolytic, diuretic, anticoagulant, anticancer, antianaphylactic antioxidant, antileishmanial, antibacterial, antifungal, hypotensive, antiviral, antiallergenic, and antitumor activities [2-7], and can be used as cognitive enhancers, for the treatment of neurodegenerative disease, including Alzheimer’s disease, amyotrophic lateral sclerosis, Huntington’s disease, Parkinson’s disease, AIDS associated dementia and Down’s syndrome as well as for the treatment of schizophrenia and myoclonus[8]. The polyfunctionalized benzopyrans were used as cosmetics, pigments and biodegradable agrochemicals [9-10].

Kinetic analyses are often performed in order to refine our understanding of what happens on the molecular level during a chemical reaction. During a kinetic study of an organic reaction, for example, experimental evidence in support of or contrary to a proposed mechanism may force us to think of a more detailed (or even alternative) curved-arrow depiction of a chemical reaction. The goal of any kinetic study is to establish a quantitative relationship between the rate of a reaction and the concentration of reagents or catalysts [11]. Thus, it is simply to follow the disappearance of the starting material or appearance of a product as a function of time. This can be done by measuring the rate of a reaction at vary concentrations of reactants and catalysts in order to determine the kinetic order with respect to a particular reactant and to establish an overall rate law for a reaction with a rate constant.

The experimental techniques that have been used in kinetics studies to accomplish these measurements are many and varied. Numerous kinetics investigations over a large area of different reactions have previously been reported using the UV-vis technique [12-30]. In our previous work, experimental and theoretical kinetics studies and mechanism

56 Younes Ghalandarzehi et al.: Insight into the Kinetics and Mechanism of the Tetrahydrobenzo[b]pyran Formation in the Presence of Caffeine as a Green Catalyst

investigation have fully been discussed for various reactions [31-43]. The synthesis of substituted tetrahydropyridines via one-pot multicomponent reaction was reported previously by H Nourallah et al [44]. Herein, for the first time, we are interested in an experimental studying regarding the detailed kinetics and mechanism of reaction between 4-methoxybenzaldehyde 1, malononitrile 2 and dimedone 3 in the presence of caffeine (1,3,7-trimethylxanthine) in a mixture of water and ethanol solvent (Fig. 1).

Cat : Caffeine

N

N

N

N

CH3

O

CH3

O

H3C

Fig. 1. The reaction between 4-methoxybenzaldehyde 1, malononitrile 2 and

dimedone 3 in the presence of caffeine (1,3,7-trimethylxanthine) in a mixture

of water and ethanol solvent.

2. Chemicals and Apparatus Used

The 4-methoxybenzaldehyde (1), malononitrile (2) dimedone (3) and caffeine were obtained from Merck (Darmstadt, Germany), Acros (Geel, Belgium) and Fluka (Buchs, Switzerland), and used without further p-urifications. All extra pure solvent also obtained from Merck (Darmstadt, Germany). A Cary UV-vis spectrophotometer model Bio-300 with a 10 mm light-path quartz spectrophotometer cell was employed throughout the current work. Optimization concentration of catalyst was chosen, (2×10-3 M), in each experiment on the basis of the report in synthesis of reaction [44].

3. Results and Discussions

3.1. Kinetics

To gain further insight into the reaction mechanism between 4-methoxybenzaldehyde 1, malononitrile 2, dimedone 3, in the presence of caffeine, a kinetics study of the reaction was undertaken by UV-vis spectrophotometry technique. Firstly, it was necessary to find the appropriate wavelength in order to follow the kinetic study of the reaction. For this purpose, in the first experiment, 10-2 M solution of each reactant 1, 2, 3 and 2×10-3 M solution of coffeine were prepared in a mixture of water and ethanol (2:1) as a solvent. Approximately 0.3 mL aliquot from each reactant was pipetted into a 1.2 mL light path quartz

