Preparation of Benzaldehyde


I. Product Nature.

Formaldehyde is widely found in the plant kingdom, especially in the Rosaceae family, mainly in the form of glycosides in the stem bark, leaves, or seeds of plants, such as amygdalin in bitter almonds. Benzaldehyde occurs naturally in essential oils such as bitter almond oil, patchouli oil, hyacinth oil, ylang ylang oil, etc. It is sometimes called bitter almond oil. It is sometimes called bitter almond oil. The pure product is a colorless liquid.

Physical properties: Appearance and properties: pure product is colorless liquid, industrial products are colorless to light yellow liquid, with a bitter almond smell. Melting point (℃): -26, relative density (water=1): 1.04, boiling point (℃): 179.62 ℃ (1.33kPa), relative vapor density (air=1): 3.66, molecular weight: 106.12, saturated vapor pressure (kPa): 0.13 (26 ℃), refractive index: 1.5455, flash point (℃): 64°, ignition temperature (℃). 192,Solubility: Slightly soluble in water, about 0.6 wt (20°C) can be miscible in ethanol, ether, benzene, chloroform.

Chemical properties: The chemical properties of benzaldehyde are similar to those of aliphatic aldehyde, but there are also differences. Benzaldehyde cannot be reduced by the Fering reagent; when benzaldehyde is reduced by the reagent used in the reduction of fatty aldehyde, in addition to the main product benzyl alcohol, some tetrasubstituted ortho-diol compounds and homodiphenylethylene glycol are produced. In the presence of potassium cyanide, two molecules of benzaldehyde generate benzoin through the transfer of hydrogen atoms.

Benzaldehyde can also undergo electrophilic substitution reactions on the aromatic nucleus, mainly to produce inter-substituted products, such as m-nitrobenzaldehyde, which is the main product of nitrification. Cyclization and neutralization by ethanolamine hydrochloride can generate piperazine hexahydrate. Benzaldehyde undergoes a disproportionation reaction in a concentrated alkali solution (Cannizarro reaction): one molecule of the aldehyde is reduced to the corresponding alcohol, and another molecule of the aldehyde is simultaneously oxidized to a carboxylate salt. The speed of this reaction depends on the substituents on the aromatic ring.

Ⅱ. Product Use.

  1. Benzaldehyde can carry out nucleophilic addition, hydroxyl aldehyde condensation, Conichalot reaction, Pankin reaction, nitrification and chlorination and other series of reactions, derived into a number of chemical products, widely used in the pharmaceutical, spice, pesticide and dye industries.
  2. Benzaldehyde is used in the manufacture of pharmaceuticals, such as phenylaminoacetic acid, N-methyl-2-methylfuranamine sulfate, 2-phenylbenzimidazole, ephedrine, and chloramphenicol.
  3. Benzaldehyde itself is used as a spice and seasoning, and is also used in the processing and synthesis of other spices and seasonings, such as cinnamic acid and its esters, cinnamyl alcohol, cinnamaldehyde, pentyl and hexyl cinnamaldehyde, phenylacetaldehyde and almondic acid.
  4. Benzaldehyde is used in the manufacture of pesticides, such as the herbicide strobilurin for the control of wild oats. In photochemistry, it is used as a photographic agent and corrosion inhibitor.
  5. Benzaldehyde is used as an intermediate in the manufacture of dyes, such as for the manufacture of triphenylmethane fuel, and the condensation of benzaldehyde and N,N-dimethylaniline can be obtained as occult malachite green. In addition, it is also used in the manufacture of pyridone dyes.
  6. Benzaldehyde is also used in the manufacture of benzylamine, benzyl alcohol, mandelic acid and 4-phenyl-3-buten-2-one, as an additive in polyamide dyeing, as an additive in electroplating baths, and as a gelling agent in reaction with polyols (such as sorbitol).
  7. At present, the use of benzaldehyde has a new extension, can be used as paint defoamer, vinyl chloride suspension polymerization kettle deterrent, anti-friction lubricants and so on.

Ⅲ. Production Method.

Benzaldehyde can be prepared in a variety of ways. In industry, toluene is mainly oxidized under catalysts (vanadium pentoxide, tungsten trioxide, or molybdenum trioxide) in the vapor phase with air or oxygen; or toluene is chlorinated to benzyl chloride under light and then hydrolyzed and oxidized; or chlorinated to dichloromethylbenzene and then hydrolyzed. It can also be chlorinated to dichloromethylbenzene and then hydrolyzed.

It can also be produced in industry by reacting benzene under pressure and aluminum trichloride with carbon monoxide and hydrogen chloride. In the laboratory, benzaldehyde is prepared by catalytic reduction of benzoyl chloride. The current main routes of preparation are liquid chlorination or oxidation of toluene. Outdated preparation methods include incomplete oxidation of benzyl alcohol, alkaline solution of benzoyl chloride, and addition of benzene to carbon monoxide.

