Method for the preparation of azomethines

Azomethines can be prepared by the condensation of cycloalkanones and anilines in the presence of acid homogeneous catalysts while the water of reaction is removed azeotropically, the condensation being carried out in a continuous reaction in a column-like reactor to which a temperature profile is applied, the starting materials being fed in in the low-temperature region, and, of the reaction products, the water of reaction to be removed azeotropically being discharged also in the low-temperature region and the azomethine produced being discharged in the high-temperature region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for the preparation of 
N-cycloalkylideneanilines by elimination of water catalysed by acid and by 
azeotropic removal of the water of reaction. 
The method comprises the reaction of optionally ring-substituted anilines 
with optionally substituted cycloalkanones in the presence of at least one 
acid catalyst which is soluble in mixtures of 
aniline/cycloalkanone/azomethine, at a raised temperature in a 
distillation column. 
The condensation of aromatic amines with cycloalkanones is an equilibrium 
reaction with a small heat of reaction. It is shifted towards the 
azomethine (Schiff base) if the water generated in the reaction is removed 
from the system. In general this is achieved by azeotropic distillation, 
an inert entrainer such as benzene, toluene etc. being used if required. 
2. Description of the Related Art 
More recently, the binding of the water of reaction by using TiCl.sub.4 (J. 
Org. Chem. 32 (1967), 3247) or molar quantities of (butyl).sub.2 
SnCl.sub.2 (Synth. Commun. 12 (1982), 495) has been described. Both these 
compounds bind the water of reaction produced while liberating HCl. 
In other developments, the species binding the water is bound covalently to 
the aniline, for example in the form of N,N-bis-(trimethylsilyl)-aniline 
(Bull. Soc. Chim. Fr. 1966, 3205), iminophosphoranes (Angew. Chem. Int. 
Ed. Eng. 5 (1966), 947) and N-(diphenylaluminium)-aniline (J. Org. Chem. 
51 (1986), 1848). 
A further method of binding effectively the water of reaction produced and 
thus being able to work under mild conditions, using small excess amounts 
of aniline or ketone, is to use molecular sieves (J. Org. Chem. 36 (1971), 
1570; German Offenlegungsschrift 2 244 238). The disadvantage of this 
last-mentioned method is the laborious and expensive regeneration of the 
molecular sieve. 
Azeotropic dehydration is certainly the method of greatest industrial 
interest if it is possible to carry out the reaction with low energy 
expenditure, a good space-time yield and selectivity, and without 
excessive quantities of azeotropic entrainer in a simple apparatus. 
High space-time yields require as a precondition a high reaction rate, and 
therefore effective catalysis. 
The condensation of cyclohexanone and aniline to give 
cyclohexylideneaniline, using zinc chloride as a catalyst, has long been 
known (Ber. 53 (1920) 345-354). In difficult cases of condensations of 
this type, the catalyst system HCl-ZnCl.sub.2 has been used (Ber. 46 
(1913) 2718). German Auslegeschrift 1 078 119 describes the condensation 
N-phenyl-p-phenylene-diamine with cyclohexanone without the addition of a 
catalyst; in such cases the use of a cyclohexanone excess of 200 to 300% 
is required. 
German Offenlegungsschrift 2 525 295 discloses that the reaction time of 
the condensation of aniline, using a 400% excess of cyclohexanone without 
a catalyst, increases sharply with increasing batch size, making it 
impossible to scale up to industrial level. It was also disclosed that 
strongly and weakly acid organic resins affect the reaction time 
favourably. 
German Offenlegungsschrift 2 901 863 describes freshly synthesised, 
anhydrous, non-calcined calcium hydrogen-phosphate, apatite of the formula 
Ca.sub.5 (PO.sub.4).sub.3 OH, dried, non-roasted aluminium oxide 
hydroxides and proton-exchanged aluminium silicates washed to neutrality 
of the montmorillonite type as effective catalysts for the reaction of 
aromatic amines with ketones. The examples in this patent application are 
limited, however, to the condensation of the reactive p-phenylenediamine 
with methyl isobutyl ketone, a 150% excess of ketone being used. 
All the abovementioned methods for azomethine synthesis by azeotropic 
dehydration have the drawback that it is both time-consuming and 
energetically expensive to take the reaction to completion. 
It was therefore desirable to develop a cost-effective and environmentally 
friendly method for the synthesis of cycloalkylideneanilines, which method 
is to be notable for high yields, simple apparatus and optimum energy 
utilisation with minimum energy consumption. 
