Effect of Reaction Conditions on Production of Catechinone Hair Dyestuff in Water / Alcohol Mixed Solution

Catechinone hair dyestuff was produced chemically from (+)-catechin by the oxidation with O2 in basic water/alcohol mixed solution. The effects of reaction conditions such as basicity, sorts of added base, temperature, alcohol molar fraction of solvent and sorts of added alcohols on the catechinone formation were studied in order to increase the efficiency of the dyestuff production. The amount of obtained catechinone increases with increasing basicity of the reaction solution. The dyestuff is obtained by adding monoethanol amine, diethanol amine, triethanol amine, L-arginine, Na2CO3, K2CO3, Na3PO4 or NaOH into the solution as a base and the most preferable base is monoethanol amine. The optimum temperature for the production in water/ethanol solution is 30 ̊C. It was found that water-soluble alcohols such as methanol, ethanol, 2-propanol, 1-propanol and tert-butyl alcohol are available for preparing the dyestuff. The optimum alcohol molar fraction of the mixed solvent used for the dye formation is 0.45 for methanol, 0.25 for ethanol and 0.20 for 2-propanol, 0.15 for 1-propanol and 0.10 for tert-butyl alcohol. The amount of the obtained dyestuff reaches a maximum at 1) 0.35 and 0.59, 2) 0.35 or 3) 0.35 mol∙kg−1 of the concentration of (+)-catechin for the 1) water/methanol, 2) water/ethanol or 3) water/1-propanol system, respectively.


Introduction
A great number of people dye their hair in the world today.Above all, human hair dyeing by using oxidation dyes is most frequently employed.The advantages of the permanent hair colouring technique using the oxidation dye are the higher dyeability for darker hair and higher colour fastness to washing [1] [2].However, many kinds of symptoms are caused for some people after using of the oxidation hair dyes (For example, [3] [4]).The problems should be solved for better human life also by scientific way.
Under such the situation, the authors invented catechinone (4-(3,4-dihydro-3α,5,7-trihydroxy-2H-1-benzopyran-2α-yl) 1,2-benzoquinone) as a safer hair dyestuff and it is prepared by oxidising (+)-catechin [5].The obtained colourant from (+)-catechin contains a small amount of byproducts but the main product is catechinone, of which structure is shown in Figure 1.Catechinone dyes human hair orange, reddish orange and deep yellowish brown.The colour fastness of the dyed hair to washing and light is high enough for practical use.Catechinone does not cause erythema or oedema on skin of rabbits.The dyestuff is obtained by enzymatic [5] and chemical [6] oxidation methods.It is produced by using a catechol oxidase such as tyrosinase and ascorbic acid oxidase, and the dye formation rate and the reaction selectivity of the enzymatic reactions are high.On the other hand, it is prepared by using O 2 gas in basic solution, where the formation rate is lower than that of the enzymatic method.However, the chemical method is easier to control and useful industrially because of the low cost.The catechinone formation occurs via proton dissociations and one electron oxidations by oxygen, thus the production efficiency of the dyestuff is improved by increasing pH of aqueous solution and the concentration of O 2 [6].
Nevertheless, the yield of catechinone is not high in one batch of the reaction solution because of the low solubility of (+)-catechin in an aqueous solution, and then it should be improved for practical use.The solubility of (+)-catechin in ethanol is over 30 times higher than that in water.Then, the chemical dyestuff preparation was attempted by adding ethanol into the reaction solution and increasing the concentration of (+)-catechin to increase the dye production efficiency in the previous study [7].The results showed that catechinone is not formed in pure ethanol solution but it is obtained in the water/ethanol mixed solution at 0.28 of the ethanol molar fraction of the solvent.The amount of the dyestuff formed in water/ethanol mixed solution is over 20 times larger than that in the aqueous solution system.It was suggested that water-ethanol mixing ratio of the solvent is an important factor for the formation efficiency.The other reaction conditions such as basicity and temperature are considered to be the key factor for the formation, too.It is also interesting to investigate the availability of other water-soluble alcohols for the reaction and the effect of the alkyl group structure of the alcohol on it.
In this study, the relationships between the amount of the formed catechinone and the dye preparation conditions such as basicity, sorts of added base, temperature, alcohol molar fraction of solvent and sorts of added alcohols were studied for the oxidation reaction of (+)-catechin in water/alcohol solution.

Dyestuff Preparation
(+)-Catechin and a base were dissolved in 100 g of distilled water/alcohol mixed solvent.Oxygen gas (≥99.5 vol%) was introduced continuously into the reaction solution through porous glass ball filter (pore size: 40 -50 μm) at 100 ml•min −1 of flow rate, and the solution was stirred for prescribed time at 20˚C -50˚C.