spectrophotometer cell and the relevant spectrum of each compound at 35°C was recorded over the wavelength range 200-600 nm. In the second experiment, 0.8 mL aliquot of 8×10-3 M solution catalyst and 0.8 mL aliquot of 4×10-2 M solution of reactants 1 and 2 were pipetted into a quartz spectrophotometer cell then 0.8 mL aliquot of 4×10-2 M solution of reactant 3 was added to the mixture according to stoichiometry of each reactant in the overall reaction. The reaction was monitored by recording scans of the entire spectra with 2 minute intervals during the whole reaction time at the ambient temperature (Fig. 2.A). As can be seen in Fig. 2, B the appropriate wavelength was discovered to be 390, 400, and 405 nm (corresponding mainly to the product). Since at these wavelengths, reactants 1, 2, 3 and caffeine have relatively no absorbance value (in comparison with the relevant spectrum of each compound), it gave us the chance to find the practical conditions that allows kinetics and a mechanistic investigation of the reaction. Herein, in all the experiments, the UV-vis spectrum of the product was measured over the concentration range (10-3M ≤ M product ≤10-2M) to confirm a linear relationship between the absorbance and concentrations values.

Fig. 2. A) The UV-vis spectra of the reaction between 4-

methoxybenzaldehyde 1 (10-2 M), malononitrile 2 (10-2 M) and dimedone 3

(10-2 M), in the presence of caffeine (2×10-3 M) in a mixture of water and

ethanol (3:1) as reaction proceeds into a 10 mm light-path cell. Herein, the

upward of direction of the arrow indicate that the progress of product versus

times. B) Expanded section of UV-vis spectra over the wavelength range

390-420 nm.

OOCHO

+CN

CNO NH2

CN

O

1 2 3

H2O:EtOH (2:1)

+ Caffeine (20 mol %)

OMe

OMe

Product

+ H2O

American Journal of Chemistry and Applications 2015; 2(3): 55-62 57

In the third experiment under same concentration of each reactant (10-2 M), experimental absorbance curve was recorded versus time at 35°C temperature and wavelength 405 nm. This is shown in Fig. 3 As can be seen, the experimental absorbance curve (dotted line) exactly fits to the second order curve (solid line) [45]. It is obvious that the reaction is second order, herein, the second order rate constant of reaction (kobs) was automatically calculated by the kinetic software associated within the programme at 35.0˚C). In this case, overall order of rate low can be written as: α+β+γ= 2.

Rate=kovr[1]α[2]γ[3]β[Cat] (1)

Fig. 3. The experimental absorbance change (dotted line) along with the

second order fit curve (solid line) against time for the reaction between 4-

methoxybenzaldehyde 1 (10-2M), malononitrile 2 (10-2M), dimedone 3 (10-

2M), and caffeine (2×10-3 M) at 405 nm, 35.0˚C and in a mixture of water

and ethanol (2:1) [45].

3.2. Effects of Concentration

In order to find the partial order of reactants 1 and 2 under pseudo-order conditions, 5×10-3 M of reactant 1, 10-2 M of reactant 2, 10-2 M of reactant 3 and 2×10-3 M of caffeine were employed in the fourth experiment.

The rate law for fourth experiment can be expressed:

rate = k ovr[1]α[2]β[3]γ[Cat]

rate = kobs[1]α[Cat] (2)

kobs = kove [2]β[3]γ

And also 10-2 M of reactant 1, 5×10-3 M of reactant 2, 10-2 M of reactant 3 and 2×10-3 M of caffeine were used in the fifth experiment, the rate law can be written:

rate = k ovr[1]α[2]β[3]γ[Cat]

rate = kobs[2]β [Cat] (3)

kobs = kovr[1]α[3]γ

In the fourth experiment the original experimental absorbance curves versus times provided a pseudo-first order (Fig. 4. A). For this experiment the experimental absorbance curve (dotted line) versus times along with a first-order fit

was recorded at 405 nm and 35.0˚C. Then, the rate constant, of the reactions were automatically obtained by the software associated within the programme. Herein, according to equation 2, partial order with respect to compound 1 is 1 (α=1). For recognizing of the partial order with respect to compound 2, in the fifth experiment, the experimental absorbance curve versus times provided a first-order fit (Fig. 4. B) at 405 nm and 35.0˚C. Hence, in accord with equation 3, partial order with respect to compound 2 is 1 (β=1).

