Process for the selective preparation of a 2-hydroxybenzoic acid and a 4-hydroxybenzaldehyde and derivatives thereof

The present invention concerns a process for the preparation of a 2-hydroxybenzoic acid and of a 4-hydroxybenzaldehyde and derivatives thereof from a mixture of two phenolic compounds, one carrying a formyl or hydroxymethyl group in the 2 position, and the other carrying a formyl or hydroxymethyl group in the 4 position. The invention also concerns the preparation of a 4-hydroxybenzaldehyde from said mixture. The invention more particularly concerns the preparation of 3-methoxy-4-hydroxybenzaldehyde and of 3-ethoxy4-hydroxybenzaldehyde, respectively known as "vanillin" and ethylvanillin". The process for the preparation of a 2-hydroxybenzoic acid and a 4-hydroxybenzaldehyde and derivatives thereof is characterised in that the formyl or hydroxymethyl group in the 2 position of compound (A) in a mixture of phenolic compounds, one (A) carrying a formyl or hydroxymethyl group in the 2 position, and the other (B) carrying a formyl or hydroxymethyl group in the 4 position, is selectively oxidised to a carboxy group and optionally a hydroxymethyl group of compound (B) in the 4 position is selectively oxidised to a formyl group, thus producing a mixture of a 2-hydroxybenzoic acid and a 4-hydroxybenzaldehyde.

This application is an application under 35 U.S.C. Section 371 of
 International Application Number PCT/FR97/01828, filed on Oct. 13, 1997.
 The present invention concerns a process for the preparation of a
 2-hydroxybenzoic acid and of a 4-hydroxybenzaldehyde and derivatives
 thereof from a mixture of two phenolic compounds, one carrying a formyl or
 hydroxymethyl group in the 2 position, and the other carrying a formyl or
 hydroxymethyl group in the 4 position.
 The invention also concerns the preparation of a 4-hydroxybenzaldehyde from
 said mixture.
 The invention more particularly concerns the preparation of
 3-methoxy-4-hydroxybenzaldehyde and of 3-ethoxy-4-hydroxybenzaldehyde,
 respectively known as "vanillin" and ethylvanillin".
 French patent application no. 95/06186 describes a process for the
 preparation of 4-hydroxybenzaldehydes, and more particularly of vanillin
 and of ethylvanillin.
 The process described consists of preparing a
 3-carboxy-4-hydroxybenzaldehyde, then of decarboxylation of said compound,
 thereby producing 4-hydroxybenzaldehyde.
 3-carboxy-4-hydroxybenzaldehyde is prepared according to FR no. 95/06186
 from one of the compounds, and mixtures thereof, having more particularly
 the following formulae (IIa), (IIb), (IIc) and (IId) shown below:
 ##STR1##
 where, in said formulae:
 M represents a hydrogen atom and/or a metal cation from group (Ia) or
 (IIa), or an ammonium cation,
 Z.sub.1, Z.sub.2 and Z.sub.3, which may be identical or different,
 represent a hydrogen atom, an alkyl, alkenyl, alkoxy, hydroxyalkyl,
 alkoxyalkyl, cycloalkyl, or aryl radical, a hydroxy group, a nitro group,
 a halogen atom, or a trifluoromethyl group.
 The patented process starts with a bi-functional phenolic compound carrying
 on the aromatic ring two functional groups of the hydroxy group, which can
 be a --CHO group and/or a --CH.sub.2 OH group, at the ortho and para
 positions.
 Firstly, the group in the ortho position is selectively oxidised to the
 carboxy group; the group in the para position being at the most oxidised
 to the formyl group. Thus, after eliminating the carboxy group in the
 ortho position, 4-hydroxybenzaldehyde is obtained.
 Vanillin and ethylvanillin are advantageously obtained according to a
 selective process but one which is also highly competitive industrially as
 it uses less expensive reactants.
 As a result of research, the applicant has found that it is possible a
 start with a mixture of monosubstituted phenolic compounds.
 A process was found, and is object of the present invention, for the
 preparation of a 2-hydroxybenzoic acid and a 4-hydroxybenzaldehyde and
 derivatives thereof, which is characterised in that in a mixture of
 phenolic compounds, one (A) carrying a formyl or hydroxymethyl group in
 the 2 position, and the other (B) carrying a formyl or hydroxymethyl group
 in the 4 position, the formyl or hydroxymethyl group in the 2 position of
 compound (A) is selectively oxidised to a carboxy group and optionally a
 hydroxymethyl group of compound (B) in the 4 position is selectively
 oxidised to a formyl group, thus producing a mixture of a 2-hydroxybenzoic
 acid and a 4-hydroxybenzaldehyde.
 In a successive step, the 4-hydroxybenzaldehyde is separated from the
 reaction medium.
 A first variation of the invention consists of separating the
 4-hydroxybenzaldehyde from the 2-hydroxybenzoic acid by pH controlled
 extraction of aldehyde.
 Another variation of the invention consists of decarboxylation of only the
 2-hydroxybenzoic acid in the mixture obtained, producing the initial
 phenolic compound which can then be recycled; the 4-hydroxybenzaldehyde is
 then recovered in a conventional manner.
