Benzoic acid is an important fine organic chemical product, the world's annual output reaches hundreds of thousands of tons. Benzoic acid is mainly used in the production of food preservative sodium benzoate, intermediates of dyestuffs, pesticides, plasticizers, medicines, and spices. It can be used as a modifier for alkyd resins and polyamide resins, and can also be used as a raw material for the production of polyester p-benzoic acid and as an antirust agent for iron and steel equipment.
Benzoic acid is a white fluorescent flake crystal, acicular crystal or monoclinic prism, molecular formula is C6H5COOH, molecular weight is 122.12, melting point is 122.4oC, relative density is 1.2659, insoluble in water, slightly soluble in hot water, soluble in ethanol, chloroform, ether, acetone, carbon disulfide and volatile, non-volatile oil, slightly soluble in hexane, chemical properties are not stable, hygroscopicity. CAS number is 65-85-0.
The molecular structure formula of benzoic acid is
Its basic structure is benzene, with a carboxyl group attached to the aromatic ring.
Synthesis Method Analysis of Benzoic Acid
It can be found from the literature that there are many methods for preparing benzoic acid, such as toluene oxidation, Grignard reagent method, diazonium salt method, carbon trihalide hydrolysis method, etc. These methods have their own advantages and disadvantages. For example, the Grignard reagent method is a reaction against water, and the water in the air will affect it; the synthesis route of the diazonium salt method is long, the operation is troublesome and the yield is not high; the hydrolysis method of the same carbon trihalide needs to use chlorine gas, which is toxic larger. Therefore, comprehensive comparison is better with the toluene oxidation method, its synthesis route is shorter, the operation is simple, and the yield is higher.
1. Toluene Oxidation
The toluene oxidation method uses toluene as a raw material and potassium permanganate as an oxidant for oxidation.
The synthesis route of toluene oxidation method is shorter, the operation is simple, and the yield is higher.
Since toluene is insoluble in potassium permanganate aqueous solution, the reaction is a two-phase reaction, so the reaction requires a higher temperature and a longer time, and the reaction often requires a heating reflux device. If at the same time mechanical stirring is used or a phase transfer catalyst is added to the reaction, the reaction time can be shortened, the reaction yield can be increased, side reactions can be reduced, selectivity can be increased and operating conditions can be simplified.
2. Grignard Reagent Method
The Grignard reagent method is a reaction against water, and the water in the air will affect it.
Synthesis of Benzoic Acid by Potassium Permanganate Oxidation
1. Reaction Mechanism
First, potassium permanganate is decomposed to produce atomic oxygen.
KMnO4 → [O]
Atomic oxygen reacts with toluene.
This is a free radical reaction. The more [O] free radicals released from the decomposition of potassium permanganate, the faster the reaction rate.
2. The Reaction Proportion and Catalyst of Potassium Permanganate as Oxidant
Take the oxidation of toluene under alkaline conditions to prepare benzoic acid as an example.
In other words, 1mol of toluene requires 2mol of KMnO4.
Except for a few solvents such as acetone, acetic acid and tert-butanol, KMnO4 is generally insoluble in organic solvents and its oxidation is mostly carried out in aqueous solution. The oxidation of organic compounds with KMnO4 aqueous solution occurs at the interface between the aqueous phase and the organic phase. The reaction rate is slow and the yield is relatively low.
If a phase transfer catalyst is added to the reactants, one of the reactants is transferred from one phase to the other through the two-phase interface, so that the two reactants react in a homogeneous system, that is, by using phase transfer catalysis, the reaction rate and yield can be increased, the occurrence of side reactions can be reduced, the selectivity can be increased and the operation conditions can be simplified. For example, in the experiment of preparing benzoic acid with toluene as raw material and potassium permanganate as oxidant, if the traditional heterogeneous reaction is replaced by phase transfer catalytic reaction, the experimental time can be reduced from 8h to 3h.
The commonly used phase transfer catalysts are:
- Onium salts: such as quaternary ammonium salts, quaternary phosphonium salts, matte salts, potash salts, etc.
- Cyclic ethers: crown ethers and hole ethers, etc.
- Acyclic polyethers: such as polyethylene glycol ethers, polyethylene glycol alkyl ethers, etc.
- Catalysts supported by polymers.
- Organometallic salts.
Synthesis Steps
1. Instruments and reagents
Instruments: 250mL three-port flask, 250mL triangle flask, spherical condensation tube, surface dish, Brinell funnel, suction bottle, water circulation vacuum pump, electric sleeve, iron frame platform.
Reagents: toluene, potassium permanganate, concentrated hydrochloric acid, sodium bisulfite, Congo red, test paper, activated carbon.
2. Construction of Experimental Equipment
The experimental device can be built with reference to figure 1.
3. Experimental Procedure
- Operation Steps
a. Feeding
Add 2.7mL toluene and 100mL water to the 250mL three-port flask and heat until boiled.
b. Add oxidizing agent
Add 8.5g potassium permanganate in batches from the top of the condensing tube, each feeding should not be too much, the whole feeding process takes about 60 minutes. Finally, use a small amount of water (about 25mL) to wash the potassium permanganate stuck to the inner wall of the condenser tube into the flask.
c. Washing, acidification, crystallization
Continue to react under stirring until the toluene layer almost disappears, and no more oil droplets appear in the reflux liquid. The reaction mixture was filtered under reduced pressure while it was hot, and the manganese dioxide on the filter residue was washed with a small amount of hot water. Combine the filtrate and washing liquid, and add a small amount of sodium bisulfite to reduce the unreacted potassium permanganate until the purple color fades and becomes a colorless and transparent solution. Filtrate under reduced pressure, cool the filtrate in an ice-water bath, then add concentrated hydrochloric acid to acidify, stir while adding, and test the pH value of the solution with pH test paper until it is strongly acidic, at which time benzoic acid crystallizes out. The precipitated benzoic acid was suction filtered and dried under pressure to obtain crude benzoic acid.
