EP0029995B1 - Process for the preparation of 4-tert.-butylbenzaldehyde - Google Patents

Description

The present invention relates to a process for the electrochemical production of 4-tert. Butylbenzaldehyde.

The electrosynthesis of substituted benzaldehydes by anodic oxidation of the corresponding alkylbenzenes is, for. B. in Helv. Chim. Acta 9, 1097 (1926). In this process, the electrolysis is carried out in sulfuric acid solution. However, the benzaldehydes are formed only in low yields, and the isolation of the aldehydes from the resulting multicomponent mixtures is difficult. From US-A 4 148 696 it is known that benzaldehydes can be prepared by electrochemical oxidation of the corresponding alkyl aromatics if the electrolysis is carried out in the presence of fatty acids, their alkali metal or alkaline earth metal salts and tetraalkylammonium salts. In addition to the benzaldehydes, the nucleic and side chain-substituted fatty acid esters also form as by-products in this process. Furthermore, the processing of the electrolysis discharges requires a number of separation operations. When the synthesis is carried out industrially, the fatty acids and fatty acid salts must be separated from the benzaldehydes and recycled, and then the benzaldehydes must be separated from the fatty acid esters.

It has now been found that 4-tert-butylbenzaldehyde is obtained by electrochemical oxidation of 4-tert. Butyltoluene in the presence of mineral acids can be produced considerably more advantageously if the electrochemical oxidation is carried out in the presence of additional alkyl, alkenyl or arylsulfonic acids. 4-tert. Butylbenzaldehyde in high yield. Another major advantage of the method according to the invention is the simple processing of the electrolysis discharges. Thus, after the electrolysis has ended, the 4-tert can be obtained from the organic phase obtained by phase separation. Isolate butylbenzaldehyde directly by distillation.

The new process can be carried out batchwise or continuously. Electrolysis does not require a special electrolysis cell. You can e.g. B. in the technically frequently used plate and frame cell. The electrolyte consists of 4-tert. Butyltoluene and an aqueous solution of a mineral acid to which small amounts of an alkyl, alkenyl or arylsulfonic acid are added. For example, sulfuric acid is used as the mineral acid. Long-chain alkyl sulfonic acids, alkenyl sulfonic acids or aryl sulfonic acids which additionally carry alkyl groups in the aryl radical are preferably used as sulfonic acid. Long-chain alkyl sulfonic acids or alkenyl sulfonic acids are understood to mean those with at least 6 carbon atoms. Examples of suitable sulfonic acids are: aliphatic sulfonic acids, such as the acids of the formulas C ₁₄ H ₂₉ SO ₃ H, C ₁₆ H ₃₃ SO ₃ H and C ₁₇ H ₃₅ SO ₃ H, or aliphatic-aromatic sulfonic acids, such as benzenesulfonic acids, on the benzene ring still carry alkyl radicals of the formulas C _10-14 H _21-29 , or butylnaphthalenesulfonic acid. Mixtures of sulfonic acids can also be used.

The electrolytes used in the electrolysis have, for example, the following composition: 5 to 50% by weight 4-tert. Butyltoluene, 0.5 to 10% by weight sulfuric acid, 40 to 90% by weight water and 0.05 to 5% by weight sulfonic acid.

Suitable anode materials are e.g. B. lead dioxide or titanium coated with lead dioxide. For example, lead, iron, steel or graphite electrodes are used as cathodes. The electrolysis itself is carried out at current densities of 1 to 10 A / dm ² and temperatures between 10 and 90 ° C. The 4-tert. Butyltoluene conversion is preferably 10 to 50%. The electrolysis discharges are preferably worked up by distillation. After phase separation, the organic phase is distilled in vacuo.

Unreacted 4-tert. Butyltoluene is returned to the electrolysis.

4-tert. Butylbenzaldehyde is a valuable intermediate for fungicides and for fragrances (Lilial).

example

• Anode : Pb0₂
• Anolyt : 300 g 4-tert. Butyltoluol
• 1 200 g 5 % ige wäßrige Schwefelsäure
• 15 g 30 %ige wäßrige Lösung von C₁sH₃₁S0₃H
• Katholyt : 5 %ige wäßrige Schwefelsäure
• Kathode : Pb
• Stromdichte : 5 A/dm²
• Temperatur: 30 bis 32 °C
• Elektrolyse mit 3,3 F/Mol 4-tert. Butyltoluol

Apparatus: divided cell with cation exchange membrane

• Anode: Pb0 ₂
• Anolyte: 300 g 4-tert. Butyltoluene
• 1,200 g of 5% aqueous sulfuric acid
• 15 g 30% aqueous solution of C ₁ sH ₃₁ S0 ₃ H
• Catholyte: 5% aqueous sulfuric acid
• Cathode: Pb
• Current density: 5 A / dm ²
• Temperature: 30 to 32 ° C
• Electrolysis with 3.3 F / mol 4-tert. Butyltoluene

The electrolyte is pumped through a heat exchanger during the electrolysis. After the end of the electrolysis, the phases are separated and the organic phase is distilled off at two torr and 60 to 120 ° C. In addition to 180 g of unreacted 4-tert. Butyltoluene 92.4 g 4-tert-butylbenzaldehyde. This corresponds to a yield of 70.4%.

Claims (4)

1. A process for the preparation of 4-tert.-butyl- benzaldehyde by electrochemical oxidation of 4-tert.-butyltoluene in the presence of an inorganic acid, wherein the electrochemical oxidation is carried out in the presence of an additional alkylsulphonic, alkenylsulphonic or arylsulphonic acid.

2. A process as claimed in claim 1, wherein the inorganic acid used is sulphuric acid.

3. A process as claimed in claim 1, wherein the sulphonic acid used is an aliphatic sulphonic acid of the formula C₁₄,₁₇H₂₉-₃₅SO₃H.

4. A process as claimed in claim 1, wherein the electrochemical oxidation is carried out with an electrolyte which contains 5-50 % by weight of 4-tert.-butyltoluene, 0.5-10 % by weight of sulphuric acid, 40-90 % by weight of water and 0.05-5 % by weight of sulphonic acid.