 According to the present invention, it was found that when starting with a
 mixture of phenolic molecules, one carrying hydroxymethyl or formyl groups
 in the ortho position of the hydroxyl group and the other carrying a
 hydroxymethyl or formyl group in the para position of the hydroxy group,
 oxidation to a carboxy group preferably takes place on the hydroxymethyl
 or formyl group carried by the (A) molecule, substituted in the ortho
 position.
 The starting substrates used in the process are mixtures of phenolic
 compounds, one carrying a formyl or hydroxymethyl group in the 2 position
 and the other in the 4 position.
 The term "phenolic compound" denotes any aromatic compound with an aromatic
 ring which carries a hydroxy group.
 In the following description of the present invention, the term "aromatic"
 denotes the conventional idea of aromaticity as defined in the literature,
 particularly in "Advanced Organic Chemistry" by Jerry MARCH, 4th edition,
 John Wiley and Sons, 1992, pp. 40 ff.
 Thus, a mixture (II) of phenolic compounds is used, more particularly with
 the following formulae:
 ##STR2##
 where, in said formulae (IIA) and IIB):
 Y.sub.1 and Y.sub.2, which may be identical or different, represent one of
 the following groups:
 a --CHO group,
 a --CH.sub.2 OH group,
 Z.sub.1, Z.sub.2 and Z.sub.3, which may be identical or different,
 represent a hydrogen atom, an alkyl, alkenyl, alkoxy, hydroxyalkyl,
 alkoxyalkyl, cycloalkyl, or aryl radical, a hydroxy group, a nitro group,
 a halogen atom, or a trifluoromethyl group.
 Particularly suitable compounds for use in the process of the invention
 have formulae (IIA) and (IIB) where Z.sub.1, Z.sub.2 and Z.sub.3, which
 may be identical or different, represent one of the following atoms or
 groups:
 a hydrogen atom,
 a linear or branched alkyl radical containing 1 to 12 carbon atoms,
 preferably 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl,
 butyl, isobutyl, sec-butyl or tert-butyl,
 a linear or branch alkenyl radical containing 2 to 12 carbon atoms,
 preferably 2 to 4 carbon atoms, such as vinyl or allyl,
 a linear or branched alkoxy radical containing 1 to 12 carbon atoms,
 preferably 1 to 4 carbon atoms, such as a methoxy, ethoxy, propoxy,
 isopropoxy, butoxy, isobutoxy, sec-butoxy or tert-butoxy radical,
 a phenyl radical,
 a halogen atom, preferably a fluorine, chlorine or bromine atom.
 The present invention does not exclude the presence on the aromatic cycle
 of substituents of a different nature, provided that they do not interfere
 with the reactions taking place in the process of the invention.
 The present invention is preferably applicable to compounds with the
 formulae (IIA) and (IIB) where Z.sub.1, represents a hydrogen atom or a
 linear or branched alkyl or alkoxy radical containing 1 to 6 carbon atoms,
 preferably 1 to 4 carbon atoms; Z.sub.2 and Z.sub.3 represent a hydrogen
 atom; and Y.sub.1, and Y.sub.2 are identical and represent a formyl group
 or a hydroxymethyl group.
 Examples of preferred mixtures of phenolic compounds which can be used in
 the process of the invention are, inter alia:
 o-hydroxymethylphenol and p-hydroxymethylphenol,
 o-hydroxymethylguaiacol and p-hydroxymethylguaiacol,
 o-formylguaiacol and p-formylguaiacol,
 o-hydroxymethylguetol and p-hydroxymethylguetol,
 o-formylguetol and p-formylguetol.
 According to the process of the invention, a mixture of phenolic compounds
 having preferably formula (II) is used as the starting compound.
 The proportion of each hydroxymethylated or formylated phenolic compound in
 the mixture depends on how they are prepared.
 By way of an example, the proportion of each isomer can vary greatly, for
 example from 10 to 90% by weight, and more preferably between 30 and 70%.
 The reaction scheme of the process according to the invention is given
 below to facilitate comprehension of the disclosure of the invention,
 without in any way binding the scope of the invention to said scheme.
 ##STR3##
 where, in said formulae (I) to (IV):
 Y.sub.1, and Y.sub.2, which may be identical or different, represent one of
 the following groups:
 a --CHO group,
 a --CH.sub.2 OH group,
 M represents a hydrogen atom and/or a metal cation from group (Ia) or (IIa)
 of the periodic classification, or an ammonium cation,
 Z.sub.1, Z.sub.2 and Z.sub.3 have the meanings given above.
 In the present text, references are made hereinafter to the periodic
 classification of the elements as published in the "Bulletin de la Societe
 Chirnique de France", no. 1 (1966).
 In accordance with the process of the invention, the Y.sub.1 group in
 position 2 of a phenolic compound (A) having preferably the formula (IIA),
 is selectively oxidized to a carboxy group, and optionally a hydroxymethyl
 group in the 4 position of a phenolic compound (B) preferably having the
 formula (IIB), is selectively oxidized to a formyl group.
 Oxidation is effected by molecular oxygen or a gas containing molecular
 oxygen, generally in the presence of a catalyst.
