Rongwen Lu*,Ziang Zhao,Xinyu Hao,Beibei Shao,Shufen Zhang

State Key Laboratory of Fine Chemicals,Dalian University of Technology,Dalian 116024,China

1.Introduction

Triphenylmethane dyes belong to the most important group of synthetic colorants and are used extensively in the textile dying,paper printing,medicine,biological staining,leather,food and cosmetic industry[1-4].The annual production of different triphenylmethane dyes is more than 8×103tons.Normally,the synthesis of triphenylmethane dyes undergoes a two step reaction,including the synthesis of leuco compoundviatandem Friedel-Crafts reaction of aromatic aldehyde with electron-rich aromatic amine using various Lewis or Br?nsted acids as catalyst,and the oxidation of leuco compound[5-10].While,this tandem Friedel-Crafts reaction suffers from certain disadvantages such as harsh reaction conditions and longer reaction time,leading to the consumption of large amount of energy and low efficacy of the production[11].Buta detailed study on this tandem Friedel-Crafts reaction is lacking,which restricts the improvement for this reaction.

In order to palliate this dearth of information,we carried out a systematic study for the synthesis of the leuco compound of triphenylmethane dyes in aqueous solution.More specifically,we selected the reaction of 2-formylbenzenesulfonic acid sodium andN-ethyl-N-(3′-sulfonic acid benzyl)aniline as a model reaction to evaluate the effect of the kinetic parameters such as temperature and concentration of reactants and hence to identify the experimental condition for the maximum yield in the shortest reaction time.

2.Experimental Section

2.1.Chemicals

Triethylamine and glacialacetic acid were purchased from Sinopharm Chemical Reagent Co.,Ltd.2-Formylbenzenesulfonic acid sodium(98%)was purchased from Chengdu Aikeda Chemical Reagent Co.,Ltd.and N-Ethyl-N-(3′-sulfonic acid benzyl)aniline(99%)was purchased from Shanghai Yiji IndustrialCo.,Ltd.These two reagents were used to establish the standard curve for measuring the reactant concentration in the reaction system.While,from the point of view of industrial practice,industrial grade 2-formylbenzenesulfonic acid sodium(95.33%,from Hangzhou Xiasha hengsheng Chemical Co.,Ltd.)and N-ethyl-N-(3′-sulfonic acid benzyl)aniline(95.70%,from Yixing City Zhongzheng Chemical Co.,Ltd.)were used for the reaction.

2.2.Friedel-Crafts reaction

In a typical procedure,a 500 ml three-necked round-bottomed flask with 30.08 g(0.1375 mol)of 2-formylbenzenesulfonic acid sodium,85.79 g(0.2819 mol)ofN-ethyl-N-(3′-sulfonic acid benzyl)aniline,325 ml of deionized water and 6.7 ml of sulphuric acid at concentration of1 g·ml-1were heated to a certain temperature by oilbath,which was accompanied by the stead dissolution of reactants under stirring and the gradual color turning of white to yellow,green and finally into deep blue.From that point,the time for reaction was recorded.Samples were drawn at preset reaction time,diluted with deionized water,and analyzed by high performance liquid chromatography(HPLC)analysis to obtain the reactant concentration at the sampling timeviastandard curve.

2.3.Analytical methods

The HPLC analysis was carried out on an Agilent 1100 series liquid chromatograph system(Agilent Technologies USA),equipped with a binary pump,an auto-sampler,a diode array detector.The analytical column was a Welch Ultimate XB-C18(250 mm × 4.6 mm,i.d.,5 μm).Water,including 0.3%triethylamine and 0.6%glacial acetic acid(V/V,solvent A)and methanol(solvent B)were used as mobile phase for the LC separation.The elution condition was isocratic,40%A and 60%B.The flow rate was at 0.8 ml·min-1and peaks were detected at 255 nm.

3.Results and Discussion

Actually,a two step reaction is needed for the synthesis of leuco compounds,the formation of diarylmethanol via Friedel-Crafts reaction of aromatic aldehyde with substituted aromatic amine and the consecutive reaction of the diarylmethanol with the same aromatic amine in the presence of Lewis or Br?nsted acids.

