Method for producing N-isopropyl(meth)acrylamide

A process which allows N-isopropyl(meth)acrylamide to be obtained in high purity and yield in a simple manner, by reacting methacrylic anhydride with isopropylamine, optionally in a solvent, is described. The amide precipitates out during the reaction and can be isolated by filtration. A further product fraction can be precipitated out of the filtrate by neutralizing the methacrylic acid obtained as a by-product and thus be obtained additionally. Alternatively, the methacrylic acid can be separated by distillation from product dissolved therein.

Surprisingly, in contrast to textbook knowledge, the equimolar conversion of the reactants forms virtually no N-isopropylammonium methacrylate.

The Ritter reaction serves to prepare amides from nitriles and substrates which can form carbenium ions (for example tertiary or secondary alcohols in the presence of strong mineral acids). For instance, N-isopropyl(meth)acryl-amide can be prepared from (meth)acrylonitrile and isopropanol. DE 31 31 096 describes this process. For workup, the acid which functions as a solvent has to be neutralized, which generates large amounts of waste salt. On top of this, the product contains impurities, for example methacrylamide.

Procedure of the Invention

The substituted (meth)acrylamide is obtained in a simple manner by initially charging the anhydride, optionally in a solvent, and metering in the amine, optionally with cooling.

After the end of the addition, a portion of the product formed has generally already crystallized out and can, optionally after cooling the product solution to complete the precipitation, be filtered off. To increase the yield, the (meth)acrylic acid present in the mother liquor can be neutralized, which leads to further precipitation of product. The product is formed virtually quantitatively. The substituted (meth)acrylamide obtained possesses a high purity, which is usually above 95%. For workup, the mother liquor can also be separated by distillation into N-isopropylmethacrylamide and methacrylic acid. Owing to the polymerization sensitivity of the monomers, it is appropriate to minimise the thermal stress in the course of this, for example by using a thin-layer evaporator. Purity and yield of the resulting monomers are higher than in the case of preparation by known processes such as the Ritter reaction or the use of carbonyl chloride as a raw material.

The Starting Materials

Anhydrides of the Unsaturated Carboxylic Acids

Useful anhydrides of the unsaturated carboxylic acids include acrylic anhydride, methacrylic anhydride or itaconic anhydride.

The methacrylic anhydride is traded, for example, by Evonik Röhm GmbH.

The Amines

The amines used may be primary amines and secondary amines. Useful primary amines include primary, optionally substituted aliphatic amines, for example methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, dodecylamine, isopropylamine, isobutylamine and benzylamine, and also allylamine.

In addition, it is possible to use primary cycloaliphatic amines, for example cyclopropylamine, cyclobutylamine, cyclopentylamine and cyclohexylamine.

The primary aromatic amines used may be aniline, the isomeric aminotoluenes, individually or in mixtures, and the isomeric xylidines, individually or in mixtures. These compounds may optionally be substituted by one or more halogens.

Secondary aliphatic amines, for example dimethylamine, methylethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, dipentylamine, dihexylamine, diheptylamine and dioctylamine can likewise be used.

The Polymerization Inhibitors

Polymerization inhibitors are already known. For example 1,4-dihydroxybenzenes can be added for stabilization. However, it is also possible to use differently substituted dihydroxybenzenes. In general, such inhibitors can be represented by the general formula (I)

in which:

R 1 is hydrogen, a linear or branched alkyl radical having one to eight carbon atoms, halogen or aryl, preferably an alkyl radical having one to four carbon atoms, more preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, Cl, F or Br;

n is an integer in the range from one to four, preferably one or two; and

R 2 is hydrogen, a linear or branched alkyl radical having one to eight carbon atoms or aryl, preferably an alkyl radical having one to four carbon atoms, more preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl.

However, it is also possible to use compounds with 1,4-benzoquinone as the parent compound. These can be described by the formula (II)

in which

R 1 is a linear or branched alkyl radical having one to eight carbon atoms, halogen or aryl, preferably an alkyl radical having one to four carbon atoms, more preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, Cl, F or Br; and

n is an integer in the range from one to four, preferably one or two.

Equally, phenols of the general structure (III) are used.

in which

R 1 is a linear or branched alkyl radical having one to eight carbon atoms, aryl or aralkyl, propionic esters with mono- to tetrahydric alcohols which may also contain heteroatoms such as S, O and N, preferably an alkyl radical having one to four carbon atoms, more preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl.

A further advantageous substance class is that of sterically hindered phenols based on triazine derivatives of the formula (IV):

where R=compound of the formula (V)

in which

R 1 ═C n H 2n+1

where n=1 or 2.

A further group of known inhibitors is that of amines, especially sterically hindered amines.