 A preferred oxidation method consists of oxidising a mixture of phenolic
 compounds having the formula (II) in the liquid phase using molecular
 oxygen or a gas containing molecular oxygen, in an aqueous medium
 comprising a basic agent, in the presence of a catalyst based on a metal
 M.sub.1 selected from metals from group 1b and 8 of the periodic
 classification of the elements, optionally comprising as an activator
 metals such as cadmium, cerium, bismuth, lead, silver, tellurium or tin.
 According to the invention, it was discovered in a totally unexpected
 manner that if the temperature was increased and the reaction was
 preferably carried out under pressure, or if the quantity of the base
 present during oxidation was increased, the formyl and/or hydroxymethyl
 groups in the 2 position of molecule (A) were selectively oxidized to a
 carboxy group, and the group in the 4 position of molecule (B) was at the
 most oxidized to a formyl group.
 The catalysts used in the process of the invention are based on a metal
 from the group 1b and 8 of the periodic classification.
 Examples of catalysts based on a metal from group 8 of the periodic
 classification are nickel, ruthenium, rhodium, palladium, osmium, iridium,
 platinum and mixtures thereof. Of the metals from group 1b, copper is
 preferred.
 Preferably, platinum and/or palladium catalysts are used, taken from all
 the available forms such as, for example: platinum black, palladium black,
 platinum oxide, palladium oxide, or the noble metal itself, deposited on
 different supports such as carbon black, calcium carbonate, activated
 aluminas and silicas, or equivalent materials. Catalytic masses based on
 carbon black are particularly suitable.
 The quantity of catalyst to be used, expressed as the weight of metal
 M.sub.1 with respect to that of the mixture of phenolic compounds with
 formula (II) can vary from 0.01 to 10%, and preferably 0.04 to 2%.
 Further details of the catalysts can be obtained from U.S. Pat. No.
 3,673,257, FR-A-2 305 420 and FR-A-2 350 323.
 An activator is preferably used in the catalysts employed in the process of
 the invention.
 The activator can be selected from all those mentioned in the above
 patents. Bismuth, lead and cadmium, in the form of the free metal or as
 cations, are preferably used. In the latter case, the associated anion is
 not critical and all derivatives of these metals can be used. Preferably,
 bismuth metal or derivatives thereof is used.
 An inorganic or organic bismuth derivative of bismuth can be used, in which
 the bismuth atom has an oxidation number greater than zero, for example 2,
 3, 4 or 5. The residue associated with the bismuth is not critical once it
 satisfies this condition. The activator can be soluble or insoluble in the
 reaction medium.
 Illustrative compounds of activators which can be used in the process
 according to the present invention are: bismuth oxides; bismuth
 hydroxides; salts of inorganic hydracids such as: bismuth chloride,
 bromide, iodide, sulphide, selenide or telluride; salts of inorganic
 oxyacids such as: bismuth sulphite, sulphate, nitrite, nitrate, phosphite,
 phosphate, pyrophosphate, carbonate, perchlorate, antimonate, arsenate,
 selenite or selenate; and salts of oxyacids derived from transition metals
 such as: bismuth vanadate, niobate, tantalate, chromate, molybdate,
 tungstate or permanganate.
 Other suitable compounds are the salts of aliphatic or aromatic organic
 acids such as: bismuth acetate, propionate, benzoate, salicylate, oxalate,
 tartrate, lactate, or citrate; and phenates such as: bismuth gallate or
 pyrogallate. These salts and phenates can also be bismuthyl salts.
 Other inorganic or organic compounds which can be used are binary compounds
 of bismuth with elements such as phosphorous or arsenic; heteropolyacids
 containing bismuth and salts thereof; and aliphatic and aromatic
 bismuthines are also suitable.