Fig.1 Reaction device diagram of toluene oxidation to benzoic acid
Fig. 2 Suction filtration device
d. Recrystallization (taking the recrystallization of 5g crude benzoic acid as an example)
To obtain pure benzoic acid, recrystallization can be carried out in water, and a small amount of activated carbon can be added for decolorization if necessary. The recrystallization process is as follows:
Operation steps: weigh 5g of crude benzoic acid and put it into a 250mL triangular flask. First add a proper amount of water, heat it to boiling, and then add a small amount of water according to the principle of a small amount of times, until the benzoic acid is just dissolved when the solution is boiling, then add about 20% of water, and reheat it to boiling. After slightly cooling, add an appropriate amount (about 0.5g~1g, depending on the impurity content) of activated carbon into the solution, boil for 5~10min, preheat the Buchner funnel, filter while hot, and collect the filtrate with a beaker. After the filtrate is cooled (cool it first, cool it with cold water, and finally cool it with ice water), the benzoic acid crystals should be separated out as much as possible, filtered by suction, squeezed the crystals with a glass stopper, and washed the crystals as much as possible. Take out the crystal, dry it in a steam bath or oven, and weigh it.
Fig. 3 Recrystallization experimental device
Calculate the yield. Finally, check whether the purity of the product meets the requirements by measuring the melting point.
- By-product Recovery and Treatment
a. Recovery of manganese dioxide
Method 1: Drain and flatten the filter cake obtained in the hot filtration operation, and wash it several times in batches with a small amount of hot water until the filtrate is neutral. Take out the filter cake and dry it to get black manganese dioxide powder.
Method 2: Take out the filter cake obtained in the hot filtration operation, add an appropriate amount of hot water, stir and wash, leave it still for clarification, and then pour out the solution. Wash several times in this way until the washing solution is neutral. Then filter by suction and dry the filter cake.
b. Potassium hydroxide
The by-product potassium hydroxide generated in the reaction can be removed by adding acid or magnesium sulfate (zinc sulfate) to keep the reaction in a medium close to neutral or weak alkaline.
2KOH+MgSO4 → K2SO4+Mg(OH)2 ↓
- Precautions
a. During vacuum filtration, the moisture in the benzoic acid should be drained as much as possible. Otherwise, boiling water bath drying takes a long time.
b. Potassium permanganate cannot stick to the pipe wall.
c. Control the reaction rate.
d. Acidification should be thorough.
e. When recrystallizing benzoic acid, a small amount of distilled water is generally added to reduce the loss of benzoic acid in the process of hot filtration. Because the solubility of benzoic acid in water is higher when the temperature is higher, there will be more benzoic acid dissolved in a smaller volume of solution. If there are droplets attached to the container wall or spilled, there will be more loss. Diluted with a small amount of water, the same volume of liquid will contain a small amount of benzoic acid, the loss will be reduced.
f. Decompression filtration.
f-1. The filter paper should be able to cover all the holes in the Brinell funnel and stick to it;
f-2. When there is no buffer bottle between the suction bottle and the water pump, special attention should be paid to removing the suction rubber tube after the filtration is completed, and then turning off the pump;
f-3. Before hot filtration, the Brinell funnel should be fully heated in the water bath pot, so that the heat filtration can be carried out quickly, so as to prevent the solution from cooling and the crystal from precipitating in advance, resulting in difficulties and losses in operation.
g. The stirring paste is about 5mm away from the bottom of the bottle.
h. Pay attention to control the amount of water brought in during the experiment to facilitate crystallization.
- Detection and Identification of Products
Observe the appearance and properties of benzoic acid, determine the refractive index of benzoic acid, determine the melting point of benzoic acid.
- Data Recording
- Calculate Yield
Brief Introduction of Industrial Production Method of Benzoic Acid
Industrially, benzoic acid is produced by liquid-phase air oxidation of toluene or by hydrolytic decarboxylation of phthalic anhydride in the presence of catalysts such as cobalt and manganese.
The liquid-phase air oxidation of toluene uses cobalt acetate as a catalyst, the dosage is about 100~150mg/kg, the reaction temperature is 150~170oC, and the pressure is 1MPa. The production process is shown in Figure 4.
Fig.4 Schematic diagram of the production process of benzoic acid by liquid-phase air oxidation of toluene
1-oxidation reactor; 2-stripping tower; 3-distillation tower
Toluene, cobalt acetate (2% aqueous solution) and air enter continuously from the bottom of the oxidation tower. The two-phase mixing between reactants not only depends on air bubbling, but also depends on the external circulation of the reaction liquid in the middle and lower part of the oxidation tower. The oxidation product from the top of the tower contains about 35% benzoic acid. The unconverted toluene in the reaction is recovered by the stripper, and the oxidation intermediates benzyl alcohol and benzaldehyde can be recovered at the top of the stripper and distillation column, and return to the oxidation tower like toluene to react. The refined benzoic acid can be collected by the side discharge of the distillation column. The residual recombination components in the tower kettle are mainly benzyl benzoate and tar-like substances, in which cobalt salts can be recycled. After cooling, the toluene in the oxidation tail gas is absorbed by activated carbon, and the toluene adsorbed on activated carbon can be blown out and recovered by steam, and the activated carbon is regenerated at the same time. The oxidation products are also separated by four distillation columns to recover toluene, light components, benzaldehyde and benzoic acid respectively. The yield of benzoic acid prepared by this method according to the consumption of toluene can reach 97%-98%, and the purity of the product can reach more than 99%.
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