Measurements
The reaction solution was sampled at several time intervals and diluted by distilled water with a dilution ratio (f).The ultraviolet-visible absorption spectra of the sampled solution were measured by a Hitachi U-3900H spectrophotometer at 25˚C.The dye formation behaviour was monitored by using the absorbance at 430 nm (A 430 ) at each reaction time (t).The obtained f•A 430 values correspond to the concentration of the formed dye [7] and were used the index of the amount of the dyestuff formed.The dyestuff formation rate (v) was calculated by v = [Δ(f•A 430 )] max /Δt, where Δt is the reaction time interval and [Δ(f•A 430 )] max the maximum increment of f•A 430 for the Δt.Constant Δt was adopted for the calculation of v for each the experiment.
The apparent basicity of the reaction mixed solution (pH * ) was determined through the measurement by a TOA-DKK MM-60R multi water quality meter with a TOA-DKK ELP-031 glass combination electrode at 25˚C.

Effects of Basicity and Sorts of Bases
The colour of reaction solution turns orange and reddish brown with reaction time showing the formation of catechinone dyestuff.Catechinone is hardly obtained in acidic and neutral reaction solution without base.The rate of catechinone formation increases with increasing the basicity of aqueous reaction solution [6] and it is increased by using a stronger organic base [7].The previous results indicate the strong influence of basicity on the reaction.Then, the effects of basicity and sort of bases on the catechinone formation were studied first.The pH is not defined in the water/organic solvent mixture, of which organic composition is high, and the apparent basicity was estimated as using pH * in the study.The amount of the formed dyestuff at 200 min of reaction time ((f•A 430 ) 200min ) and the dyestuff formation rate (v) value in the water/ethanol mixed solution for each of the experiment with a variety of bases changing their concentration (molarity: m B ) were measured and determined.The alcohol molar fraction of the mixed solvent (x A ) was 0.28.
Figure 2 shows the relationship between pH * and (f•A 430 ) 200min for the water/ethanol solution with MEA.The resulting values are also summarised in Table 1.The (f•A 430 ) 200min and v increase with increasing pH * and m B of MEA as shown in the figure and for experimental number 2 -5 in the table.The results show that the reaction is  promoted by higher basicity of the reaction solution.This relationship is similar to that between pH and the rate of catechinone formation in aqueous solution [6].The MEA is also more effective for the formation than DEA and TEA.The deprotonated (+)-catechin in a basic medium works as an electron donor and reacts with O 2 easily [6] [9].Therefore, the higher basicity should contribute to promote the formation reaction also in the water/ ethanol solution.Furthermore, the preparation of the dyestuff was made in the mixed solution with basic amino acid L-arginine or inorganic bases such as Na 2 CO 3 , K 2 CO 3 , Na 3 PO 4 or NaOH (as listed in Table 1).The solubility of Na 3 PO 4 in the water/ethanol mixed solution is low and its saturated concentration is 0.0068 mol•kg −1 .The m B of other inorganic bases was 0.10 mol•kg −1 .The results in Table 1 show that f•A 430 for the reaction system with Na 2 CO 3 , K 2 CO 3 or NaOH is larger than that with same concentration of MEA.However, it is lower than that with 0.50 mol•kg −1 of MEA, and the solubility of inorganic bases in the water/ethanol mixed solution is lower as compared with that of MEA.The solution pH * decreases sharply at the earlier stage of the reaction for the inorganic base systems.This results in the decrease of the reactivity.In contrast, the pH * decreases gradually in the MEA system.Consequently, using MEA as the base is of greater advantage for the production.