 Specific examples are:
 oxides: BiO; Bi.sub.2 O.sub.3 ; Bi.sub.2 O.sub.4 ; Bi.sub.2 O.sub.5,
 hydroxides: Bi(OH).sub.3,
 salts of inorganic hydracids: bismuth chloride BiCl.sub.3 ; bismuth bromide
 BiBr.sub.3 ; bismuth iodide BiI.sub.3 ; bismuth sulphide Bi.sub.2 S.sub.3 ;
 bismuth selenide Bi.sub.2 Se.sub.3 ; bismuth telluride Bi.sub.2 Te.sub.3,
 salts of inorganic oxyacids: basic bismuth sulphite Bi.sub.2
 (SO.sub.3).sub.3,Bi.sub.2 O.sub.3,5H.sub.2 O; neutral bismuth sulphate
 Bi.sub.2 (SO.sub.4).sub.3 ; bismuthyl sulphate (BiO)HSO.sub.4 ; bismuthyl
 nitrite (BiO)NO.sub.2,O.5H.sub.2 O; neutral bismuth nitrate
 Bi(NO.sub.3).sub.3,5H.sub.2 O; double nitrate of bismuth and magnesium
 2Bi(NO.sub.3).sub.3,3Mg(NO.sub.3).sub.2,24H.sub.2 O; bismuthyl nitrate
 (BiO)NO.sub.3 ; bismuth phosphite (Bi.sub.2 (PO.sub.3 H).sub.3,3H.sub.2 O;
 neutral bismuth phosphate BiPO.sub.4 ; bismuth pyrophosphate Bi.sub.4
 (P.sub.2 O.sub.7).sub.3 ; bismuthyl carbonate (BiO).sub.2
 CO.sub.3,0.5H.sub.2 O; neutral bismuth perchlorate
 Bi(ClO.sub.4).sub.3,5H.sub.2 O; bismuthyl perchlorate (BiO)ClO.sub.4 ;
 bismuth antimonate BiSbO.sub.4 ; neutral bismuth arsenate
 Bi(AsO.sub.4).sub.3 ; bismuthyl arsenate (BiO)AsO.sub.4,5H.sub.2 O;
 bismuth selenite Bi.sub.2 (SeO.sub.3).sub.3,
 salts of oxyacids derived from transition metals: bismuth vanadate
 BiVO.sub.4 ; bismuth niobate BiNbO.sub.4 ; bismuth tantalate BiTaO.sub.4 ;
 neutral bismuth chromate Bi.sub.2 (CrO.sub.4); bismuthyl dichromate
 [(BiO).sub.2 ]Cr.sub.2 O.sub.7 ; acid bismuthyl chromate H(BiO)CrO.sub.4 ;
 double chromate of bismuthyl and potassium K(BiO)CrO.sub.4 ; bismuth
 molybdate Bi.sub.2 (MoO.sub.4).sub.3 ; bismuth tungstate Bi.sub.2
 (WO.sub.4).sub.3 ; double molybdate of bismuth and sodium
 NaBi(MoO.sub.4).sub.2 ; basic bismuth permanganate Bi.sub.2 O.sub.2
 (OH)MnO.sub.4,
 salts of aliphatic or aromatic organic acids: bismuth acetate Bi(C.sub.2
 H.sub.3 O.sub.2).sub.3 ; bismuthyl propionate (BiO)C.sub.3 H.sub.5 O.sub.2
 ; basic bismuth benzoate C.sub.6 H.sub.5 CO.sub.2 Bi(OH).sub.2 ; bismuthyl
 salicylate C.sub.6 H.sub.4 CO.sub.2 (BiO)(OH); bismuth oxalate (C.sub.2
 O.sub.4).sub.3 Bi.sub.2 ; bismuth tartrate Bi.sub.2 (C.sub.4 H.sub.4
 O.sub.6).sub.3,6H.sub.2 O; bismuth (C.sub.6 H.sub.9 O.sub.5)OBi,7H.sub.2
 O; bismuth citrate C.sub.6 H.sub.5 O.sub.7 Bi, phenates: basic bismuth
 gallate C.sub.7 H.sub.7 O.sub.7 Bi; basic bismuth pyrogallate C.sub.6
 H.sub.3 (OH).sub.2 (OBi)(OH).
 Other suitable inorganic or organic compounds are: bismuth phosphide BiP;
 bismuth arsenide Bi.sub.3 As.sub.4 ; sodium bismuthate NaBiO.sub.3 ;
 bismuth-thiocyanic acids H.sub.2 [Bi(BNS).sub.5 ], H.sub.3 [Bi(CNS).sub.6
 ] and sodium and potassium salts thereof; trimethylbismuthine
 Bi(CH.sub.3).sub.3, triphenylbismuthine Bi(C.sub.6 H.sub.5).sub.3.
 The bismuth derivatives which are preferably used in the process according
 to the invention are: bismuth oxides; bismuth hydroxides; bismuth or
 bismuthyl salts of inorganic hydracids; bismuth or bismuthyl salts of
 inorganic oxyacids; bismuth or bismuthyl salts of aliphatic or aromatic
 organic acids; and bismuth or bismuthyl phenates.
 A particularly suitable group of activators for implementing the process of
 the invention is constituted by: bismuth oxides Bi.sub.2 O.sub.3 and
 Bi.sub.2 O.sub.4 ; bismuth hydroxide Bi(OH).sub.3 ; neutral bismuth
 sulphate Bi.sub.2 (SO.sub.4).sub.3 ; bismuth chloride BiCl.sub.3 ; bismuth
 bromide BiBr.sub.3 ; bismuth iodide BiI.sub.3 ; neutral bismuth nitrate
 Bi(NO.sub.3).sub.3,5H.sub.2 O; bismuthyl nitrate BiO(NO.sub.3); bismuthyl
 carbonate (BiO).sub.2 CO.sub.3,0.5H.sub.2 O); bismuth acetate Bi(C.sub.2
 H.sub.3 O.sub.2).sub.3 ; and bismuthyl salicylate C.sub.6 H.sub.4 CO.sub.2
 (BiO)(OH).
 The quantity of activator used, expressed as the quantity of metal
 contained in the activator with respect to the weight of metal M.sub.1
 used, can vary between wide limits. For example, this quantity can be as
 little at 0.1% and can reach the weight of metal M.sub.1 used, or even
 exceed it without any problems.
 More particularly, this quantity is selected so that it provides the
 oxidation medium with 10 to 900 ppm by weight of activator metal with
 respect to the mixture of phenolic compounds having the formula (II). In
 this respect, higher quantities of activator, of the order of 900 to 1500
 ppm, can naturally be used, but with no significant additional advantage.
