Reac Kinet Mech Cat (2010) 101:41–48 DOI 10.1007/s11144-010-0202-2
The reaction of o-hydroxybenzyl alcohol with phenyl isocyanate in polar solvents Yong De Han • Peng Fei Yang • Jun Ying Li Cong De Qiao • Tian Duo Li
•
Received: 9 December 2009 / Accepted: 8 May 2010 / Published online: 22 May 2010 Ó Akade´miai Kiado´, Budapest, Hungary 2010
Abstract The reaction of o-hydroxybenzyl alcohol with phenyl isocyanate has been investigated in different polar solvents with in situ FT-IR. The rate constants for the reactions of the phenolic hydroxyl group and the aliphatic hydroxyl group were calculated as k1 and k2, respectively. It is found that the phenolic hydroxyl group reacts more easily than the aliphatic hydroxyl group. It is also found that k1 increases with increasing solvent polarity, while the trend of k2 is the opposite. Moreover, the reaction kinetics is second-order with respect to toluene, butyl acetate, cyclohexanone and pyridine, but is first-order with respect to NMP and DMF without distinction for the two kinds of hydroxyl groups. Keywords
Urethane reaction Kinetics Polar solvent In situ FT-IR
Introduction The reaction of isocyanates with alcohols has been widely studied and successfully used as the basic reaction during polyurethane synthesis [1–7]. There are several factors such as the reactant properties [8], the catalysts [9–11], the reactant concentration [12–14] and temperature [15, 16] affecting the urethane reaction rate. The reaction medium is also one of the important factors in the urethane reaction. Ephraim [17] found that the rate constants in the methanol– isocyanate reaction decreased in the order of toluene, nitrobenzene, di-n-butylether, n-butyl acetate, methyl ethyl ketone, dioxane and acetonitrile. Chen [18] also studied the influence of solvent polarity on the reaction of phenyl isocyanate with iso-butanol by adjusting the polar component in mixed solvents, and found that the Y. D. Han P. F. Yang J. Y. Li C. D. Qiao T. D. Li (&) Shandong Provincial Key Laboratory of Fine Chemicals, Shandong Institute of Light Industry, Jinan, Shandong 250353, China e-mail:
[email protected]
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rate constants substantially increased with the ratio of the non-polar solvent increasing. Furthermore, Kothandaraman et al. [19] also investigated the effect of solvent polarity on the reaction between isocyanate and phenol. However, no investigator has studied the effect of solvent polarity on the reaction between isocyanate and a compound containing both an aliphatic hydroxyl group and a phenolic hydroxyl group, only the reaction sequence of the two kinds of hydroxyl groups was reported in these papers [20, 21]. In this paper, the effect of solvent polarity on the kinetics of the o-hydroxybenzyl alcohol–phenyl isocyanate reaction has been studied systematically with in situ FTIR. The rate constants for the phenolic hydroxyl group (k1), for the aliphatic hydroxyl group (k2) and their ratio (k1/k2) were calculated. Furthermore, a possible mechanism for the reaction was proposed.
Experimental Materials Phenyl isocyanate (Sigma-Aldrich, A.R.) was purified by distillation under vacuum before use. o-Hydroxybenzyl alcohol (Sigma-Aldrich, A.R.) was dried at 40 °C under vacuum before use. All solvents (Sinopharm Chemical Reagent Co, A.R.) were purified by distillation or vacuum distillation and stored over molecular sieves ˚ ). (5 A Calibration for calculating concentrations from absorbances A series of phenyl isocyanate solutions with different –NCO concentrations in toluene, butyl acetate, cyclohexanone, pyridine, N-methyl pyrrolidone (NMP), DMF was prepared, and the absorbance of –NCO at 2273 cm-1 was measured with in situ FT-IR. The relationship between –NCO absorbance (A) and concentration (C) was described. The reaction of o-hydroxybenzyl alcohol with phenyl isocyanate In situ FT-IR (React IR IC10, manufactured by Mettler Toledo) was used as a main tool to monitor the whole reaction of o-hydroxybenzyl alcohol with phenyl isocyanate. Clean and dry nitrogen was flown into the instrument continuously until the absorbance of all impurities was constant. Thus, the background spectra were recorded in order to eliminate interference by air. After that, 0.172 g (1.385 mmol) o-hydroxybenzyl alcohol and 7 mL solvent were put into a three-necked flask fitted with a magnetic stirrer, and placed in a thermostat bath at the set temperature. When the temperature became stable, 0.321 g (2.77 mmol) phenyl isocyanate was added. The reaction was immediately monitored in order to get the relationship between – NCO absorbance (A) and time (t).