SUMMARY OF THE INVENTION 
A method has been found in which the starting materials, with a minimum 
excess of carbonyl component or aniline component, are fed into a reactor 
which operates continuously and from which the azomethine compound can be 
taken directly in high yield and with high purity. 
A method for the preparation of azomethines of the formula 
##STR1## 
in which 
R.sup.1 , R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8, 
independently of one another, represent hydrogen, straight-chain or 
branched C.sub.1 -C.sub.6 -alkyl, C.sub.3 -C.sub.6 -cycloalkyl or aryl and 
R.sup.5, R.sup.6, R.sup.7 and R.sup.8, independently of one another, 
additionally represent halogen, straight-chain or branched C.sub.1 
-C.sub.6 -alkoxy, hydroxyl, amino, C.sub.1 -C.sub.6 -alkylamino, 
di-C.sub.1 -C.sub.6 -alkyl-amino, aryloxy or arylamino, in which aryl 
represents phenyl or 5- or 6-membered heteroaryl having 1 or 2 heteroatoms 
from the group N, O and S which is linked in the 2-, 3- or 4-position, and 
X represents --CH.sub.2 -- or a bond connecting the neighbouring C atoms, 
by condensation of cycloalkanones of the formula 
##STR2## 
with anilines of the formula 
##STR3## 
in which R.sup.1 to R.sup.8 and X have the abovementioned meaning, has 
been found, which is characterised in that the condensation is carried out 
in a continuous reaction in a distillation column in the presence of acid 
homogeneous catalysts having a pK.sub.a value measured in H.sub.2 O of 1 
to 6, while the water of reaction is removed azeotropically, the starting 
materials, the azeotropic agent and the catalyst or the catalyst mixture 
being fed separately at different locations, or together as a mixture at 
the same location, into the distillation column, and, of the reaction 
products, the water to be removed azeotropically being discharged from the 
low-temperature region and the azomethine produced being discharged from 
the high-temperature region. 
DETAILED DESCRIPTION OF THE INVENTION 
In a preferred embodiment, starting materials are used in which aryl 
represents phenyl. 
In another preferred embodiment, cycloalkanones are used in which R.sup.4 
represents hydrogen. 
In yet another preferred embodiment, anilines are used in which R.sup.8 
represents hydrogen. 
In a particularly preferred embodiment, cycloalkanones are used in which 
R.sup.3 and R.sup.4 represent hydrogen. 
In a similarly particularly preferred embodiment, anilines are used in 
which R.sup.7 and R.sup.8 represent hydrogen. 
Furthermore, cycloalkanones of the formula 
##STR4## 
are preferred in which 
R.sup.11 and R.sup.12, independently of one another, represent hydrogen, 
straight-chain or branched C.sub.1 -C.sub.4 -alkyl, cyclopropyl, 
cyclopentyl, cyclohexyl or phenyl and 
X represents --CH.sub.2 -- or a bond connecting the neighbouring C atoms. 
In a particularly preferred embodiment, cycloalkanones of the formula 
##STR5## 
are used in which 
R.sup.21 and R.sup.22, independently of one another, represent hydrogen, 
methyl or ethyl and 
X has the abovementioned meaning. 
In a preferred embodiment, furthermore, anilines of the formula 
##STR6## 
are used in which 
R.sup.15 and R.sup.16, independently of one another, represent hydrogen, 
straight-chain or branched C.sub.1 -C.sub.4 -alkyl, phenyl, fluorine, 
chlorine, bromine, straight-chain or branched C.sub.1 -C.sub.4 -alkoxy, 
hydroxyl, amino, C.sub.1 -C.sub.4 -alkylamino, di-C.sub.1 -C.sub.4 
-alkyl-amino, phenoxy or phenylamino. 
In a particularly preferred embodiment, anilines of the formula 
##STR7## 
are used in which 
R.sup.25 and R.sup.26, independently of one another, represent hydrogen, 
methyl, ethyl, fluorine, chlorine, methoxy, ethoxy, methylamino, 
ethylamino, dimethylamino or diethylamino. 
In a most particularly preferred embodiment, anilines of the formula 
##STR8## 
are used in which 
R.sup.35 and R.sup.36, independently of one another, represent hydrogen, 
fluorine, chlorine, methyl, ethyl, methoxy, methylamino or dimethylamino. 