 According to the process of the invention, oxidation is carried out in an
 aqueous medium containing a basic agent in solution, and more particularly
 ammonium hydroxide, alkaline or alkaline-earth bases, for example
 hydroxides such as sodium, potassium, lithium and barite hydroxide;
 alkaline alkanolates such as sodium or potassium methylate, ethylate,
 isopropylate and tert-butylate, sodium or potassium carbonates or
 bicarbonates, and in general, the salts of alkaline or alkaline-earth
 bases and weak acids.
 Thus, the compounds with formula (III) and (IV) can be completely or
 partially turned into salts depending on the quantity of basic agent used.
 It follows that in said formulae, M symbolises a hydrogen atom and/or a
 metal cation from group (Ia) or (IIa), or an ammonium cation.
 Sodium or potassium hydroxide is used for reasons of economy. The
 proportion of inorganic base to be used can be between 0.5 to 10 moles,
 preferably between 1 and 4 moles, and still more preferably between 2 and
 4 moles of inorganic base per mole of phenolic compounds with formula
 (II).
 The concentration by weight of the mixture of phenolic compounds with
 formula (II) in the liquid phase is usually between 1% and 60%, preferably
 between 2% and 30%.
 In practice, one manner of implementing the process consists of bringing
 the solution comprising the mixture of phenolic compounds with formula
 (II), the basic agent, the catalyst based on metal Ml, and any activator,
 into contact with molecular oxygen or a gas containing molecular oxygen,
 for example air, in the proportions indicated above.
 Atmospheric pressure can be used, but it is preferable to work under a
 pressure of between 1 and 20 bar.
 The mixture is then stirred at the desired temperature until a quantity of
 oxygen corresponding to that necessary for transforming the hydroxymethyl
 or formyl group of the compound (A) into a carboxy group, and optionally
 the hydroxymethyl group of the compound (B) into a formyl group, has been
 consumed.
 The temperature of the reaction to be used varies according to the thermnal
 stability of the products to be prepared.
 In accordance with the invention, the temperature is preferably selected in
 a temperature range going from 30.degree. C. to 200.degree. C., preferably
 between 40.degree. C. and 160.degree. C.
 The skilled person will adapt the temperature according to the reaction
 conditions (in particular the quantity of base, nature of the metal
 M.sub.1, pressure and stirring). It has been found, in particular, that
 the lower the temperature, the greater the quantity of basic agent which
 must be used.
 By way of examples, the preferred conditions for the preferred metals,
 platinum and palladium, will be given. For platinum, as the temperature
 selected is between 100.degree. C. and 160.degree. C., the quantity of
 base to be used is advantageously between 1 and 3 moles per mole of
 phenolic compounds with formula (II). In the case of palladium, the
 temperature can be selected between 30.degree. C. and 200.degree. C.,
 preferably between 30.degree. C. and 150.degree. C., and for this latter
 range, the quantity of base is preferably 2 to 4 moles per mole of
 phenolic compounds.
 Thus, the quantity of base has to be sufficient to oxidise the Y1 group, in
 the ortho position, to a carboxy group. It is determined by the skilled
 person according to the temperature and the metal selected.
 At the end of the reaction, which preferably lasts between 30 minutes and 6
 hours, 2-hydroxybenzoic acid which can be partially or totally in its salt
 form, and preferably has formula (III), and a 4-hydroxybenzaldehyde
 preferably having formula (IV), can be recovered.
 After any necessary cooling, the catalytic mass is then separated from the
 reaction medium, for example by filtration.
 4-hydroxybenzaldehyde will be recovered from the reaction medium.
 A first method for treatment of the reaction medium consists of pH
 controlled extraction of the 4-hydroxybenzaldehyde.
 To this end, the reaction medium is placed in contact with an organic
 solvent able to extract aldehyde.
 A solvent is chosen which is non-miscible with water.
 Examples of solvents suitable for the invention are, in particular, ketones
 such as methylethylketone, methylisobutylketone, cylcohexanone; esters
 such as ethyl acetate, isopropyl acetate, butyl acetate; ether oxides such
 as diethylether, diisopropylether, methyl-tert-butylether,
 ethyl-tert-butylether, di-n-butylether; heavy alcohols (containing
 preferably at least 4 atoms of carbon) such as butanol, hexanol, octanol,
 cyclohexanol; aliphatic hydrocarbons such as n-pentane, hexane, heptane
 and cyclohexane; halogenated aliphatic hydrocarbons such as
 dichloromethane, dichloroethane; aromatic hydrocarbons such as toluene,
 xylenes; halogenated aromatic hydrocarbons such as monochiorobenzene,
 dichlorobenzene, and mixtures thereof.
 The reaction medium is thus placed in contact with the reaction solvent,
 generally at one volume of solvent per volume of medium.
 1 or more extractions can be carried out, for example 5, and more
 preferably 1 to 3.
 Prior, or at the same time as the addition of the solvent, the pH is
 returned to between 4 and 9 by adding a protonic acid of inorganic origin,
 preferably hydrochloric acid or sulphuric acid such as, for example,
 trifluoromethanesulphonic acid or methanesulphonic acid. The concentration
 of the acid is immaterial and commercially available forms are preferably
 used.
