Preparation method of 4-chlorophthalic acid

Abstract

The invention relates to a preparation method of 4-chlorophthalic acid, and relates to the technical field of organic synthesis.The preparation method of 4-chlorophthalic acid comprises the following steps that a compound 1 and alcohol are subjected to an alkoxylation reaction, and a compound 2 is obtained; in a reaction system containing acetic acid, the compound 2 and pypocholoride are subjected to a chlorination reaction, and a compound 3 is obtained; and sequentially carrying out hydrolysis reaction and acidification reaction on the compound 3 to obtain the 4-chlorophthalic acid. Compared with the existing 4-chlorophthalic acid monosodium salt technology, the preparation method of 4-chlorophthalic acid provided by the invention has the advantages that by adding acetic acid in the chlorination reaction, the reaction temperature is increased, the reaction time is shortened, the cost is reduced, and the defects in the prior art are made up.

Description

Technical Field

[0001] The invention relates to the technical field of organic synthesis, and in particular to a method for preparing 4-chlorophthalic acid. Background Art

[0002] 4-Chlorophthalic anhydride and 4-chlorophthalic acid are important chemical intermediates, widely used in dyes, plastics, medicines and pesticides. In the field of plastics, it is mainly used to synthesize biphenyl dianhydride (BPDA), an important monomer of special engineering plastic polyimide (PI). The industrial production method of 4-chlorophthalic anhydride is: phthalic anhydride is nitrated with concentrated sulfuric acid and concentrated nitric acid to obtain a mixture of 4-nitrophthalic acid and 3-nitrophthalic acid. Then chlorine gas is passed to denitrate and chlorinate to produce a mixture of 4-chlorophthalic acid and 3-chlorophthalic acid. Finally, after the two are separated by distillation, they are dehydrated intramolecularly at high temperature or by chemical methods to form 4-chlorophthalic anhydride. From the perspective of the entire process, nitration and chlorination are both dangerous reactions, so the cost of 4-chlorophthalic anhydride is not low, and the market price is 60 to 80 yuan / kg. When synthesizing biphenyl dianhydride, 4-chlorophthalic anhydride must first be hydrolyzed into diacids and then coupled.

[0003] It is the responsibility of scientific and technological personnel to find a synthetic route with reasonable process, cheap and easy-to-obtain raw materials, and prospects for industrial large-scale production. Therefore, the technology of 4-chlorophthalic acid monosodium salt appeared. Phthalic anhydride is hydrolyzed into phthalic acid monosodium salt under alkaline conditions, and then sodium hypochlorite aqueous solution is added dropwise (temperature does not exceed 35°C, time is not less than 10 hours) to generate 4-chlorophthalic acid monosodium salt, and finally acidified into 4-chlorophthalic acid (such as CN110563678A). The market price of 4-chlorophthalic acid monosodium salt is 30 to 40 yuan / kg. This technology has a long chlorination reaction time, low purity, and many impurities (containing dichloro products and unreacted raw materials). For example, U.S. Patent US2006004223 uses a liquid phase oxidation method to prepare 4-chlorophthalic acid, first chlorination and then oxidation. The by-product 3-chlorophthalic acid needs to be separated by distillation, oxidation is difficult to operate, and potassium permanganate wastewater is difficult to treat, so the entire process is costly. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a method for preparing 4-chlorophthalic acid.

[0005] The present invention provides a method for preparing 4-chlorophthalic acid, which comprises the following steps:

[0006] Compound 1 and alcohol are subjected to an alkoxylation reaction to obtain compound 2;

[0007] In a reaction system containing acetic acid, the compound 2 and hypochlorite are subjected to a chlorination reaction to obtain a compound 3;

[0008] The compound 3 is subjected to a hydrolysis reaction and an acidification reaction in sequence to obtain the 4-chlorophthalic acid;

[0009] The chemical structural formula of the compound 1 is:

[0010] The chemical structural formula of the compound 2 is:

[0011] The chemical structural formula of the compound 3 is: Wherein, R is methyl or ethyl.

[0012] Furthermore, the alcohol is methanol or ethanol, and the weight ratio of the alcohol to the compound 1 is (1.5-2.0):1.

[0013] Furthermore, the weight ratio of the alcohol to the compound 1 is 1.875:1.

