Purification of alkenyl compounds

Details Purification of alkenyl compounds Purification of alkenyl compounds

2001-08-09

2002-05-07

Davis, Brian J. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Unsubstituted hydrocarbyl chain between the ring and the -c-...

C548S335100, C564S437000, C564S497000, C568S057000, C568S616000, C568S621000

Reexamination Certificate

active

06384216

ABSTRACT:

The present invention relates to a process for purifying alkenyl compounds having a divalent or trivalent heteroatom in the &agr; position relative to the double bond, in particular alkenyl compounds of the formula (Ia) or (Ib)
where X is a divalent heteroatom, R
1
, R
2
and R
3
are each, independently of one another, a carbon-containing organic radical, where R
2
and R
3
may also be joined to one another, and R
4
, R
5
, R
6
and R
7
are each, independently of one another, hydrogen or a hydrocarbon radical, by distillation.
Alkenyl compounds are used, inter alia, as monomeric building blocks for oligomers, polymers and copolymers. Thus, alkenyl compounds are employed, for example, in the production of paper coatings, adhesives, printing inks, laundry detergents, motor oil additives, textile assistants, radiation-curing surface coatings, cosmetics, pharmaceuticals, auxiliaries for crude oil recovery or chemicals for photographic applications.
Alkenyl compounds are obtained industrially by means of various processes, for example by addition of alkynes (alkenylation), transfer of alkenyl groups, elimination to form the double bond or oxidative addition of alkenes. An overview of the preparation of vinyl ethers and vinyl esters is given in Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 1999 Electronic Release, Chapter “VINYL ETHERS” and Chapter “VINYL ESTERS”.
W. Reppe et al., Justus Liebigs Ann. Chem., Vol. 601, 1956, pages 81 to 138, describe the preparation of vinyl ethers, vinyl esters, vinyl amines, vinyl-N-heterocycles and vinyl amides by, reaction of ethyne with the corresponding alcohols, carboxylic acids, amines, NH-heterocycles and amides in the presence of basic catalysts.
As indicated in the documents cited, the actual synthesis step is usually followed by purification by distillation, in which the desired products can be obtained in high purity by condensation from the gas phase. In this way, purities of significantly above 99% can be achieved without problems for very many alkenyl compounds, which is completely satisfactory for many applications.
However, in applications in which the products should have as little color as possible, for example in the cosmetic or photographic areas or in the case of paper coatings, it is not only a high chemical purity of the alkenyl compounds but also a very high purity in respect of color-imparting impurities which is demanded. Amounts of only a few ppm by weight of color-imparting impurities are generally sufficient to cause significant discoloration of the product.
As a measure of the color of transparent compounds, it is usual to report the APHA color number which is defined in DIN EN 1557 (March 1997). The lower the APHA color number, the more colorless is the product.
In the industrially customary distillation of alkenyl compounds immediately after their preparation, it is, depending on the type of alkenyl compounds and the color-imparting impurities present therein, impossible or possible only with great difficulty (e.g. large number of theoretical plates, high reflux ratio, additional purification steps such as adsorptive processes) to bring the APHA color number to below 40.
It is an object of the present invention to find a process for purifying alkenyl compounds which no longer has the abovementioned disadvantages and leads, without incurring high costs, to purified alkenyl compounds having a very low color number.
We have found that this object is achieved by a process for purifying alkenyl compounds having a divalent or trivalent heteroatom in the &agr;-position relative to the double bond by distillation, which comprises carrying out at least two distillations in which the purified alkenyl compounds are obtained from the gas phase by condensation, where the time between the first distillation after the synthesis of the alkenyl compounds and at least one further distillation is at least one day and the purified alkenyl compounds have an APHA color number of <30.
For the purposes of the present invention, the term distillation encompasses quite generally all processes in which the alkenyl compounds are obtained from the gas phase by condensation. In general, the distillation is carried out in a distillation column having distillation packing and/or distillation trays as column internals. Examples of suitable column internals are ordered packing, random packing elements, valve trays, sieve trays and bubble cap trays. Preference is given to column internals which lead to a low pressure drop, for instance ordered packing and random packing elements. The second distillation and any further distillations are very particularly preferably carried out without column internals. The vaporization of the alkenyl compounds to be purified can in principle be achieved by a variety of constructions. Preference is given to using falling film and thin film evaporators, since these make particularly gentle vaporization possible. The purified alkenyl compounds are preferably isolated at the top.
The distillations in the process of the present invention can be carried out with or without reflux.
In the first distillation after the synthesis of the alkenyl compound, the alkenyl compound is separated from the by-products formed. To enable the separation task to be achieved reliably, it is preferably carried out with reflux. In this way, a purity of the alkenyl compound of over 99% can generally be achieved in the first distillation after the synthesis of the alkenyl compound.
Since the alkenyl compounds to be purified generally have a purity of significantly above 99% before the second distillation and any further distillations and the traces of color-imparting impurities to be removed preferably remain in the bottoms, the second distillation and any further distillations are preferably carried out without reflux. The absence of reflux brings with it a series of economic advantages, for example simpler construction of the apparatus, a lower instrumentation requirement and a lower energy and time requirement.
The distillations in the process of the present invention can be carried out batchwise, semicontinuously or continuously. In the case of batchwise distillation, the alkenyl compound to be purified is generally introduced all at once into the still pot and is vaporized by heating and/or lowering the pressure. The distilled, purified alkenyl compound is taken off continuously or passed to a receiver. A semicontinuous distillation is generally commenced in a manner similar to the batchwise distillation, but the still pot is refilled during the distillation process. In the case of a continuous distillation, the alkenyl compound to be purified is fed continuously into the distillation apparatus and the distilled, purified product is taken off continuously.
The distillations in the process of the present invention can in principle be carried out at atmospheric pressure, at subatmospheric pressure (e.g. as a vacuum distillation) or at superatmospheric pressure. The choice of the pressure and thus the distillation temperature generally depends on the product to be distilled and the impurities present therein. Preference is given to carrying out both the first distillation after the synthesis of the alkenyl compounds and the second and any further distillations under a pressure lower than or equal to atmospheric pressure, particularly preferably at subatmospheric pressure.
An essential aspect of the process of the present invention is that the time between the first distillation after the synthesis of the alkenyl compounds and at least one further distillation is at least one day. In each case, the distillation is considered to be complete at the time at which the alkenyl compounds have been condensed from the gas phase and isolated. In general, the condensed, purified alkenyl compounds are isolated by being passed to a receiver.
In general, a total of two distillations are sufficient for achieving the desired APHA color number of <30 in the process of the present invention. The first distillation is generally carri

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