 The aqueous and organic phases are separated.
 The aqueous phase comprises 2-hydroxybenzoic acid in salt form.
 The organic phase comprises 4-hydroxybenzaldehyde which is perhaps then
 recovered according to conventional techniques, particularly by
 distillation.
 The aqueous phase can be treated by carrying out decarboxylation of the
 2-hydroxybenzoic acid obtained, the description of which is set out in
 detail in the second variation, which allows regeneration of the starting
 phenolic compound which can then be recycled.
 According to a second variation of the process of the invention, at the end
 of the reaction producing 2-hydroxybenzoic acid partially or completely in
 its salt form and 4-hydroxybenzaldehyde, a decarboxylation reaction is
 carried out on the reaction medium.
 This is effected by acidifying the resulting medium by adding a protonic
 acid of inorganic origin, particularly those previously described, until a
 pH of less than or equal to 3 is obtained.
 The reaction medium is heated to a temperature varying, for example,
 between 120.degree. C. and 350.degree. C., and preferably between
 150.degree. C. and 220.degree. C.
 The process is preferably carried out under the autogenous pressure of the
 reactants.
 At the end of the reaction, the reaction medium is cooled between
 20.degree. C. and 80.degree. C.
 A two-phase medium is obtained, constituted by an organic phase comprising
 on the one hand 4-hydroxybenzaldehyde, preferably with formula (IV) and
 the starting phenolic compound with formula (I), and on the other hand a
 saline aqueous phase.
 The organic and aqueous phases are separated and the 4-hydroxybenzaldehyde
 is recovered from the organic phase using conventional separation
 techniques, for example by extraction using a suitable solvent and then by
 distillation. Reference may be made to the description of the first
 variation.
 In accordance with the process of the invention, the mixture used is of two
 phenolic compounds, one carrying a formyl or hydroxymethyl group in the 2
 position, and the other carrying a formyl or hydroxymethyl group in the 4
 position.
 The starting compounds (IIA) and (IIB) therefore have more particularly the
 following formulae:
 ##STR4##
 in said formulae, M.sub.1, Z.sub.1, Z.sub.2 and Z.sub.3 have the meanings
 previously described.
 The mixtures of phenolic compounds to which the process according to the
 invention can be applied are generally known products which can be
 prepared by various methods of organic synthesis.
 Thus, mixtures with formula (IIa.sub.1) and (IIb.sub.1) can be obtained by
 a process of hydroxymethylation of a phenol by condensation thereof with
 formaldehyde or a formaldehyde generator in an aqueous phase in the
 presence of an alkaline or alkaline-earth base.
 More precisely, there is an unsubstituted phenol on the ortho and para
 positions with respect to the hydroxy group, with the general formula (I):
 ##STR5##
 in which Z.sub.1, Z.sub.2 and Z.sub.3 have the meanings previously
 described.
 Examples of phenols with formula (I) which may act as a starting point for
 the synthesis of compounds with formula (II) are phenol, pyrocatechin,
 guaiacol, guetol, 3-methoxyphenol, 3-ethoxyphenol, 3-isopropoxyphenol,
 3-t-butoxyphenol, m-cresol and o-cresol.
 The conditions selected for this hydroxymethylation step are those taught
 by the prior art listed hereinafter: see in particular H. G. PEER, Rec.
 Trav. Chim. Pays-Bas [Netherlands] 79 825-835 (1960); GB-A-774 696;
 GB-A-751 845; EP-A-165; J. H. FREEMAN, J. Am. Chem. Soc. 74 6 257-6 260
 (1952) and 76 2080-2087 (1954); H. G. PEER, Rec. Trav. Chim. Pays-Bas
 [Netherlands] 78 851-863 (1959); H. EULER et al Arkiv fur Chem. 13 1-7
 (1939); P. CLAUS et al Monath. Chem. 103 1178-11293 (1972).
 Formaldehyde or any formaldehyde generator can be used such as, for
 example, trioxane or paraformaldehyde used as linear paraformaldehydes of
 any degree of polymerisation, preferably containing 8 to 100 (CH.sub.2 O)
 units.
 Formaldehyde can be used in the form of an aqueous solution of non-critical
 concentration. It can vary between 20 and 50% by weight; preferably
 commercial solutions are used which have a concentration of about 30 to
 40% by weight.
 The quantity of formaldehyde expressed as moles of formaldehyde per mole of
 phenol can vary between wide limits. The formaldehyde/phenol molar ratio
 can vary between 0.5 and 2.0, and preferably between 0.5 and 1.5.
 The quantity of base present in the hydroxymethylation medium, expressed as
 the number of moles of base/phenolic hydroxy group of the phenol to be
 hydroxymethylated, can vary between broad limits. In general, this ratio,
 which is variable depending on the nature of the base, can vary between
 0.1 and 2, and preferably between 0.5 and 1.1. The base used can be one of
 those cited above for the oxidation phase. The use of alkaline hydroxides
 in aqueous solution is particularly convenient.