[0014] Furthermore, the condition parameters of the alkoxylation reaction include: when R is methyl, the temperature is 65-67° C. (methanol reactant), or when R is ethyl, 78-80° C. (ethanol reactant), and the time is 6-7 hours.

[0015] Furthermore, the molar ratio of the acetic acid to the compound 2 is (1.05-1.10):1.

[0016] Furthermore, the molar ratio of the hypochlorite to the compound 2 is (1.4-1.5):1.

[0017] Furthermore, the hypochlorite includes sodium hypochlorite.

[0018] Furthermore, the condition parameters of the chlorination reaction include: a reaction temperature of 48° C. to 52° C., and a reaction time of 6.0 hours to 8.0 hours.

[0019] Further, the hydrolysis reaction is an alkaline hydrolysis reaction, and the conditions and parameters of the alkaline hydrolysis reaction include: a temperature of 90 to 95°C, a time of 1.5 to 2.0 hours, and an added weight of the base of 0.25 to 0.28 times the weight of the compound 3. Further, the conditions and parameters of the acidification reaction include: a temperature of room temperature, a time of 20 to 30 minutes, and an added amount of the acid to make the pH value of the reaction system 2.

[0020] The above technical solution provided by the embodiment of the present invention has at least the following advantages compared with the prior art:

[0021] The embodiment of the present invention provides a method for preparing 4-chlorophthalic acid. Compared with the existing technology for 4-chlorophthalic acid monosodium salt, the chlorination reaction of the method for preparing 4-chlorophthalic acid provided by the present invention increases the reaction temperature, shortens the reaction time, reduces the cost, and makes up for the shortcomings of the prior art. Specifically:

[0022] The present invention adopts methoxylation or ethoxylation of a carboxyl group of phthalic anhydride, methanol or ethanol is both a reaction raw material and a reaction medium, generates monomethyl phthalate or monoethyl phthalate, and then reacts with sodium hypochlorite aqueous solution to generate 4-chloro-2-methoxycarbonyl benzoic acid or 4-chloro-2-ethoxycarbonyl benzoic acid, and finally hydrolyzes and acidifies to form 4-chlorophthalic acid. Due to the electronic effect, chlorine substitution reaction will not occur at the para and ortho positions of the alkoxycarbonyl group, and chlorine substitution reaction may occur at the ortho position of the carboxyl group, but rarely. Dichloro product impurities can be washed off during water washing, and of course 4-chlorophthalic acid also has losses during water washing. Relative to the 4-chlorophthalic acid monosodium salt technology, the chlorination reaction temperature of the invention is higher, the reaction time is shortened, and therefore the cost is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0025] Figure 1 A schematic flow chart of a method for preparing 4-chlorophthalic acid provided in an embodiment of the present invention.

[0026] Figure 2 This is the hydrogen spectrum of 4-chlorophthalic acid obtained in Example 1 of the present invention.

[0027] Figure 3 This is the hydrogen spectrum of 4-chlorophthalic acid obtained in Example 5 of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0030] The present invention provides a method for preparing 4-chlorophthalic acid, such as Figure 1 As shown, the preparation method of 4-chlorophthalic acid comprises the following steps:

[0031] Compound 1 and alcohol are subjected to an alkoxylation reaction to obtain compound 2;

[0032] In a reaction system containing acetic acid, the compound 2 and hypochlorite are subjected to a chlorination reaction to obtain a compound 3;

[0033] The compound 3 is subjected to a hydrolysis reaction and an acidification reaction in sequence to obtain the 4-chlorophthalic acid;

[0034] The chemical structural formula of the compound 1 is:

[0035] The chemical structural formula of the compound 2 is:

[0036] The chemical structural formula of the compound 3 is: Wherein, R is methyl or ethyl.

[0037] The embodiment of the present invention provides a method for preparing 4-chlorophthalic acid. Compared with the existing technology for preparing 4-chlorophthalic acid monosodium salt, the method for preparing 4-chlorophthalic acid provided by the present invention increases the reaction temperature, shortens the reaction time, reduces the cost, and makes up for the deficiencies of the prior art by adding acetic acid in the chlorination reaction.