As a result of third, fourth and fifth experiments, we can write:

α+β+ γ =2 from the third experiment, α=1 on the basis of the forth experiment, β=1 based on the fifth experiment, Therefor γ =0 in related to compound 3. So the

experimental rate law can be stated:

rate=kovr[1][2][Cat]

rate=kobs[1][2] (4)

kobs = kovr[Cat]

A

B

Fig. 4. A). Pseudo first order fit curve (solid line) along with the original

experimental curve (dotted line) in relation to 4-methoxybenzaldehyde 1, for

the reaction between 1 (5×10-3 M), 2 (10-2M), 3 (10-2 M) and caffeine (2×10-

3 M) which was processed in a mixture of water and ethanol (2:1) at

35�and 405 nm. B). Pseudo first order fit curve (solid line) along with the

original experimental curve (dotted line) in relation to malononitrile 2 for

the reaction between 1 (10-2 M), 2 (5×10-3 M), 3 (10-2 M) and caffeine

(2×10-3M) which was processed in a mixture of water and ethanol (2:1) at

35�and 405 nm.


3.3. Effect of Solvent and Temperature

In order to determine the effect of temperature and solvent environment on the reaction rate, previous experiments were repeated under different temperatures and solvents. For this purpose, a mixture of ethanol and water (water/ethanol, 2:1, 58.6 D) and another mixture of ethanol and water (water/ethanol, 1:1, 52.7 D) have been used in the experiment. The results showed that rate of reaction speeds up in solvent with high dielectric constant (water/ethanol, 2:1) with respect to lower dielectric constant (water/ethanol, 1:1) at all temperatures investigated (Table 1).

Table 1. kobs (M-2min-1) for the reaction between 1 (10-2 M), 2 (10-2M) and 3

(10-2 M) and caffine (2×10-3 M) in different solvent media and temperatures.

Solvent: mix of water/ethanol (1:1) (52.7)a

T T=308.15 K T=313.15 K T=318.15 K T=323.15 K

kobs 1.13 (0.0011)b

1.81 (0.0009)

3.7 (0.0008)

6.12 (0.0008)

Solvent: mix of water/ethanol (2:1) (58.6)a

T T=308.15 K T=313.15 K T=318.15 K T=323.15 K

kobs 1.61 2.51 4.6 7.19

(0.0008)b (0.0011) (0.0013) (0.0012)

ln ( k 1c / T ) 0.96 1.39 1.98 2.41

T × ln ( k1 /

T) 295.90 434.72 629.35 778.53

a: dielectric constant (D) b: standard deviation (SD). c: kovr = k1 = kobs / [Cat]

As can be seen in Table 1, the rate of reaction increases in each solvent at higher temperatures. In the studied temperature range, the second-order rate constant (lnk1) of the reaction was inversely proportional to the temperature, which is agreement with the Arrhenius equation. This behavior is shown in Fig. 5. The activation energy, for the reaction between 1, 2, 3 and caffeine was obtained in a mixture of water and ethanol (2:1) (Ea = 84.35 kJ ⁄ mol) form the slope of Fig. 5.

Fig. 5. The Dependence of the second order rate constant (ln kobs) on

reciprocal temperature for the reaction between compounds 1, 2, 3 and

caffeine measured in a mixture of ethanol and water (2:1) at wavelength of

405 nm

The activation parameters which involve ∆Gǂ, ∆Sǂ and ∆Hǂ can be now calculated for the first step (RDS, k1 = kovr, see mechanism section), as an elementary reaction, on the basis of Eyring equation (Fig. 6A, equation (5)). Fig. 6B shows also a different linearized form of Eyring equation (equation (6)) [46]. Statistical analysis of the Eyring equation clearly confirms that the standard errors of ∆Sǂ and ∆Hǂ correlate (Tav is the center of the temperature range used): σ(∆Sǂ) =1/Tavσ(∆Hǂ)

It follows that in most solution phase studies σ(∆Sǂ) ≈σ(∆Hǂ)×0.0034 K-1. This correlation has been mentioned elsewhere [46, 47].The standard errors for activation parameters have been calculated according to above instructions [46-48] and they have been reported along with these parameters in Fig. 6(A,B).