Since the direct oxidation of toluene in the electrolytic cell to produce benzaldehyde causes serious electrical pollution and low yield, the indirect electrosynthetic oxidation method is generally used to produce benzaldehyde. The so-called indirect electrosynthesis method, is the selection of electronic carriers with oxidation or reduction activity, so that it and the organic oxidation or reduction reaction.

After the reaction of the electronic carrier itself and then through electrolysis, the loss or gain of electrons, and then revert to the electronic carrier with oxidation or reduction activity, that is, the electronic carrier can be regenerated on the electrode. Thus, the electron carriers can be recycled in the organic electromechanical synthesis, continuously making the organic matter undergo the desired reaction. Indirect electro-oxidative synthesis of benzaldehyde is used here.

Chemical reaction equation.

IV. Processes

Indirect organic electromechanical synthesis is the use of water-soluble inorganic redox pairs as a medium, and organic reaction, and then the water phase and the organic phase will be separated to obtain products. The mordant in the aqueous phase, which has lost its oxidizing (or reducing) ability, can be regenerated and reused through the electrode reaction after oxidation (or reduction).

If organic synthesis and mordant regeneration in the same reactor, known as in-tank indirect electromechanical synthesis; if organic synthesis and mordant regeneration in the chemical reactor and electrochemical reactor, respectively, known as out-of-tank indirect electromechanical synthesis. The electrolytic cell is separated by a perfluorine ion exchange membrane and the electrodes are platinum and lead oxide, etc. Mn(lll) is another commonly used oxidant.

Mn(lll) is another commonly used oxidant. Mn(ll)/Mn(lll) is used as the electrolytic medium, a diaphragm-less electrolytic cell is selected, the anode is lead oxide/titanium electrode, the cathode is pure lead, and the electrolytic manganese sulfate/sulfuric acid system is used.

Ⅴ. Process Characteristics

Compared with the traditional organic synthesis method, the indirect electro-oxidation method has significant advantages. The method is an environmentally friendly green production process with mild reaction conditions, simple operation, high product yield, and closed-circuit operation; however, it requires large amounts of electrical energy and high production costs.

The benzaldehyde series compounds can be produced by the anodic oxidation of acryl, hydroxyl and methyl groups on the benzene ring, the cathodic reduction of carboxyl groups on the benzene ring or the anodic substitution of methyl groups on the benzene ring, especially the indirect electro-oxidation of methyl groups on the benzene ring.

Electrolytic medium: Indirect electrolytic oxidation uses an electron carrier with oxidation or reduction activity, and carries out oxidation or reduction reactions with organic substances. The research of indirect electrochemical synthesis of benzaldehyde first focused on the anode process.

In this process, the selected electrolytic coal must be highly selective for the oxidation of toluene or benzyl alcohol, producing more aldehydes and fewer by-products. The anionic electrolytic mordants include S2O82- / SO42-, BrO- / Br-, ClO / Cl-, and Cr2O72- / Cr3+, while the ionic electrolytic mordants include Co3+ / Co2+, Ce4+ / Ce3+ and other pairs. In order to make full use of the cathode, it is found that the cathode-catalyzed pairs Fe3+/Fe2+, V5+/V4+, V4+/V3+, Cu2+/Cu+ and Mn3+/Mn2+ can also be used for the electrolytic reduction of benzaldehyde.

Electrode: In the synthesis of benzaldehyde by indirect electro-oxidation using manganese sulfate as medium, pure lead is used as cathode in the electrolysis of manganese sulfate. The anode is easily corroded, so platinum or platinum-based precious metal materials are used as the anode, but these materials are scarce and expensive, so it is difficult to produce them industrially in China.

Although the coated electrode has high oxygenation overpotential, antioxidant corrosion resistance and good experimental performance, the coating has some peeling phenomenon, which affects the service life of the electrode. It is reported that the alloy electrode made by adding a small amount of noble metals to the cheap metal can not only be cheaper, but also solve the problem of short electrode life caused by the shedding of the coating.

Reaction conditions: In the non-homogeneous electrolysis of manganese sulfate in a diaphragm-less electrolyzer, the temperature and sulfuric acid concentration have a great influence on the viscosity of the electrolyte and the disproportionation of Mn3+, which affects the current efficiency of the electrolysis process and the subsequent toluene oxidation process. High current efficiency is obtained at low temperature and low acidity and high temperature and high acidity, while the current efficiency decreases at low temperature and high acidity. High-temperature low-acid electrolysis has high current efficiency, but it is not suitable for high-temperature low-acid electrolysis because of the severe disproportionation.

The reason for low electrolysis efficiency at low temperature and high acidity may be due to the increase of acidity, increase of solution viscosity, and decrease of manganese sulfate solubility, which causes adhesion of manganese sulfate on the electrode and decreases the electrolysis efficiency. If the temperature is increased, the solubility of manganese sulfate increases, the viscosity of the electrolyte decreases, and the electrolysis efficiency increases. Choosing the concentration of sulfuric acid for the electrolysis reaction that is consistent with the oxidation reaction can avoid the adjustment of acidity during electrolysis and oxidation and simplify the steps.

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