3.1.Determination of reaction order

For the synthesis of C.I.Acid blue 9 leuco compound,the reaction of 2-formylbenzenesulfonic acid sodium(A)withN-ethyl-N-(3′-sulfonic acid benzyl)aniline(B)yields a diarylmethanol(D),which has been confirmed by the presence of an unknown component in the liquid chromatogram of reaction mixture.The other is the consecutive reaction of the diphenylmethanol withN-ethyl-N-(3′-sulfonic acid benzyl)aniline,leading to the formation of leuco compound(E)(Fig.1).Both of the reactions are catalyzed by the sulfonic group on component B.

Keeping steady state approximation in mind,we firstly checked the concentration of intermediate D during the tandem Friedel-Crafts reaction of 2-formylbenzenesulfonic acid sodium withN-ethyl-N-(3′-sulfonic acid benzyl)aniline conducted at 100°C and found that the relative concentration of D kept in the range of 1.32%-1.99%.According to steady state approximation,if the concentration of the intermediate is very low,the derivative of the concentration of the intermediate could be set to zero:[12]

Then,the reaction equation could be simplified as:

Hence,the derivative of the concentration of2-formylbenzenesulfonic acid sodium(A)withN-ethyl-N-(3′-sulfonic acid benzyl)aniline(B)could be in the following forms:

Moreover,the initial molar ratio of A to B is 1/2 according to the reaction equation of 2-formylbenzenesulfonic acid sodium andN-ethyl-N-(3′-sulfonic acid benzyl)aniline,meaning the initial concentration of A and B could be written as:

Due to the very low concentration of intermediate D,we could presume that:

Based on the stoichiometric relation of the reaction,the reaction rate coefficient could be assumed as:

Thus,the evolution of the concentration of A and B could be modeled according to Eqs.(8)and(9).

After making a measurement of concentration of A(CA)during the reaction at four different temperatures ranging between 85 and 100 °C with an interval of 5 °C,we sketched theCA-t,lnCA-t,1/CA-t,1/-t,and 1/-t(CAis the concentration of 2-formylbenzenesulfonic acid sodium in the reaction system),respectively and found that 1/CAdecays linearly with time(see Fig.2).Then,it is reasonable to claim that α + β =2,reflecting the fact that the reaction followed a second-order rate kinetics[13].

3.2.Determination of reaction rate coefficient

Based on the fact that the dependence of the reaction rate on the concentration of both A and B,we presumed that:

Hence,the following equations are easily derived from Eqs.(8)and(9):

Fig.1.Reactions taking place during the synthesis of leuco compound of triphenylmethane dyes in aqueous solution.

Fig.2.Time courses for 1/C A changes at 85°C.

After the separation of variables and applying integration,the relation of concentration of A or B with corresponding reaction rate coefficient and reaction time could be in the following forms:

The reaction rate coefficient(k)was calculated as arithmetical mean values ofkAandkBto minimize the error in the calculation.Thus,reaction rate coefficient could be obtained based on Eq.(7)and in the following form:

Definitely,Eq.(14)will be used for the calculation of reaction rate coefficient after getting thekAandkBfrom experiments.Thiskvalue will be used for the calculation of the reaction rate in the kinetic equation.

Next,we made a measurement of concentration of A(CA)during the reaction with a time interval of 0.5 h at four different temperatures ranging between 85 and 100°C and sketched the 1/CA-t(Fig.3).Then four corresponding reaction rate coefficientskAwere calculated according to Eq.(13).With the same method,reaction rate coefficientkBcould also be obtained and thenkwas calculated from Eq.(14).The overall results are listed in Table 1.