[0038] In the present invention, temperature has a great influence on the chlorination reaction. When the temperature is low, the reaction time is long. When the temperature is high, 4-chloro-2-alkoxycarbonylbenzoic acid is easily hydrolyzed. Adding acetic acid can protect the alkoxycarbonyl group and prevent hydrolysis, so the chlorination reaction temperature can be increased.

[0039] In some specific embodiments, the alcohol is methanol or ethanol, and the weight ratio of the alcohol to the compound 1 is (1.5-2.0):1.

[0040] In some specific embodiments, the weight ratio of the alcohol to the compound 1 is 1.875:1.

[0041] In some specific embodiments, the alkoxylation reaction conditions include: a temperature of 65-67° C. (methanol reactant) or 78-80° C. (ethanol reactant), and a reaction time of 6-7 hours.

[0042] In some specific embodiments, the molar ratio of the acetic acid to the compound 2 is (1.05-1.10):1.

[0043] In some specific embodiments, the molar ratio of the hypochlorite to the compound 2 is (1.4-1.5):1.

[0044] In some embodiments, the hypochlorite comprises sodium hypochlorite.

[0045] In some specific embodiments, the condition parameters of the chlorination reaction include: a reaction temperature of 48° C. to 52° C., and a reaction time of 6.0 hours to 8.0 hours.

[0046] In some specific embodiments, the hydrolysis reaction is an alkaline hydrolysis reaction, and the conditions and parameters of the alkaline hydrolysis reaction include: a temperature of 90 to 95° C., a time of 1.5 to 2.0 hours, and an added base of 0.25 to 0.28 times the weight of the compound 3.

[0047] In some specific embodiments, the conditions and parameters of the acidification reaction include: the temperature is room temperature, the time is 20 to 30 minutes, and the amount of acid added is such that the pH value of the reaction system is 2.

[0048] In some specific embodiments, the chemical formula of a method for preparing 4-chlorophthalic acid provided by the present invention is as follows:

[0049]

[0050] (1) Alkoxylation reaction: Phthalic anhydride reacts with alcohol to obtain monoalkyl phthalate.

[0051] (2) Chlorination reaction: Monoalkyl phthalate reacts with sodium hypochlorite in the presence of acetic acid at a certain temperature for a certain period of time to obtain 4-chloro-2-alkoxycarbonylbenzoic acid.

[0052] (3) Hydrolysis and acidification reaction: 4-Chloro-2-alkoxycarbonylbenzoic acid is hydrolyzed under alkaline conditions and then acidified under acidic conditions to form 4-chlorophthalic acid.

[0053] It should be noted that the components and raw materials involved in the preparation method of 4-chlorophthalic acid provided in the embodiment of the present invention, unless otherwise specified or specifically described, can be directly commercially available products or homemade using existing public preparation methods; at the same time, the steps and parameters involved, unless otherwise specified or specifically described, can be carried out according to the existing preparation method of 4-chlorophthalic acid or directly carried out using existing equipment, and the present invention document will not repeat them one by one.

[0054] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples that do not specify specific conditions are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0055] Example 1

[0056] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0057] Add 380 ml (300 g) of methanol to a three-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer and a reflux device, start stirring, add 160 g (1.08 mol) of phthalic anhydride, raise the temperature to about 65°C, and reflux for 6 hours. Distill under normal pressure to recover 250 g of methanol, and dry the residue to obtain 194 g of monomethyl phthalate, with a yield of 100%.

[0058] Add 500ml of pure water and 60ml (63g, 1.05mol) of acetic acid to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 180g (1mol) of monomethyl phthalate, heat to 50°C, start dropping 1110g of 10% sodium hypochlorite aqueous solution (NaClO: 111g, 1.5mol), keep the temperature at 48-52°C, and add the solution in about 4 hours. Continue to keep the temperature at about 50°C for 2 hours. Cool to room temperature, stand and separate, and separate 220g of crude 4-chloro-2-methoxycarbonylbenzoic acid as the lower oily substance.

