DE2360494A1 - PROCESS FOR THE PREPARATION OF ALKYL-SUBSTITUTED HYDROCHINONES - Google Patents

Description

BASF Aktiengesellschaft

Our reference: O. Z. 30 25O Bk / GP 6700 Ludwigshafen, December 3rd, 1973

Process for the preparation of alkyl-substituted

Hydroquinones

The invention relates to a process for the preparation of alkyl hydroquinones by electrolytic oxidation of alkyl phenols and subsequent electrolytic reduction of the quinones thus obtained.

The anodic oxidation of phenols to the corresponding benzoquinones in a divided cell and the electrochemical synthesis of hydroquinones in an undivided cell have been known for some time (see reports of the German Chemical Society Volume 47, page 2003 (191 ² I-), HeIv . Chim. Acta volume 2, page 583, (1919), loc.cit. Volume 8, page 7 ² ^ (1925) loc.cit. Volume 10, page-40 (1927) and loc.cit. Volume 10, page (1927)), In these processes, however, the material yield and the current yield are not very high and a number of undesirable by-products are also formed. These by-products make expensive cleaning operations necessary.

From the German laid-open specification 1 643 558 it is also known that unsubstituted phenol can be obtained ^{in 90 percent yield by choosing the electrolysis parameters such as current density, depolarizer and electrolyte concentration 1.} The current yields here are approx. Βθ%. If one tries now to transfer these conditions to the anodic oxidation of alkyl-substituted phenols, the corresponding hydroquinones are obtained, but only in very low material yields, the current yields even drop below 20 % .

For each it is in Chemical Communications 1971, pages 1643 ff. described that dimethylphenol can be used electrolytically with concomitant use from acetonitrile as a solvent to dimethylquinone oxy-

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can date. However, if one tries to reduce the alkylquinone obtained in this way to the corresponding hydroquinone, the yields drop still much to be desired. A major disadvantage, however, is that the hydroquinones obtained are not suitable for further processing have the necessary purity and thus require a complex cleaning process.

It has now been found that one can alkylhydrochinones of the general formula

OH

in which R denotes an alkyl radical having 1 to 4 carbon atoms and η stands for an integer from 1 to 3, by electrolytic oxidation of alkylphenols of the general formula

II,

in which R and η have the above meaning, with the proviso that the para position is not substituted and subsequent electrolytic Reduction of the alkylquinones thus obtained in the presence of an aqueous solution of an electrolyte and in the presence of a solvent is more advantageous than before when one water soluble ketones used as solvents.

The new method has the advantage that it works with both good Material as well as with good current yields. A decisive advantage of the new process, however, is that the obtained Hydroquinones are produced in over 90 percent purity and no longer have to be cleaned for further processing.

Preferred alkylphenols of the formula II are those in which R, for represents a methyl radical. It goes without saying that in each case the

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oxidizing para division in the starting phenol II unsubstituted is. Suitable phenols are, for example, 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol or 2,3,5-trimethylphenol.

2,6-Dimethylphenol and 2,3 * 6-Trimethylphenol are of particular technical importance attained.

The electrolysis is carried out using an aqueous solution of an electrolyte. The electrolytes used are preferably non-oxidizing mineral acids, in particular sulfuric acid. In general, the mineral acid is used in a concentration of 1 to 20 percent by weight, in particular 5 to 10 wt.

During the anodic oxidation and the cathodic reduction, room temperatures or slightly elevated temperatures are maintained, e.g. from 20 to 40 ° C. The boiling temperatures should be on each pail the solvent used is not exceeded will.

In the case of electrolysis, it is advantageous to maintain current densities

ρ
5 amps per dm a. In general, the range of appropriate

applied current densities between 5 and 20 amperes per dm.

Lead dioxide or coated with lead dioxide are used as anodes Electrodes or noble metal electrodes such as platinum, platinum-plated Titanium or gold. Lead dioxide anodes are preferably used.

The cathodes are lead, mercury, cadmium, tin, zinc, Copper, nickel, and also silver or lead amalgam electrodes are possible. Lead electrodes have become particularly important.

The anodic oxidation of the phenols is expediently carried out in one partitioned anode compartment through and then reduces the so obtained

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Alkylquinones in a subsequent stage in one as well partitioned cathode compartment. It has proven to be particularly useful to carry out the oxidation in a divided cell in the anode compartment the alkylphenols II carries out the solution thus obtained conducts into the cathode compartment and there carries out the electrochemical reduction to the corresponding hydroquinone.