Fig. 6. (A, B). Eyring plots according to equations (5 and 6), for the reaction

between 1, 2, 3 and caffeine in a mixture of water and ethanol (2:1).

ln �� ln � � �∆�ǂ� �

∆�ǂ� (5)


∆� ǂ = 75.45 ± 4.17 J mol�

∆ ǂ = 81.72 ± 1.31 kJ mol�

T × ln �� = T × �ln� � +

∆�ǂ� −

∆�ǂ� (6)

∆� ǂ = 75.57 ± 4.54 J mol�

∆ ǂ = 81.76 ± 1.31 kJ mol�

With respect to the values of ∆Sǂ and ∆Hǂ (Fig. 6 (A)), the value of ∆Gǂ for the reactions between 1, 2, 3 and caffeine in a mixture of water and ethanol (2:1), and 308.15 K is 58.47 ± 1.31 kJ mol-1 (Table 2).

Table 2. Activation parameters for the reaction between compounds 1, 2, 3 and caffeine measured in different solvent media.

water/ethanol ∆∆∆∆Hǂǂǂǂ(kJ mol-1) ∆∆∆∆Sǂǂǂǂ (Jmol-1) ∆∆∆∆Gǂǂǂǂ(kJ mol-1) Ea (kJ ⁄ mol)

(2:1) (58.6)a 81.72 ± 1.3 75.45 ± 4.54 58.47 ± 1.31 84.35b And 84.36 ± 1.31c

(1:1) (52.7)a 93.17 ± 1.3 109.52 ± 4.54 59.42 ± 1.31 95.7b And 95.46 ± 1.31c

a: dielectric constant (D) b: on the basis of Arrhenius c: According to the this equation Ea = ∆Hǂ+ RT

It can be seen from Table 2 that activation enthalpy (∆Hǂ) and free energy of activation (∆Gǂ) are positive and have prominent and large values which suggest that the energy required for the reaction is high, The high enthalpy of activation energy from the broken bonds before the generation transition state. That's not favorable and makes the reaction harder. However, positive value of activation entropy (∆Sǂ) shows that energy must be partitioned into the more states at TS.

Nevertheless, both the activation enthalpy and entropy are lower in solvent with high dielectric constant (water/ethanol, 2:1) with respect to lower dielectric constant (water/ethanol, 1:1) at same temperature, and this give rise to lower Ea and ∆Gǂ for the reaction in solvent with high dielectric constant because Ea is directly proportional to the activation enthalpy (Ea = ∆Hǂ + RT) and also ∆Hǂ proportional to the activation entropy (∆Gǂ + T∆S = ∆Hǂ).

3.4. Speculative Mechanism

Utilizing the above results, the simplified scheme of the proposed reaction mechanism is shown in Fig. 7.

To investigate which steps of the proposed mechanism is a rate-determining step, the rate law was written using the final step of reaction:

rate=k4[I3][Cat] (7)

The steady state approximation can be applied for obtaining the concentration of [I3] which is generated from the following equations:

()*+,(- =k3[I2][Cat]-k4[I3][Cat]=0 (8)

k3[I2]=k4[I3] (9)

The value of equation (9) can be replaced in the equation (7) so the rate equation becomes:

rate=k3[I2] (10)

For obtaining the concentration of intermediate [I2] the

following equation is yielded by applying the steady state assumption:

()*.,(- =k2[I1][3][Cat]-k3[I2][Cat]=0 and k2[I1][3]=k3[I2] (11)

And with the replacement of the equation (11) in (10) the following equation is obtained:

rate=k2[I1][3] (12)

