Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1998-11-25
2001-09-11
Shippen, Michael L. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06288288
ABSTRACT:
SUMMARY OF THE INVENTION
The present invention relates to a process for preparing saturated alcohols by means of an aldolcondensation of alkyl methyl ketones of 6 to 8 carbon atoms which are branched at the &bgr;-carbon atom with aldehydes of 4-15 carbon atoms which are branched at the &agr;-carbon atom to form &agr;,&bgr;-unsaturated ketones and subsequent hydrogenation of the &agr;,&bgr;-unsaturated ketones to give the corresponding saturated alcohols.
STATE OF THE ART
The aldol addition is the base- or acid-catalyzed addition of activated methylene groups onto the carbonyl groups of aldehydes or ketones with formation of &bgr;-hydroxy carbonyl compounds. After a certain reaction time, an equilibrium is established between the &bgr;-hydroxy carbonyl compounds and the unreacted starting materials. If the aldol addition is followed by elimination of water, which occurs easily and is the rule when acid catalysts are used, the overall reaction is known as an aldol condensation. Products of the aldol condensation are &agr;,&bgr;-unsaturated carbonyl compounds.
In the aldol addition between aldehydes and ketones,the ketones always function as the methylene component as a result of their lower carbonyl activity. The condensation of the keto group with the &agr;-methylene group of an aldehyde is possible only in a few cases via particular intermediates. For this reason, the first products are usually &bgr;-hydroxy ketones whose keto group originates from the ketone used. The subsequent elimination of water the &bgr;-hydroxy ketones forms the corresponding &agr;,&bgr;-unsaturated ketones.
In the reaction between aldehydes and ketones, only a single &agr;,&bgr;-unsaturated ketone is obtained as the reaction product if saturated, aliphatic aldehydes without an &agr;-hydrogen atom are used. The self-condensation of the ketone is observed only to a minimal extent. In contrast, when aldehydes having an &agr;-hydrogen atom are used, the self-condensation of the aldehydes has to be expected as an undesired secondary reaction.
While the use of symmetrical ketones always forms only one &bgr;-hydroxyketone in the aldol addition, the use of unsymmetrical ketones in which the &agr;-hydrogen atoms are not equivalent makes it possible to form two structurally different &bgr;-hydroxyketones according to the reaction scheme I below, where R
1
is not hydrogen,
Rules can be formulated for the course of such aldol condensations involving unsymmetrical ketones. Here, a distinction first has to be made between the alkali-catalyzed and the acid-catalyzed aldol condensation. Both the CH-acidity and steric factors determine the course of the reaction.
In the alkali-catalyzed aldol condensation of unbranched aldehydes and &agr;-branched ketones, the abstraction of the proton occurs at the &agr;-carbon atom of the ketone which bears the most substituents (case A). In contrast, when aldehydes which are branched at the &agr;-carbon atom are used, the abstraction of the proton occurs preferentially at the &agr;-carbon atom of the ketone which bears the least substituents (case B). Thus, in case A, the aldol addition forms a &bgr;-hydroxyketone in accordance with equation IA, while in case B, the reaction is expected to proceed in accordance with equation IB.
Exceptions to the above rule occur when the ketone is branched in the &bgr; position. As a result of the additional steric effect, the condensation here always occurs in accordance with equation IA, regardless of the aldehyde structure. In contrast to alkaline catalysis, the aldol condensation using acid catalysts always, i.e. regardless of the type of branching of the aldehydes and ketones, proceeds in accordance with equation IB.
Undesired secondary reactions of the ketone and aldehyde starting materials which occur concurrently with the aldolization can be, for example, the Cannizzaro reaction and the Claisen-Tishtshenko reaction. Usually, only aromatic and non-aldolizable aliphatic aldehydes, i.e. aldehydes without an &agr;-hydrogen atom, undergo the Cannizzaro reaction and disproportionate in the presence of strong alkalis to form equimolar amounts of alcohol and carboxylic acid. In contrast, in the case of aldolizable aldehydes, the aldol addition and condensation generally occurs exclusively because its rate is higher than that of the Cannizzaro reaction. However, under particular reaction conditions, aldolizable aldehydes having a certain structure with a hydrogen atom in the &agr; position can also undergo the Cannizzaro reaction.
Thus, it is known from U.S. Pat. No. 3,398,166 that C4-16 aldehydes having only one hydrogen atom in the &agr; position disproportionate into the corresponding alcohol and the associated carboxylic acid at 40-250° C. in the presence of a 30-50% strength aqueous solution of an alkali metal hydroxide or alkaline earth metal hydroxide. In the case of 2-ethylhexanal, 48.5% of 2-ethylhexanol and 49% of sodium ethylhexanoate, based on 100% of aldehyde, are formed in the presence of a 50% strength sodium hydroxide solution.
The Claisen-Tishtshenko reaction occurs as a further secondary reaction to the aldol condensation if the catalysts used are aluminum alkoxides which are too weakly basic to catalyze the aldol reaction. In this case, even aldolizable aliphatic aldehydes are converted into the corresponding alkoxide and an ester of the corresponding carboxylic acid in a reaction similar to a Cannizzaro reaction.
Further secondary reactions in the form of subsequent reactions of the reaction products of the aldolization are known. An example of such a reaction is the reaction of the &agr;,&bgr;-unsaturated ketone present after the aldol condensation with further aldehyde molecules to form more highly condensed compounds. on this subject, U.S. Pat. No. 3,291,821 reports that C
4-10
-aldehydes having only one hydrogen atom in the a position form glycol monoesters by trimerization even in the presence of an only 5-20% strength aqueous solution of a strong inorganic base at 50-125° C.
Only a small amount of literature exists on the subject of the aldol condensation of ketones which are branched at the &bgr;-carbon atom and aldehydes which are branched at the &agr;-carbon atom. British Patent No. 446,026 discloses the aldol condensation of methyl isobutyl ketone with aliphatic aldehydes having at least 8 carbon atoms to form &agr;,&bgr;-unsaturated ketones. In particular, the aldol condensation of methyl isobutyl ketone with 2-ethylhexanal at a temperature of less than 25° C. in the presence of a catalyst solution in the form of methanolic potassium hydroxide is described. As an alternative to methanol as solvent for the catalyst, it is also possible to use other substances which are inert toward the reactants but are volatile at the same time, e.g. ethanol. In this case, the described use of alkali metal hydroxides as catalyst has the disadvantage that, as a result of the presence of methanol, a relatively large amount of the alkali metal goes over into the organic product phase formed in the reaction. This product phase has to be appropriately worked up afterwards to remove the alkali metal, e.g. by costly scrubbing with water which in turn leads to large amounts of wastewater contaminated with alkali.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide an improved process for preparing saturated alcohols by aldol condensation of ketones which are branched at the &bgr;-carbon atom and aldehydes which are branched at the &agr;-carbon atom and subsequent hydrogenation.
This and other objects and advantages of the invention will become obvious from the following detailed description.
THE INVENTION
This object is achieved by a process for preparing saturated alcohols comprising an aldol condensation of alkyl methyl ketones of 6 to 8 carbon atoms which are branched at the &bgr;-carbon atom and aldehydes of 4 to 15 carbon atoms which are branched at the &agr;-carbon atom to form &agr;,&bgr;-unsaturated ketones and subsequent hydrogenation of the &agr;,&bgr;-unsaturated ketones to give the corresponding satura
Bierman, Muserlian and Lucas
Celanese GmbH
Shippen Michael L.
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