Alkyl groups to be mentioned in the said substituents (alkyl, alkoxy, 
alkylamino, dialkylamino) are straight-chain or branched C.sub.1 -C.sub.6 
-alkyl groups, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, 
tert-butyl and the isomeric pentyls and hexyls. Preferably to be mentioned 
are the said C.sub.1 -C.sub.4 -alkyl groups, particularly preferably 
methyl and ethyl, most particularly preferably methyl. 
Aryl to be mentioned in the said substituents (aryl, aryloxy, arylamino) is 
phenyl or a 5- or 6-membered heteroaryl having 1 or 2 heteroatoms from the 
group N, O and S which is linked in the 2-, 3- or 4-position. Examples of 
such a heteroaryl are: furanyl, pyrrolyl, thienyl, pyrazolyl, imidazolyl, 
oxazolyl, thiazolyl, pyridyl and others. In a preferred embodiment, aryl 
represents phenyl. 
C.sub.3 -C.sub.6 -cycloalkyl to be mentioned is, for example, cyclopropyl, 
cyclobutyl, cyclopentyl and cyclohexyl, preferably cyclopropyl, 
cyclopentyl and cyclohexyl. The cycloalkyl substituents, in their turn, 
may be singly or doubly substituted by methyl or ethyl. 
Halogen to be mentioned is, for example, fluorine, chlorine, bromine, 
iodine, preferably fluorine, chlorine, bromine, particularly preferably 
fluorine and chlorine. 
The expression "X" in the cycloalkyl ring in the formulae (I), (II), (Iv) 
and (V) may represent the methylene group --CH.sub.2 -- or a bond 
connecting the neighbouring C atoms. In the first case, this denotes the 
cyclohexane skeleton, in the second case the cyclopentane skeleton. In a 
preferred embodiment, X represents the methylene group and thus 
constitutes the cyclohexane skeleton. 
The following specific cycloalkanones may be mentioned as starting 
compounds by way of example: cyclopentanone, 2-methyl-cyclopentanone, 
2-ethyl-cyclopentanone, 2-phenyl-cyclopentanone, cyclohexanone, 
2-methyl-cyclohexanone, 4-methyl-cyclohexanone, 2-ethylcyclohexanone, 
4-ethyl-cyclohexanone, 4-phenyl-cyclohexanone, 
2-phenyl-cyclohexanone,4-cyclohexyl-cyclohexanone, 
2-cyclohexyl-cyclohexanone. 
The following specific anilines may be mentioned as starting compounds by 
way of example: aniline, 2-methylaniline, 3-methylaniline, 
4-methylaniline, 2,4-dimethyl-aniline, 2-ethylaniline, 4-ethylaniline, 
3-methoxyaniline, 3-ethoxyaniline, 4-aminoaniline, 4-methylamino-aniline, 
4-dimethylamino-aniline, 4-phenylaniline, 2-phenylaniline, 
4-phenoxyaniline, 2-phenoxyaniline, 4-phenylamino-aniline, 
2-phenylaminoaniline, 2-fluoroaniline, 4-fluoroaniline, 2-chloroaniline, 
4-chloroaniline. 
The molar ratio in which the cycloalkanone and the aniline are metered in 
is 2:1 to 1:2, preferably 1.5:1 to 1:1.5, particularly preferably 1.2:1 to 
1:1.2, most particularly preferably 1.1:1 to 1:1.1. 
In order to remove the water of reaction azeotropically, azeotropic agents 
may be used from the group of the aliphatic and aromatic hydrocarbons, the 
aliphatic and aromatic ethers and the aliphatic alcohols. In these 
compounds, hydrogen atoms may be replaced by halogen atoms. These 
azeotropic agents, their mode of action and their use are, in principle, 
known to those skilled in the art. Azeotropic agents to be mentioned by 
way of example are the following: 
benzene, toluene, xylene, anisole, chlorobenzene, butanol, hexanol, methyl 
butyl ether, diphenyl ether, p-chloroanisole. The weight ratio in which an 
azeotropic agent and the sum of the starting materials are metered in is 
0.001-1:1, preferably 0.001-0.5:1, particularly preferably 0.001-0.2:1, 
most particularly preferably 0.001-0.1:1. In a way known to those skilled 
in the art, the azeotrope which has been distilled off can be condensed 
outside the column-type reactor and can separated into the water of 
reaction and the azeotropic agent. The azeotropic agent thus recovered can 
again be recycled into the reaction of the method according to the 
invention. 
It is also possible to use, as the azeotropic agent, one of the starting 
materials, preferably the cycloalkanone, which makes it possible to 
dispense with an azeotropic agent foreign to the system. 