[0059] Add 300ml pure water and 150g (NaOH: 60g, 1.5mol) of 40% liquid caustic soda to a three-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer and a reflux device, start stirring, add 220g of crude 4-chloro-2-methoxycarbonylbenzoic acid oil, heat to reflux, distill for 1hr, and steam out a mixture of 50g methanol and water. Cool to room temperature, add 30% hydrochloric acid to adjust the pH to below 2, precipitate a large amount of white solid, filter, and rinse the filter cake with 100ml water. Dry to obtain 135g of 4-chlorophthalic acid, purity 96.5%, melting point 158.2°C, ignition residue 2.8%, and two-step total yield 67.5%.

[0060] The hydrogen spectrum of 4-chlorophthalic acid obtained in this example is as follows Figure 2 shown.

[0061] Example 2

[0062] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0063] Add 380 ml (300 g) of ethanol to a three-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer and a reflux device, start stirring, then add 160 g (1.08 mol) of phthalic anhydride, raise the temperature to about 78°C, and reflux for 7 hours. Distill under normal pressure to recover 230 g of ethanol, and dry the residue to obtain 210 g of monoethyl phthalate, with a yield of 100%.

[0064] Add 500ml of pure water and 60ml (63g, 1.05mol) of acetic acid to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 194g (1mol) of monoethyl phthalate, heat to 50°C, start dropping 1110g of a 10% sodium hypochlorite aqueous solution (NaClO: 111g, 1.5mol), keep the temperature at 48-52°C, and add the solution in about 4 hours. Continue to keep the temperature at about 50°C for 2 hours. Cool to room temperature, stand and separate, and separate 235g of crude 4-chloro-2-ethoxycarbonylbenzoic acid as the lower oily substance.

[0065] Add 320ml pure water and 150g (NaOH: 60g, 1.5mol) of 40% liquid caustic soda to a three-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer and a reflux device, start stirring, add 235g of crude 4-chloro-2-methoxycarbonylbenzoic acid oil, heat to reflux, distill for 1hr, and steam out a mixture of 65g of ethanol and water. Cool to room temperature, add 30% hydrochloric acid to adjust the pH to below 2, separate out a large amount of white solid, filter, and rinse the filter cake with 100ml water. Dry to obtain 132g of 4-chlorophthalic acid, purity 96.3%, melting point 158.4°C, ignition residue 2.9%, and two-step total yield 66.0%.

[0066] Example 3

[0067] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0068] The preparation of monomethyl phthalate is the same as in Example 1.

[0069] Add 500ml of pure water and 63ml (66g, 1.1mol) of acetic acid to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 180g (1mol) of monomethyl phthalate, heat to 50°C, start dropping 1110g of 10% sodium hypochlorite aqueous solution (NaClO: 111g, 1.5mol), keep the temperature at 48-52°C, and add the solution in about 4 hours. Continue to keep the temperature at about 50°C for 2 hours. Cool to room temperature, stand and separate, and separate 222g of crude 4-chloro-2-methoxycarbonylbenzoic acid as the lower oily substance.

[0070] The following steps are the same as those in Example 1. 137 g of 4-chlorophthalic acid is obtained with a purity of 96.4%, a melting point of 158.2° C., an ignition residue of 2.9%, and a two-step total yield of 68.5%.

[0071] Example 4

[0072] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0073] The preparation of monoethyl phthalate is the same as in Example 2.

[0074] Add 500ml of pure water and 60ml (63g, 1.05mol) of acetic acid to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 194g (1mol) of monoethyl phthalate, heat to 50°C, start dripping 1040g of 10% sodium hypochlorite aqueous solution (NaClO: 104g, 1.4mol), keep the temperature at 48-52°C, and complete the dripping in about 4 hours. Continue to keep the temperature at about 50°C for 2 hours. Cool to room temperature, stand and separate, and separate 236g of crude 4-chloro-2-ethoxycarbonylbenzoic acid as the lower oily substance.

[0075] The following steps are the same as those in Example 2. 134 g of 4-chlorophthalic acid is obtained with a purity of 96.6%, a melting point of 158.1° C., an ignition residue of 2.7%, and a two-step total yield of 67.0%.

[0076] Example 5

[0077] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0078] The preparation of monomethyl phthalate is the same as in Example 1.