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Results and discussion It is known that Beer’s law can only be used for solutions of low concentration. As far as high concentration solutions are concerned, it is necessary to examine the relationship between –NCO absorbance (A) and concentration (C). Fig. 1 shows the relationship in toluene. It follow Beer’s law when the concentration of phenyl isocyanate is lower than 0.4 mol/L (Eq. 1). A ¼ k0 C
ð1Þ
Parameter k0 is the slope of the straight line in Fig. 1. The calibration curves in other solvents are similar. The values of k0 in various solvents are listed in Table 1. In the urethane reaction, solvents are usually divided into three categories: nonpolar solvents, common polar solvents and highly polar solvents. The polarity of the solvent has a great influence on the urethane reaction [17]. We had planned to study all those categories of solvents, but o-hydroxybenzyl alcohol did not dissolve in the non-polar solvents very well and could not be monitored with in situ FT-IR. Thus, only the effects of the last two categories of solvents on the urethane reaction were studied. Common polar solvents as reaction medium Yang et al. [21] have studied the urethane reaction between p-hydroxybenzyl alcohol and phenyl isocyanate in 1,4-dioxane, and found that the isocyanate group reacted with the phenolic hydroxyl group first, followed by the aliphatic hydroxyl group. It is believed that there are two stages in the reaction of o-hydroxybenzyl alcohol with phenyl isocyanate, which can be seen from the 3D FT-IR spectra for the reaction in toluene at 60 °C (Fig. 2).
Fig. 1 The calibration curve of phenyl isocyanate in toluene solution
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Table 1 Values of k0 in different solvents Solvents
Toulene
Butyl acetate
Cyclohexanone
Pyridine
NMP
DMF
k0 (L mol-1)
0.4998
0.4951
0.4451
0.4769
0.4544
0.3399
Fig. 2 Absorbance profiles of –NCO vs. time at 60 °C in toluene
Fig. 3 Relationship between 1/C and t for the reaction at 60 °C in toluene
According to Eq. 1 and the relationship between A and t in toluene at 60 °C, the rate constants in the two stages were calculated from the slopes of the kinetic curves (Fig. 3), which was obtained on the basis of the second-order kinetic equation. A reasonable linear fit of the reaction profile can be achieved with the proposed model if the reaction is divided into two stages, and both stages fit well. The values of k1, k2 and k1/k2 at different temperatures in toluene are shown in Table 2.
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Table 2 Second-order rate constants in toluene at different temperatures 50 °C k1 (L mol-1 min-1) -1
k2 (L mol
-1
min )
k1/k2
60 °C
65 °C
70 °C
0.84 9 10-2
1.74 9 10-2
3.07 9 10-2
4.76 9 10-2
-2
-2
-2
1.05 9 10
1.53 9 10-2
2.92
3.11
0.54 9 10
0.72 9 10
1.56
2.42
Compared with the rate constants in toluene, the rate constants for the urethane reaction in butyl acetate and cyclohexanone were also calculated according to the second-order kinetic equation. From Tables 2, 3 and 4, we can find that k2 decreases in the order of toluene, butyl acetate and cyclohexanone, which is in accordance with the conclusions of Ephraim in the phenyl isocyanate–methanol reaction [17] and those of Oberth in the butyl isocyanate–butanol reaction [22]. However, k1 increases in the order of toluene, butyl acetate and cyclohexanone, which may be similar to the reaction mechanism between a thiol and phenyl isocyanate [23]. In addition, k1 increases much faster than k2 with increasing temperature, which indicates the phenolic hydroxyl group is more easily affected by temperature than the alcoholic hydroxyl group. The reason may be in the reaction order and the steric effect. Highly polar solvents as reaction medium In order to completely study the effects of the solvent polarity on the reaction, some highly polar solvents such as DMF, NMP and pyridine were used as reaction media. Because of the urethane reaction running too fast in highly polar solvents, the reaction temperature had to be lower. Tables 5, 6 and 7 show the rate constants for the reaction in DMF, NMP and pyridine. In contrast to the reaction in common polar solvents, the reaction in DMF and NMP follows a first-order rate law. Furthermore, there is no distinction between k1 and k2 in the whole reaction, although there are two different kinds of hydroxyl groups in the reaction system (Fig. 4). The reaction in pyridine follows a second-order rate law, but the reaction rates are much higher, which may be due to the catalytic effect of highly polar solvents [24]. Moreover, k1/k2 varies slowly with the increase of temperature, which may be attributed to a difference in mechanisms.