 In general, the hydroxymethylation step is carried out at a temperature
 between 0 and 100.degree. C., preferably between 20 and 70.degree. C.
 The process is preferably carried out under the pressure which is
 autogenous for the reactants to avoid any possible losses of
 paraformaldehyde, which may be gaseous at the temperatures used.
 Preferably, the reaction is carried out under a controlled atmosphere of
 inert gases such as nitrogen or noble gases, for example argon.
 The reaction time can be very variable. It is usually between 30 minutes
 and 24 hours, preferably between 4 hours and 8 hours.
 In practice, the reaction is readily carried out by placing the phenol and
 formaldehyde, and any base, in the equipment, then stirring and heating
 the reaction mixture to the temperature desired for the time required to
 complete the reaction.
 The order of introduction of the reactants is not critical and can thus be
 different.
 A mixture of phenolic compounds with formula (Ia.sub.1) and (IIb.sub.1) is
 obtained.
 The compounds with formula (IIa.sub.2) and (IIb.sub.2) can be prepared by
 oxidation of hydroxymethylated phenolic compounds with formula (IIa.sub.1)
 and (IIb.sub.1), by oxidation with molecular oxygen or a gas containing
 molecular oxygen, in an alkaline aqueous phase in the presence of a
 catalyst based on a metal from group 8 of the periodic classification,
 preferably platinum and palladium, optionally containing metals such as
 cadmium, cerium, bismuth, lead, silver, tellurium or tin as an activator.
 Such processes have been described in U.S. Pat. No. 3,673,257, FR-A-2 305
 420, and in FR-A-2 350 323.
 If necessary, the pH of the solution is brought to a value between 8 and 13
 by optional addition of an alkaline or earth-alkaline base. The optimum pH
 value depends on the nature of the hydroxymethylated phenols.
 For example, in the case of a platinum catalyst, the quantity of base to be
 used is advantageously between 1 and 3 moles per mole of hydroxymethylated
 phenolic compounds, and between 0.5 and 2 in the case of a paliadium
 catalyst.
 The temperature of the oxidation reaction is between 10.degree. C. and
 60.degree. C., and preferably between 20.degree. C. and 50.degree. C.
 More specifically again, the process according to the present invention is
 very suitable for the preparation of compounds with formula (IIa.sub.2)
 and (IIb.sub.2) from phenolic compounds with formula (IIa.sub.1) and
 (IIb.sub.1) resulting from the first step, by molecular oxygen or a gas
 containing molecular gas in the presence of a catalyst based on a metal
 from group 8 of the periodic classification, optionally containing a metal
 such as those used as an activator, without intermediate separation of the
 hydroxymethylated phenolic compounds.
 It appears particularly advantageous for industry to implement the process
 according to the present invention using compounds with formula
 (IIa.sub.2) and (IIb.sub.2) obtained by a two-step process comprising:
 hydroxymethylation of a phenol in an aqueous medium in the presence of an
 alkaline or earth-alkaline base, by formaldehyde or a formaldehyde
 generator, producing a mixture of hydroxymethylated phenolic compounds,
 one hydroxymethylated in the 2 position, the other in the 4 position,
 and oxidation, without intermediate separation, of phenolic compounds
 obtained by molecular oxygen or a gas containing molecular oxygen in an
 alkaline aqueous phase in the presence of a catalyst based on a metal from
 group 8 of the periodic classification, with optionally as an activator, a
 metal such as those previously listed.
 A supplementary importance of the process of the invention is that it
 allows the use of mixtures of phenolic compounds directly produced from
 the preceding steps of hydroxymethylation and optionally of oxidation.
 As previously mentioned, the process of the invention is particularly
 suitable for the preparation of vanillin and of ethylvanillin from a
 mixture of phenolic compounds obtained by hydroxymethylation of guaiacol
 or of guetol.
 Thus, vanillin can be prepared by selective oxidation of the hydroxymethyl
 group in the 2 position of compound (A) of a mixture of phenolic
 compounds, o-hydroxymethylguaiacol (A) and p-hydroxymethylguaiacol (B), to
 the carboxy group, and of the hydroxymethyl group of compound (B) in the 4
 position to the formyl group, thus producing a mixture of a
 2-hydroxy-3-methoxybenzoic acid and vanillin, then of recovering the
 latter.
 Another variation consists of selective oxidation of the formyl group in
 the 2 position of compound (A) of a mixture of phenolic compounds,
 o-formylguaiacol (A) and p-formylguaiacol (B), to the carboxy group, thus
 producing a mixture of a 2-hydroxy-3-methoxybenzoic acid and vanillin,
 then of recovering the latter.
 With regard to the preparation of ethylvanillin, in accordance with the
 invention, the hydroxymethyl group in the 2 position of compound (A) of a
 mixture of phenolic compounds, o-hydroxymethylguaiacol (A) and
 p-hydroxymethylguaiacol (B) is selectively oxidised to the carboxy group,
 and the hydroxymethyl group of compound (B) in the 4 position to the
 formyl group, thus producing a mixture of a 2-hydroxy-3-ethoxybenzoic acid
 and ethylvanillin, then of recovering the latter.