Effects of Temperature
The rate of oxidation reactions are generally influenced by temperature.It is instructive to study the effect of reaction temperature on catechinone formation for obtaining better condition to produce the dyestuff efficiently.Figure 3 indicates the relationship between the reaction temperature (T ) and f•A 430 at 140 min ((f•A 430 ) 140min ) for the catechinone preparation in water/ethanol solution (x A = 0.28) with MEA.Here, the values of f•A 430 at 140 min are adopted, because f•A 430 increases and then decreases with reaction time at higher temperature and the downward turn begins before 200 min as described at the next section.The amount of the formed dye increases with increasing T up to 30˚C and it decreases with increasing T over 30˚C.The formation of colourless products and/or the precipitation of colourants may be caused by the further reactions of (+)-catechin and catechinone, and such the side reactions give rise to the decrease in f•A 430 at higher temperature [6].In fact, the maximal value of f•A 430 decreases with T. This indicates that higher temperature promotes the reactions too much.It can be said in conclusion that 30˚C is the most preferable for producing the dyestuff in the water/alcohol mixed solution.Figure 5 depicts the relationship between x A and f•A 430 obtained at 140 min 430 ) 140min ) in water/ethanol mixed solution with MEA.The reason why the value of f•A 430 obtained at 140 min is also adopted here, is that the same experiments were made in water and other alcohols as described below and the reactivities can be compared by using (f•A 430 ) 140min .The (f•A 430 ) 140min increases steeply from x A = 0.10 to 0.25 and decreases from 0.28 to 0.97.The results show that the amount of formed dye depends strongly on the ethanol molar fraction and reaches its peak at x A = 0.25.The results are interesting and it is thought that the effect of alcohol content for the reaction consists of two factors at least such as promotion and depression.Further, the same experiments were carried out using other alcohols being miscible with water such as methanol, 1-propanol, 2-propanol and tert-butyl alcohol.The results exhibit that the catechinone can be produced efficiently by adding all the alcohols used here and time change of f•A 430 for the alcohols is in a similar manner as in the water/ethanol solution.The f•A 430 for each reaction in the water/alcohol solutions increases monotonously with time, or reaches a peak and decreases.The f•A 430 value for water/1-propanol solution at x A = 0.15 takes a downward turn after 140 min as an earliest beginning of the decrease.Therefore, the value of (f•A 430 ) 140min is adopted in order to make better comparison of the whole results of f•A 430 under increasing.Figure 6 shows the relationships between the alcohol molar fraction for each of the mixed solvents (x A ) and (f•A 430 ) 140min for each of the reactions made in each water/ alcohol mixed solution.The (f•A 430 ) 140min for all the systems shows maximum against x A .The (f•A 430 ) 140min represents the maximal value at 0.45, 0.25, 0.20, 0.15 or 0.10 of x A for the MeOH, EtOH, 2-PrOH, 1-PrOH or t-BuOH-mixed system, respectively.It is, then, important to clarify the basis for the order of the x A values.The change in (f•A 430 ) 140min as shown in Figure 6 might relate with the concentration of O 2 in the solution, the polarity of the mixed solvent, and/or a particular microstructure formed in the reaction solution.The authors study the mechanism and the reason for obtaining the results as shown in Figure 5 and Figure 6 at present.

Effect of Alcohol Content and Sorts of Alcohols
Next, the dependence of the maximal amount of formed dye on the concentration of (+)-catechin in the reaction solution with the alcohols was investigated to elucidate the optimum condition for producing the dyestuff.MeOH, EtOH and 1-PrOH are convenient linear alcohols and they were employed in the experiments as at each x A = 0.45, 0.25 or 0.15, respectively for obtaining maximum.Figure 7 exhibits the relationships between the concentration of (+)-catechin (molarity: m S ) and the maximal amount of formed dye ((f•A 430 ) max ) in the three     The results show that the amount of formed catechinone depends on the sort of alcohols, the alcohol molar fraction and the concentration of (+)-catechin, and there are optimum x A and m S values to obtain the dyestuff efficiently.It can be said that the EtOH is most preferable as the solvent added for the production of the dyestuff, because its safety is highest, its handling is easiest during and after the production and it is most economical.

Conclusion
The relationships between the amount of the formed catechinone and the dye preparation conditions such as basicity, sorts of added base, temperature, alcohol molar fraction of solvent and sorts of added alcohols were studied for the oxidation reaction of (+)-catechin in water/alcohol solution.The amount of catechinone obtained increases with increasing basicity and the dyestuff is obtained by adding MEA, DEA, TEA, L-arginine, Na 2 CO 3 , K 2 CO 3 , Na 3 PO 4 or NaOH.The optimum temperature for the production in water/ethanol solution is 30˚C.The optimum alcohol molar fraction of the mixed solvent for the dye formation is 0.45, 0.25, 0.20, 0.15 or 0.10, with the methanol, ethanol, 2-propanol, 1-propanol or tert-butyl alcohol system, respectively.The amount of the obtained dyestuff reaches maximum at 1) 0.35 and 0.59, 2) 0.35 or 3) 0.35 mol•kg −1 of the fed concentration of (+)-catechin for the 1) water/MeOH, 2) water/EtOH or 3) water/1-PrOH system, respectively.

Figure 2 .
Figure 2. The pH * dependence of the (f•A 430 ) 200min for the catechinone produced in water/ethanol solution (x A = 0.28) with MEA at 30˚C.