The overall rate constant(k)increased with the increase of temperature,reflecting the benefit of high temperature for the reaction.The reaction rate,which conducted at 100°C,could be written as follows:

Table 1Reaction rate coefficient k A,k B and k calculated from Eq.(14)

3.3.Determination of apparent activation energy

Fig.3 shows the time courses for consumption of 2-formylbenzenesulfonic acid sodium(A)andN-ethyl-N-(3′-sulfonic acid benzyl)aniline(B)during the formation of C.I.Acid Blue 9 leuco compound at various temperatures in the range of 85-100°C.It is apparent that temperature has a great influence on the synthesis of C.I.Acid Blue 9 leuco compound.The concentration of A decreased sharply from 0.18 to 0.08 mol·L-1when the temperature rose from 85 °C to 100 °C.

The activation energy(Ea)was calculated as 70.96 kJ·mol-1from the slope of corresponding Arrhenius plots and shown in Fig.4.

3.4.Analysis of reactant concentration effect

From the point of view of industrial practice,it is interesting to know if the concentration of the reactant significantly changes the rate of the condensation process.Due to the benefit of high temperature for the reaction,we investigated the condensation reaction in details at100°C and the results are shown in Fig.5(the corresponding data were listed in Table S1).It is clearly shown that the slop of the curves decreased gradually as the reaction proceeded,meaning that the reaction rate decreased gradually with the substrate concentration.For example,the reaction rates at 5 h,10 h,and 15 h,which could be calculated from Eq.(15),were 11.08 × 10-3mol·L-1·h-1,4.173 × 10-3mol·L-1·h-1,and 2.125 × 10-3mol·L-1·h-1,respectively.These results mean that the reaction rate at 15 h was only one fifth of the rate at 5 h,implying that reactant concentration had great effect on this reaction.Hence,it was not enough to improve the conversion of 2-formylbenzenesulfonic acid sodium andN-ethyl-N-(3′-sulfonic acid benzyl)aniline by only prolonging reaction time in the late period of the reaction.The better way is to increase the concentration of the substrate concentration,which could be realized by evaporating the solvent in the late period of the reaction.

Fig.3.Time courses for 1/C A and 1/C B changes at four different temperatures.

Fig.4.Time courses for consumption of 2-formylbenzenesulfonic acid sodium(A)and N-ethyl-N-(3′-sulfonic acid benzyl)aniline(B)and plot of ln k versus 1/T(C).

Fig.5.Evolution of the concentration of 2-formylbenzenesulfonic acid sodium(A)and N-ethyl-N-(3′-sulfonic acid benzyl)aniline(B)during condensation conducted at 100 °C.

3.5.DFT calculation for the first step of the tandem Friedel-Crafts reaction

To gain more insight into the mechanism of the reaction,a DFT calculation for the first step of the tandem Friedel-Crafts reaction was performed with a simplified model process.Surprisingly,according to the DFT calculation,water is necessary for the dissociation of hydrogen in transition state.

Fig.6.Energy profile for the reaction of benzaldehyde with N,N-dimethylaniline.

While,itis worthy to note that water will also associate with sulfonic acid group onN-ethyl-N-(3′-sulfonic acid benzyl)anilineviahydrogen bonding and one molecule of water will be associated with at least two sulfonic acid groups.Then,the reaction mechanism will change from a sulfonic acid catalysis into a hydrated proton catalysis with the presence of more water,resulting in the decrease of reaction rate.So,the water needs to be evaporated with reactant consumption(see Fig.6).

4.Conclusions

In summary,the tandem synthesis of C.I.Acid Blue 9 leuco compound from 2-formylbenzenesulfonic acid sodium andN-ethyl-N-(3′-sulfonic acid benzyl)aniline proceeded by second order rate kinetics,and the reaction orders with respect to 2-formylbenzenesulfonic acid sodium andN-ethyl-N-(3′-sulfonic acid benzyl)aniline are both 1.The value of overall rate constantkwas found to be increased with the increase of temperature.On the basis of the value ofk,evaluated activation energy(Ea)was 70.96 kJ·mol-1.Also,the reactant concentration has great effect on the formation of C.I.Acid Blue 9 leuco compound.This means that evaporation of solvent in the late period of the reaction would be a better solution to shorten the reaction time,preventthe consumption of a large amountofenergy and enable low efficacy of the production.The current investigation provides a convenient and practical way to the production of leuco compound of triphenylmethane dyes.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2017.01.004.

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