It is an essential feature of the invention that one as a solvent water-soluble ketones are also used. Suitable water-soluble ketones are, for example, acetone or methyl-ethyl Diet hy! Ketone. Acetone has proven particularly useful as a solvent. The electrolysis mixture to be used advantageously contains 20 to 80 percent by weight, in particular 40 to βθ percent by weight, Acetone.

It is expedient to start from an electrolysis mixture which contains 1 to 10 percent by weight of alkylphenols of the formula II.

The alkylhydroquinones produced as end products are in the Usually obtained by evaporating the solvent. You can also use a suitable water-immiscible solvent extracted and obtained from this in the usual way, e.g. precipitation or fractional crystallization.

Alkyl substituted hydroquinones made by the process of the invention are useful in making Plant protection products, dyes or biologically active agents, e.g. vitamin E (see S.F. Dyke, The Chemistry of the Vitamins, p. 256 ff. Interscience Publishers 1965).

The process according to the invention is illustrated by the following examples.

Alkyl hydroquinones are also suitable as polymerization inhibitors (see Belgian patent specification 779 388).

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example 1

Anodic oxidation of 2,6-dimethylphenol

Device: divided cell with cation exchange membrane

Anode: PbO ₂ electrode; F: Ο, ββ dm

Anolyte: 2M-Λ g (0.2 mol) 2,6-dimethylphenol

550 ml HgO

450 ml of acetone

49 g H ₂ SO ₂ ; conc.
Catholyte: 1 η HgSO ₂₁ cathode: Pb electrode
Charge Q: 0.8 P
Current I: 10 A -

After the end of the electrolysis, a sample was taken from the anolyte, the acetone was distilled off and the residue was extracted several times with ether. After the ether has been distilled off, 2,6-dimethyl-p-benzoquinone and unreacted starting material are determined by gas chromatography. According to this, the material yield of 2,6-dimethyl-p-benzoquinone (based on converted 2,6-dimethylphenol) is 86/2 %, the current yield is 5.8%.

Example 2

Electrochemical synthesis of 2,6-dimethyl! Hydroquinone contraption

Anolyte Js * as in Example 1 cathode

I and Q

Catholyte: anolyte from example 1.

After the electrolysis has ended, acetone is distilled off and the residue for separating off the unreacted 2,6-dimethylphenol

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steam distilled. After the residue from the steam distillation has cooled, 2,6-dimethylhydroquinone precipitates. The yield, based on converted 2,6-dimethylphenol, is $ 65. The purity of the 2,6-dimethylhydroquinone obtained is 90 %.

Example 3

Anodic oxidation of 2,3,6-trimethylphenol

Contraption; Anode: anolyte:

Catholyte cathode:

divided cell with cation exchange membrane

PbO ₂ ; Q: 0.66 dm ²

27.2 g (0.2 mole) 2,3,6-trimethylphenol

500 ml H ₂ O

500 ml of acetone

49 g conc. InH Pb

Q: 0.8 F; 1: 5 A

If a sample of the electrolysis discharge is worked up analogously to Example 1, trimethyl-p-benzoquinone is obtained in 84.6 percent yield (based on converted 2,3,6-trimethylphenol). The current efficiency is 54 %.

Example 4

Electrochemical synthesis of trimethylhydroquinone

as in example j5

contraption

Anolyte cathode I and Q, catholyte: anolyte from example 3

-7-

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After the electrolysis has ended, the catholyte is worked up analogously to rinse 1 2. TrimethylhydroeMnon is obtained in over 90 percent purity. The material yield (based on reacted 2,5,6-rTrimethylphenol ·) is 77, ² I- $.

Comparative example

If one proceeds as described in examples JJ 'and 4.;,. However, use acetonitrile instead of acetone, s: o contributes to the yield of 2,3,6-tr ime thy! hydroquinone 45 - 55 %, ΊΜ &: Purity is: only 70 - 75 %.

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Claims (1)

Claim Process for the preparation of alkyl hydroquinones of the general formula I, in which R denotes an alkyl radical having 1 to 4 carbon atoms and η represents an integer from 1 to J5 by electrolytic Oxidation of alkylphenols of the general formula II, in which R and η have the above meaning, with the proviso that the para position is not substituted and subsequent electrochemical reduction of the alkylquinones thus obtained in the presence of an aqueous solution of an electrolyte and in the presence of a solvent, characterized in that water-soluble ketones are used as solvents used. BASF Aktiengesellschaft 509824/0759