[0079] Add 500ml of pure water and 63ml (66g, 1.1mol) of acetic acid to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 180g (1mol) of monomethyl phthalate, heat to 50°C, start dropping 1110g of 10% sodium hypochlorite aqueous solution (NaClO: 111g, 1.5mol), keep the temperature at 48-52°C, and complete the dropwise addition in about 4 hours. Continue to keep the temperature at about 50°C for 4 hours. Cool to room temperature, stand and separate, and separate 223g of crude 4-chloro-2-methoxycarbonylbenzoic acid as the lower oily substance.

[0080] The following steps are the same as those in Example 1. 138 g of 4-chlorophthalic acid is obtained with a purity of 96.5%, a melting point of 158.3° C., an ignition residue of 2.8%, and a two-step total yield of 69.0%.

[0081] The hydrogen spectrum of 4-chlorophthalic acid obtained in this example is as follows Figure 3 shown.

[0082] Comparative Example 1

[0083] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0084] The preparation of monomethyl phthalate is the same as in Example 1.

[0085] Add 500 ml of pure water to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 180 g (1 mol) of monomethyl phthalate, heat to 35 ° C, start dropping 1110 g of a 10% sodium hypochlorite aqueous solution (NaClO: 111 g, 1.5 mol), keep the temperature at about 35 ° C, and add the solution in about 4 hours. Continue to keep the temperature at about 35 ° C for 2 hours. Cool to room temperature, stand and separate, and separate the lower layer of oily 4-chloro-2-methoxycarbonylbenzoic acid crude product 158 ​​g.

[0086] Comparative Example 2

[0087] This example provides a method for preparing 4-chlorophthalic acid, comprising the following steps:

[0088] The preparation of monomethyl phthalate is the same as in Example 1.

[0089] Add 500 ml of pure water to a four-necked flask equipped with a nitrogen conduit, a thermometer, a stirrer, a dropping device and a reflux device, start stirring, then add 180 g (1 mol) of monomethyl phthalate, heat to 35 ° C, start dropping 1110 g of a 10% sodium hypochlorite aqueous solution (NaClO: 111 g, 1.5 mol), keep the temperature at about 35 ° C, and add the solution in about 4 hours. Continue to keep the temperature at about 35 ° C for 10 hours. Cool to room temperature, stand and separate, and separate 197 g of crude 4-chloro-2-methoxycarbonylbenzoic acid as the lower oily substance.

[0090] Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present invention; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0091] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing 4-chlorophthalic acid, It is characterized in that The preparation method of the 4-chlorophthalic acid comprises the following steps: Compound 1 and alcohol are subjected to an alkoxylation reaction to obtain compound 2; In a reaction system containing acetic acid, the compound 2 and hypochlorite are subjected to a chlorination reaction to obtain a compound 3; The compound 3 is subjected to a hydrolysis reaction and an acidification reaction in sequence to obtain the 4-chlorophthalic acid; The chemical structural formula of the compound 1 is: The chemical structural formula of the compound 2 is: The chemical structural formula of the compound 3 is: Wherein, R is methyl or ethyl.

2. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The alcohol is methanol or ethanol, and the weight ratio of the alcohol to the compound 1 is (1.5-2.0):

1.

3. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The weight ratio of the alcohol to the compound 1 is 1.875:

1.

4. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The condition parameters of the alkoxylation reaction include: when R is methyl, the temperature is 65-67° C.; or, when R is ethyl, the temperature is 78-80° C. and the time is 6-7 hours.

5. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The molar ratio of the acetic acid to the compound 2 is (1.05-1.10):

1.

6. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The molar ratio of the hypochlorite to the compound 2 is (1.4-1.5):

1.

7. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The hypochlorite includes sodium hypochlorite.

8. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The condition parameters of the chlorination reaction include: a reaction temperature of 48° C. to 52° C., and a reaction time of 6.0 hours to 8.0 hours.

9. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The hydrolysis reaction is an alkaline hydrolysis reaction, and the conditions and parameters of the alkaline hydrolysis reaction include: a temperature of 90 to 95° C., a time of 1.5 to 2.0 hours, and an added weight of the base of 0.25 to 0.28 times the weight of the compound 3.

10. The method for preparing 4-chlorophthalic acid according to claim 1, It is characterized in that The conditions and parameters of the acidification reaction include: the temperature is room temperature, the time is 20 to 30 minutes, and the amount of acid added is such that the pH value of the reaction system is 2.

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