Possible acid homogeneous catalysts are, in principle, all inorganic or 
organic compounds are used which have a pK.sub.a value, measured in 
H.sub.2 O, from 1-6, preferably from 2-5.5, particularly preferably from 
3-5. Preferably, alkylcarboxylic acids or arylcarboxylic acids are used in 
their pure form or as mixtures. 
Particularly preferably, C.sub.1-10 -, most particularly preferably 
C.sub.2-6 -carboxylic acids, are used in their pure form or as mixtures. 
Examples to be mentioned are: formic acid, acetic acid, propionic acid, 
butyric acid, benzoic acid, valeric acid and isobutyric acid. 
If the sum of the acid groups of the metered catalytically active acids is 
considered in proportion to the sum of the metered starting material 
molecules, the metered amount of acid varies within the range from 10 to 
0.001 mol %, preferably from 1 to 0.01 mol %, particularly preferably from 
0.5 to 0.02 mol %. 
Acid mixtures are used in this context which are removed, to at least 50%, 
preferably to at least 70%, particularly preferably to at least 90%, 
together with the azeotropic gas phase, from the low-temperature region of 
the reactor. 
This is expedient because of the fact that large amounts of acid in the 
high-temperature region may give rise to the condensation of azomethine 
molecules with the elimination of aniline, and because they should 
therefore be kept away to a large extent from this region of the reactor. 
This is achieved by making use of the distillation process taking place in 
the reactor in the method according to the invention; accordingly, for 
each of the substrate combinations claimed, an optimum solution in each 
individual case is to be found by experimental means from the acid 
combinations claimed. 
The method according to the invention is characterised by the condensation 
being carried out in a distillation apparatus suitable for continuous 
operation. 
Such an apparatus, in the simplest case, comprises an isolated column with 
a condenser and flow divider at the column head (end of the 
low-temperature region) and an evaporator of any design arranged for 
continuous product discharge at the bottom of the column (end of the 
high-temperature region of the distillation apparatus). 
The column may, according to the processes taking place therein, by 
definition be divided into three sections: in the upper section (the 
low-temperature region), the separation of starting materials and water of 
reaction is the main process taking place; in the lower section (the 
high-temperature region), the product is separated from unreacted starting 
material and catalyst residues: in the middle section, the reaction range, 
the starting materials and, where possible, the catalysts from the other 
two regions are combined and reacted with the elimination of water. 
As the reaction is, at best, weakly exothermic, and the water produced has 
to be transferred into the gas phase with a relatively high energy 
expenditure and, at the same time, starting material molecules, according 
to their partial vapour pressures, must also be evaporated, the reaction 
section requires an energy infeed which, in general, is greater than the 
energy required for the separation of unreacted starting material from the 
azomethine. 
In a special embodiment of the method, below the reaction zone a heat 
exchanger is therefore installed which evaporates most of the unreacted 
starting material in order to supply the insulated reaction part with 
energy and to relieve the high-temperature region of this requirement. If 
necessary, the reaction part may, at a suitable location, additionally be 
supplied with more energy, generally at that location where most of the 
reaction takes place and where, therefore, the largest amount of water is 
formed. These additional energy input locations not only relieve the 
high-temperature region of the column: they also allow the input of a 
considerable fraction of the energy required at a low thermal level. 
Above the reaction part, a heat exchanger may be installed, if required, 
which separates unreacted starting material by condensation, with energy 
recovery if possible, from the water of reaction and which thus relieves 
the low-temperature region in which the azeotrope is formed. 
The design of the individual regions is a standard process engineering 
problem which is solved by known methods for each individual case by 
experiments and calculations. 
The three sections of the column, according to the design of the apparatus, 
may have different cross-sectional areas, in order to take account of the 
different gas and liquid loads prevailing therein. 
Without an evaporator part below the reaction zone, all of the energy 
required for reaction and separation must be fed into the system at the 
bottom of the column by the evaporation of azomethine and must be 
transported upwards from there as evaporation enthalpy, being used, 
successively, for the separation of starting material and product, and of 
water and starting material. This means that the lower volume part, in 
accordance with its high liquid and gas loading, has at least the same 
cross-sectional area, preferably 1.5 times the cross-sectional area, and 
at most three times, as a preferred maximum twice, the cross-sectional 
area of the reaction part. 
If the reaction part is fitted at the bottom with an evaporator and at the 
top with a dephlegmator, both the low- and the high-temperature region are 
relieved and accordingly have at most the same cross-sectional area, 
preferably only two thirds, but at least one third, preferably at least 
half of the cross-sectional area of the reaction part. 