And we can obtain the value of [I1] as below:

()*�,(- =k1[1][2][Cat]-k-1[I1][Cat][H2O]-k2[I1][3] [Cat]=0 (13)

k1[1][2][Cat]=[I1][Cat](k-1[H2O]+k2[3]), [I1] = ��)/,)0,

�1�)203,4�.)5, (14)

With the replacement of the equation (14) in (12) the general rate law (15) can be deduced:

rate = ��.)/,)0,)5,):;-,�1�)203,4�.)5,

(15)

Equation (15) is not compatible with the UV/vis experimental data, because the rate law is proportional to compound 3 while the experimental result slow that the rate law is independent from concentration of compound 3. Nevertheless, this equation does not involve the rate constants (k3) and (k4), therefore, steps 3 and 4 are not rate determining step (RDS). On contrary, this is possible for steps 1and 2 (see equation 15). If step 2 is a RDS, as a result, the following speculation is logical: k2[3]<<k-1[H2O][Cat]

In this case, the rate law becomes:

rate = ��.)/,)0,)5,):;-,�1�)203,

(16)

Equation (16) is not consistent with the experimental rate law (equation 4) because the overall order of reaction is three, in addition, it depends on concentration to compound 3.

If step 1 is a RDS, in this case:

k2[3]>>k-1[H2O][Cat]


The rate law can be expressed:

rate=k1[1][2][Cat] (17)

<=>?=k1[Cat] (18)

rate=kobs[1][2] (19)

The final equation (19) which obtained from the steady state assumption and the mechanism of the reaction is agreement with the experimental equation (4) and indicates that the overall order of the reaction is two. In addition, the partial order with respect to each reactant 1, 2 and 3 is one,

one and zero, respectively. Due to the presence of k1 in the rate low (equation (17)), it obvious that first the step (k1) is overall rate constant (k1 = kovr, deduced from comparison between equation 4 and 17). Therefor step 1 (k1) is a rate determining step (RDS), and step 2 (k2) is a fast step. Herein, the transition state (step 1, Fig. 5) carries a dispersed charge in reaction, so solvents with higher dielectric constant speed up the reaction rate by stabilizing the species at the transition state more than reactants, and therefore Ea would be lower (Table 3, 2 and 1).

MeO

O

N N

MeO

CN

CNO

O

HH

OMe

O

CN

C N

O

OMe

CN

O

O NH

OMe

CN

O

O NH

OMe

CN

O

O NH2

H

H

+

12

k1

k-1MeO

CN

CN

3 I1

I1

+k2

I2

OMe

O

CN

C N

O

I2

k3

I3

I3

k4

Product

Cat : CaffeineN

N

N

N

CH3

O

CH3

O

H3C

H H

+ H2O

[1,3] H-Shift

+ Cat

caffeine

+ Cat

Caffeine

+ Cat

H

Caf

fein

e

Caffeine

+ Cat

Fig. 7. The speculative mechanism of the tetrahydrobenzo[b]pyran formation


4. Conclusion

The overall order of reaction for the formation of tetrahydrobenzo[b]pyran formation in the presence of caffeine as a catalyst followed second-order kinetics and the partial orders with respect to 4-methoxybenzaldehyde 1, malononitrile 2 and dimedone 3 were one, one and zero, respectively. The results showed that rate of reaction speeds up in solvent with high dielectric constant (water/ethanol, 2:1) and decreases with respect to lower dielectric constant (water/ethanol, 1:1) at all temperatures. In the studied temperature range, the second-order rate constant of the reaction was proportional to the temperature, which was agreement with the Arrhenius equation. The first step of proposed mechanism was recognized as a rate-determining step (k1) and this was confirmed based upon the steady-state approximation. The activation parameters involving ∆Hǂ, ∆Gǂ and ∆Sǂ were reported for step 1 as an elementary step.

Acknowledgement

We gratefully acknowledge financial support from the Research Council of the University of Sistan and Baluchestan.

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insight into the kinetics and mechanism of the

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