Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Patent
1991-10-31
1993-01-05
Lone, Werren B.
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
568458, 568459, C07C 4500, C07C 4721
Patent
active
051772655
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the preparation of citral starting from prenol and prenal.
It relates more particularly to an improved process for the preparation of citral starting from prenol and prenal.
It is known, according to French Patent No. 72.40678 published under the number 2,160,525, to prepare .alpha.,.beta.-ethylenically-linked aldehydes by reaction at elevated temperature in liquid phase of an .alpha.,.beta.-ethylenically-linked aldehyde and an allylic alcohol. The .alpha.,.beta.ethylenically-linked [sic] aldehyde can be 3,3-dimethylacrolein, known under the name prenal, and the ethylenic alcohol can be 3-methyl-2-buten-1-ol known under the designation prenol.
The condensation reaction is preferably carried out in the presence of an acid catalyst at temperatures between 100.degree. and 250.degree. C. The acid used as a catalyst is chosen from amongst the inorganic acids such as sulphuric acids, phosphoric acids, halogenated acids, nitric acid, sulphurous acid, phosphorous acid, perchloric acid, boric acid, silicic acid, acid salts with dissociable hydrogen and heterogeneous acids. Organic acids can also be used. The quantity of acid used, if it is a mineral acid having a PK [sic] of 0 to approximately 3, is between 0.01 and 0.5% during the entire reaction, which can be divided into two reaction steps: formation of the acetal and decomposition of the latter by cracking.
It is additionally known according to U.S. Pat. No. 4,288,636 to prepare citral by heating a diprenylic acetal in the presence of an inert liquid having a boiling point higher than prenol and lower than citral at the reaction pressure. The inert liquid is preferably 3,3,7-trimethyl-4-oxaocta-1,6-diene.
The quantity of phosphoric acid used is between 0.001 and 0.5% by weight and preferably between 0.005 and 0.05% by weight.
It is likewise known according to the Patent FR 2,353,512 to prepare the acetals by condensation of an aldehyde with an alcohol in the presence of a distillable acid which is preferably nitric acid. In this case, the quantity of acid used varies between 10.sup.-6 % and 1% by weight with respect to the starting compounds and preferably between 10.sup.-4 and 10.sup.-2 %. It is even specified in this text that when a less strong acid is used, for example phosphoric acid, the quantity of acid can extend to 10%, that is to say is multiplied by ten with respect to nitric acid.
Nothing indicates, even on gathering together these three documents, of which certain and particularly the third have very high areas of concentration, that the concentrations of acid may be different in the two steps: acetalisation and cracking, since the concentration ranges indicated have a certain area of overlap.
In comparing these three texts, it even seems that the acidity necessary for acetalisation would be less than or equal to the acidity necessary for cracking.
In employing these processes at the laboratory level, it was apparent to us that the acidity necessary for the formation of the acetal and the acidity necessary for the cracking step were different and that the yield was improved on carefully controlling the acidity of the reaction medium during the process.
The present invention therefore relates to an improved process for the preparation of citral starting from prenol and prenal, characterised in that, in the same reaction space: the presence of a mineral acid at a concentration of approximately 5.times.10.sup.-3 mol per mole of prenal introduced and of a solvent forming an azeotrope with water, are removed and recycled to the first step, then the citral is distilled off.
The use of a very weak acidity during the cracking step allows the yields of citral to be increased.
Thus, when in the cracking step a concentration of acid of 10.sup.-3 mol per mole of acetal is used (corresponding to 1 mol of starting prenal), citral is not obtained (Example 2). On the other hand, when in the acetalisation step a concentration of acid of 10.sup.-4 mol per mole of prenal is used, the acetalisation yield reaches 30% at th
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patent: 4933500 (1981-06-01), Chabardes et al.
patent: 4933550 (1990-06-01), Chabardes et al.
Chabardes Pierre
Chazal Jacques
Lone Werren B.
Rhone-Poulenc Sante
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