Table 3 Second-order rate constants in butyl acetate at different temperatures 30 °C k1 (L mol-1 min-1) -1
k2 (L mol k1/k2
-1
min )
40 °C
50 °C
60 °C
1.56 9 10-2
2.71 9 10-2
0.67 9 10-1
0.78 9 10-1
-3
-3
-3
2.94 9 10
3.32 9 10-3
22.79
23.49
1.39 9 10
1.94 9 10
11.22
13.97
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Table 4 Second-order rate constants in cyclohexanone at different temperatures 30 °C k1 (L mol-1 min-1) -1
k2 (L mol
-1
min )
k1/k2
40 °C
50 °C
60 °C
1.59 9 10-2
3.63 9 10-2
1.25 9 10-1
1.58 9 10-1
-3
-3
-3
2.15 9 10
2.60 9 10-3
58.14
60.76
0.58 9 10
0.93 9 10
27.41
39.03
Table 5 First-order rate constants in DMF at different temperatures
k (min-1)
5 °C
10 °C
15 °C
20 °C
1.43 9 10-2
1.71 9 10-2
2.01 9 10-2
3.73 9 10-2
Table 6 First-order rate constants in NMP at different temperatures
k (min-1)
5 °C
10 °C
15 °C
20 °C
1.26 9 10-2
1.63 9 10-2
1.89 9 10-2
1.98 9 10-2
Table 7 Second-order rate constants in pyridine at different temperatures 5 °C k1 (L mol-1 min-1) -1
k2 (L mol k1/k2
-1
min )
10 °C
15 °C
3.51 9 10-1
5.10 9 10-1
6.15 9 10-1
9.59 9 10-1
-2
-1
-1
1.16 9 10
1.76 9 10-1
5.30
5.49
7.77 9 10
1.03 9 10
4.52
4.95
Fig. 4 Relationship between ln C and t for the reaction at 20 °C in DMF
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Reaction mechanism in highly polar solvents Although the reaction mechanism for the urethane reaction in common polar solvents is well studied, the mechanism is more complicated in highly polar solvents. The reaction rate increases and the activation energy is brought down [25, 26]. Dabi [27] proposed that the possible mechanism for the urethane reaction of butanol with phenyl isocyanate in highly polar solvents included two steps: first, the isocyanate forms a complex with the solvent, and then the complex reacts with the alcohol whereby the actual urethane reaction occurs. The mechanism for the reaction of o-hydroxybenzyl alcohol with phenyl isocyanate according to this proposal in polar solvents is shown in Scheme 1. According to Scheme 1, a steady state concentration of the complex is calculated out. And thus the kinetic equation could be written as: t¼
k3 ½OH k1 ½NCO ½S: k2 þ k3 ½OH
ð2Þ
In Eq. 2, if k2 k3 [OH], then t ¼ k1 ½NCO ½S:
ð3Þ
Suppose the concentration of the solvent is stable in all reaction processes. According to Eq. 3, the reaction follows first-order kinetics and the reaction rate is only related to the concentration of –NCO. The reaction rate in the initial and the last stage must be unchanged even if there are two different kinds of hydroxyl groups in the reaction system. The data in Tables 5 and 6 can be explained by Eq. 3 well. However, the reaction in pyridine is very surprising. It follows the second-order rate law, which can be explained by another assumption in Scheme 1, that is, k2 k3 [OH]. The kinetic equation could be written as: t ¼ Kk3 ½OH ½NCO ½S:
ð4Þ
where K is the equilibrium constant for step a (K = k1/k2).
Conclusions In this paper, the reaction of o-hydroxybenzyl alcohol with phenyl isocyanate was investigated. The rate constants k1 and k2 for the phenolic hydroxyl group and aliphatic hydroxyl group were calculated. It shows that the solvent polarity affects the reaction rate very much: the phenolic hydroxyl group reacts more easily than the
Scheme 1 Reaction mechanism in highly polar solvents
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aliphatic hydroxyl group; k1 increases with increasing solvent polarity, while k2 decreases with increasing solvent polarity. Moreover, the reaction kinetics obeys a second-order rate law in toluene, butyl acetate, cyclohexanone and pyridine, but a first-order rate law is valid in NMP and DMF, and there is no distinction for the two kinds of hydroxyl groups. A reasonable mechanism in highly polar solvents is proposed. In the first step, isocyanate forms a complex with highly polar solvents, then follows the reaction between the complex and the hydroxyl group. When the formation of the complex is the rate determining step (such as in DMF and NMP), it follows the first-order rate law. When the formation of the complex is not the rate determining step (such as in pyridine), it follows the second-order rate law. Acknowledgment The work is supported by the National Natural Science Foundation Committee of China (Grant 20676074).
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