 Another variation is in the fact that in a mixture of phenolic compounds,
 o-formylguetol (A) and p-formylguetol (B), there is selective oxidation of
 the formyl group in the 2 position of compound (A) to the carboxy group,
 thus producing a mixture of a 2-hydroxy-3-ethoxybenzoic acid and
 ethylvanillin, then of recovering the latter.
 Examples of implementations of the invention will be given below. These
 examples are given by way of illustration are in no way limiting.
 In the examples, the degree of conversion and the yield obtained is
 defined.
 The degree of conversion (DC) corresponds to the ratio between the number
 of moles of substrate transformed and the number of moles of substrate
 used.
 The yield (YY) corresponds to the ratio between the number of moles of
 product formed and the number of moles of substrate used.
 The yield (YT.sub.vanillin) corresponds to the ratio between the number of
 moles of vanillin formed and the number of moles of guaiacol transformed
 in the sequence.
 In the examples, the abbreviations are:
 o-hydroxymethylguaiacol=OMG
 p-hydroxymethylguaiacol=PMG
 o-vanillin=3-methoxy-2-hydroxybenzaldehyde=OVA
 p-vanillin=3-methoxy-4-hydroxybenzaldehyde=PVA
 o-vanillic acid=2-hydroxy-3-methylbenzoic acid=AOV
 p-vanillic acid=4-hydroxy-3-methylbenzoic acid=APV

EXAMPLES
 Example 1
 In this example, a mixture of o-hydroxymethylguaiacol and
 p-hydroxymethylguaiacol is oxidised.
 2700 g of an aqueous solution containing 28.5 g of o-hydroxymethylguaiacol
 (OMG) and 33.72 g of p-hydroxymethylguaiacol (PMG) and 148 g of sodium
 carbonate is introduced into a pressurised 3.91 autoclave provided with an
 automatic exhaust turbine.
 This aqueous solution is the product of condensation of guaiacol on formol
 in an aqueous base solution, and prepared as described in the prior art
 (particularly according to example 4 in U.S. Pat. No. 4,351,962).
 0.54 g of bismuth trioxide and 22 g of palladium catalyst, deposited on
 charcoal in an amount of 3% by weight of metal, is added to this reaction
 mixture.
 The reaction mixture is stirred at 1500 rpm and the temperature thereof
 increased to 45.degree. C. in nitrogen.
 A pressure of 3 bar is established and air introduced into the reaction
 medium at a rate of 300 g/h.
 The reaction mixture is kept under these conditions for 6 hours.
 The reaction mixture is cooled and the pressure returned to atmospheric
 pressure, then the catalyst is filtered.
 The reaction medium is then analysed using high performance liquid
 chromatography.
 The results obtained are as follows:
 DC OMG=100%
 YY o-vanillin 7%
 YY o-vanillic=93%
 DC PMG=100%
 YY p-vanillin=89%
 YY p-vanillic acid=7%
 Examination of these results shows that o-vanillin was selectively
 oxidised, compared to p-vanillin.
 Example 2
 In this example, a mixture of o-vanillin/p-vanillin is oxidised.
 50.26 g of o-vanillin, 49.88 g of p-vanillin, 2003 g of water and 142.5 g
 of an aqueous solution of 30% by weight of sodium carbonate is introduced
 into a 3.91 autoclave provided with an automatic exhaust turbine.
 22 g of palladium catalyst, deposited on charcoal in an amount of 3% by
 weight of metal, and 0.96 g of bismuth trioxide is added to the reaction
 mixture.
 The mixture is stirred at 1500 rpm and the temperature thereof is increased
 to 140.degree. C. in nitrogen.
 A pressure of 13 bar is established and air introduced at a rate of 300 g/h
 for 15 minutes.
 The reaction mixture is cooled and the pressure returned to atmospheric
 pressure, then the catalyst is filtered.
 The reaction medium is then analysed using high performance liquid
 chromatography.
 The results obtained are as follows:
 DC o-vanillin=100%
 YY o-vanillic acid=90%
 DC p-vanillin=20%
 YY p-vanillic acid=16%
 This clearly demonstrates the selectivity of the oxidation.
 Example 3
 Example 2 is repeated, but using a platinum catalyst.
 50.5 g of o-vanillin, 50.1 g of p-vanillin, 2003 g of water and 142.5 g of
 an aqueous solution of 30% by weight of sodium carbonate is introduced
 into a 3.91 autoclave provided with an automatic exhaust turbine.
 22 g of platinum catalyst, deposited on charcoal in an amount of 5% by
 weight of metal, and 1.5 g of bismuth trioxide is added to this reaction
 mixture.
 The mixture is stirred at 1500 rpm and the temperature thereof increased to
 140.degree. C. in nitrogen.
 A pressure of 13 bar is established and air introduced at a rate of 300 g/h
 for 30 minutes.
 The reaction mixture is cooled and the pressure returned to atmospheric
 pressure, then the catalyst is filtered.
 The reaction medium is then analysed using high performance liquid
 chromatography.
 The results obtained are as follows:
 DC o-vanillin 100%
 YY o-vanillic acid 89%
 DC p-vanillin=15%
 YY p-vanillic acid=12%
 This also clearly demonstrates the selectivity of the oxidation.