These include especially phenylenediamines which can be represented by formula (VI)

in which R 1 , R 2 , R 3 and R 4 are each independently hydrogen and alkyl, aryl, alkaryl, aralkyl radicals having in each case up to 40 and preferably up to 20 carbon atoms, where preferably at least one of the R 1 , R 2 , R 3 and R 4 radicals is hydrogen.

Illustrative p-phenylenediamines include p-phenylenediamine in which the R 1 , R 2 , R 3 and R 4 radicals are each hydrogen; N-phenyl-N′-alkyl-p-phenylenediamines, for example N-phenyl-N′-methyl-p-phenylenediamine, N-phenyl-N′-ethyl-p-phenylenediamine, N-phenyl-N′-propyl-p-phenylenediamine, N-phenyl-N′-isopropyl-p-phenylenediamine, N-phenyl-N′-n-butyl-p-phenylenediamine, N-phenyl-N′-isobutyl-p-phenylenediamine, N-phenyl-N′-sec-butyl-p-phenylenediamine, N-phenyl-N′-tert-butyl-p-phenylenediamine, N-phenyl-N′-n-pentyl-p-phenylenediamine, N-phenyl-N′-n-hexyl-p-phenylenediamine, N-phenyl-N′-(1-methylhexyl)-p-phenylenediamine, N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine, N-phenyl-N′-(1,4-dimethylpentyl)-p-phenylenediamine; N-phenyl-N′,N′-dialkyl-p-phenylenediamines, for example N-phenyl-N′,N′-dimethyl-p-phenylenediamine, N-phenyl-N′,N′-diethyl-p-phenylenediamine, N-phenyl-N′,N′-di-n-butyl-p-phenylenediamine, N-phenyl-N′,N′-di-sec-butyl-p-phenylenediamine, N-phenyl-N′-methyl-N′-ethyl-p-phenylenediamine; N,N-dialkyl-p-phenylenediamines, for example N,N-dimethyl-p-phenylenediamine and N,N′-diethyl-p-phenylenediamine; N,N′-dialkyl-p-phenylenediamines, for example N,N′-diisopropyl-p-phenylenediamine, N,N′-diisobutyl-p-phenylenediamine; N,N′-diarylphenylenediamines, for example N,N′-diphenyl-p-phenylenediamine; N,N,N′-trialkyl-p-phenylenediamines, for example N,N,N′-trimethyl-p-phenylenediamine, N,N,N′-triethyl-p-phenylenediamine.

In addition, phenazine dyes constitute a further preferred group. These include especially induline and nigrosine. Nigrosine forms as a result of heating of nitrobenzene, aniline and aniline in hydrochloric acid solution with metallic iron and FeCl 3 . Preference is given here to alcohol-soluble aniline dyes which may include, for example, 5 benzene rings, such as dianilido-N,N-diphenylphenosafranin. These substances are widely known and can be obtained commercially.

The compounds 1,4-dihydroxybenzene, 4-methoxyphenol, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone, 1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 2,6-di-tert-butyl-4-methylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,2-bis[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl-1-oxopropoxymethyl]1,3-propanediylesters, 2,2′-thiodiethyl bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)]propionate, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 3,5-bis(1,1-dimethylethyl-2,2-methylenebis(4-methyl-6-tert-butyl)phenol), tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-s-triazine-2,4,6-(1H,3H,5H)trione, tris(3,5-di-tert-butyl-4-hydroxy)-s-triazine-2,4,6-(1H,3H,5H)trione, tert-butyl-3,5-dihydroxybenzene or diphenyl-p-phenylenediamine (DPPD) and also 4-hydroxy-2,2,6,6,-tetramethylpiperidine 1-oxyl, are used particularly successfully, hydroquinone monomethyl ether (4-methoxyphenol) being very particularly appropriate in turn among these.

The inhibitors mentioned are commercially available.

As base stabilization for ethylenically unsaturated compounds, the compounds mentioned can be used alone or in a mixture of two or more compounds. When the compounds are phenolic compounds, the presence of oxygen in the reaction mixture is required in order to ensure sufficient efficacy against polymerization. The use of air as an oxygen source is particularly preferred.

The Solvent

The solvents used may be all inert organic solvents, for example aliphatic hydrocarbons such as hexane, heptane, octane, cyclohexane, or mixtures of aliphatic hydrocarbons, for example petroleum ether, ligroin, decalin or benzine.

In addition, it is possible to use aromatic solvents, for example benzene, toluene or the isomeric xylenes, and mixtures of aforementioned compounds.

In addition, oxygen-containing hydrocarbons are useful, for example diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethyl ether or methyl tert-butyl ether. Water is also a possible inert solvent, since it brings about only a low level of hydrolysis of the (meth)acrylic anhydride under the reaction conditions of amide preparation.