Figure 3 .
Figure 3. Relationship between reaction temperature (T) and (f•A 430 ) 140min for catechin preparation in water/ethanol mixed solution with MEA at x A = 0.28.

Figure 4
Figure4shows the time course of change in the amount of the formed dye (f•A 430 ) for catechinone preparation in water/ethanol mixed solution with MEA.The x A was changed from 0.10 to 0.97.The dye formation behaviour strongly depends on the x A as shown in the figure.The f•A 430 increases monotonously with t at x A = 0.10 -0.20 and 0.69 -0.97.On the other hand, f•A 430 increases and then decreases with t at x A = 0.25 -0.59.The maximal value of f•A 430 also changes with x A of 0.25 -0.59 and the maximum of f•A 430 is given at x A = 0.25.The formation of colourless products and/or the precipitation of colourants as described above may be caused also here leading to the decrease in f•A 430 at x A = 0.25 -0.59.Figure5depicts the relationship between x A and f•A 430 obtained at 140 min 430 ) 140min ) in water/ethanol mixed solution with MEA.The reason why the value of f•A 430 obtained at 140 min is also adopted here, is that the same experiments were made in water and other alcohols as described below and the reactivities can be compared by using (f•A 430 ) 140min .The (f•A 430 ) 140min increases steeply from x A = 0.10 to 0.25 and decreases from 0.28 to 0.97.The results show that the amount of formed dye depends strongly on the ethanol molar fraction and reaches its peak at x A = 0.25.The results are interesting and it is thought that the effect of alcohol content for the reaction consists of two factors at least such as promotion and depression.Further, the same experiments were carried out using other alcohols being miscible with water such as methanol, 1-propanol, 2-propanol and tert-butyl alcohol.The results exhibit that the catechinone can be produced efficiently by adding all the alcohols used here and time change of f•A 430 for the alcohols is in a similar manner as in the water/ethanol solution.The f•A 430 for each reaction in the water/alcohol solutions increases monotonously with time, or reaches a peak and decreases.The f•A 430 value for water/1-propanol solution at x A = 0.15 takes a downward turn after 140 min as an earliest beginning of the decrease.Therefore, the value of (f•A 430 ) 140min is adopted in order to make better comparison of the whole results of f•A 430 under increasing.Figure6shows the relationships between the alcohol molar fraction for each of the mixed solvents (x A ) and (f•A 430 ) 140min for each of the reactions made in each water/ alcohol mixed solution.The (f•A 430 ) 140min for all the systems shows maximum against x A .The (f•A 430 ) 140min represents the maximal value at 0.45, 0.25, 0.20, 0.15 or 0.10 of x A for the MeOH, EtOH, 2-PrOH, 1-PrOH or t-BuOH-mixed system, respectively.It is, then, important to clarify the basis for the order of the x A values.The change in (f•A 430 ) 140min as shown in Figure6might relate with the concentration of O 2 in the solution, the polarity of the mixed solvent, and/or a particular microstructure formed in the reaction solution.The authors study the mechanism and the reason for obtaining the results as shown in Figure5and Figure6at present.Next, the dependence of the maximal amount of formed dye on the concentration of (+)-catechin in the reaction solution with the alcohols was investigated to elucidate the optimum condition for producing the dyestuff.MeOH, EtOH and 1-PrOH are convenient linear alcohols and they were employed in the experiments as at each x A = 0.45, 0.25 or 0.15, respectively for obtaining maximum.Figure7exhibits the relationships between the concentration of (+)-catechin (molarity: m S ) and the maximal amount of formed dye ((f•A 430 ) max ) in the three

Figure 5 .
Figure 5. Relationship between alcohol molar fraction (x A ) of the water/ethanol mixed solvent and the amount of the catechinone at 140 min ((f•A 430 ) 140min ).

Table 1 .
The reaction condition and the resulting rate and amount of catechinone dye.The reactions are started in water/ethanol solution (x A = 0.28) at m S = 0.172 mol•kg −1 at 30˚C.
[8]revious work[7]; b The base dissociation constant in water at 25˚C[8]; c Apparent pH value in the water/ethanol mixed solvent; d f•A430 at 80 min.
of water/alcohol systems.The maximum value of f•A 430 is ca.1600 in all the reaction systems.The m S value to give the maximum f•A 430 for water/EtOH and water/1-PrOH systems is 0.35 mol•kg −1 and the values for water/MeOH system are 0.35 and 0.59 mol•kg −1 . kinds