The sizes depend not least on the column fillings selected for the various 
column zones (packings or fixed fillings which may be identical or 
different). 
The packings to be used, which may be arranged packings, are those which 
are per se common for distillations, as they are described, for example, 
in Ullmanns Encyclopadie der Techn. Chemie [Ullmann's Encyclopedia of 
Industrial Chemistry] Vol. 2, p. 528 et seq. (4th Edition), or in company 
publications, for example from Sulzer, Montz, Raschig, Kuhni or Norton. 
To be mentioned, for example, are: 
Raschig.RTM. or Pall.RTM. rings, Berl or Intalox.RTM. toroidal saddles, 
Interpack.RTM. packings made of various materials, such as glass, ceramic, 
porcelain, carbon, alloy steel, plastics, which, particularly in the case 
of metal, may be in a woven or mesh-like form. 
Preferred are arranged packings and poured packings which have a large 
surface area and good wetting properties, in addition to sufficient 
residence time of the liquid, for example Pall.RTM. and Novolax.RTM. 
rings, Berl.RTM. saddles, BX.RTM. packings, Montz-Pak.RTM., Mellapak.RTM., 
Melladur.RTM., Kerapak.RTM., CY.RTM. packings, Ralu-Pak.RTM. and 
Rombo-Pak.RTM.. 
Suitable for the method according to the invention, however, are not only 
packed columns, but also, in particular, columns with fixed fillings. 
Suitable are plate columns having sieve plates, bubble-cap plates, valve 
plates, tunnel-cap plates and centrifuge plates, which again may have 
various designs. 
Preferred are, for example, bubble-cap and valve plates having high 
residence times accompanied by effective mass exchange; this applies, in 
particular, to the reaction part of the column. 
The method according to the invention is carried out in a pressure range 
from 0.5 mbar to 3 bar, preferably from 1 mbar to 1 bar, particularly 
preferably at 2-250 mbar, most particularly preferably at 3-180 mbar. For 
the purpose of working in the preferred low-pressure range, the 
column-type reactor is equipped, at its upper low-temperature end, with a 
vacuum pump which may be connected upstream or downstream of the condenser 
for the water to be removed azeotropically. 
In the distillation column a steady-state temperature profile is 
established. This temperature profile comprises the temperature range from 
10.degree. to 300.degree. C., preferably 15.degree. to 250.degree. C., 
particularly preferably 20.degree. to 200.degree. C. 
The temperature range of the reaction zone, the zone in which more than 90% 
of the water of reaction is liberated and which, if required, is fitted at 
its upper end with a dephlegmator and its lower end with an evaporator, is 
preferably 25.degree. to 180.degree. C., particularly preferably 
35.degree. to 160.degree. C., most particularly preferably 45.degree. to 
140.degree. C. 
The pressure applied to a distillation column, together with the pressure 
build-up due to the flow resistance in the apparatus, determines the 
boiling points of the liquid mixtures inside the column. 
If the reaction is slow, and if, in order to improve the space-time yield, 
the pressure in the reaction part and therefore the temperature in the 
reaction part is raised, the thermal load for the high-boiling reaction 
product at the end of the column increases at the same time. 
If this creates problems, that is to say decomposition reactions occur, the 
high-temperature range may be subdivided if required by selecting the 
temperature at the lower end of the column in such a way that 
decomposition still does not take place, but that unreacted aniline and 
cycloalkanone are discharged at the same time. This mixture is freed, in a 
column (the second part of the high-temperature range) working in parallel 
at distinctly lower pressure, from residual aniline and cycloalkanone 
which, as the top product of this second distillation apparatus, are fed 
to the reaction column either after condensation or directly after 
compression. 
The infeed position can be chosen at will anywhere in the reaction region 
(e.g. in the first part of the high-temperature region) or in the 
low-temperature region, preferably at that location where the ratio of 
aniline to cycloalkanone of the liquid phase in the reaction column is 
equal to that of the top product. 
The pressure in this second part of the high-temperature region is 
preferably 1/1000 to 1/10, particularly preferably 1/100 to 1/10, of the 
working pressure of the reaction column. 
The input materials may be fed into the reactor, individually or as a 
mixture, above the reaction zone at a location chosen at will in the 
low-temperature region. They are preferably fed in as a mixture 
immediately above the reaction zone. In a particularly preferred 
embodiment, the mixture is fed into the reactor at that point where the 
ratio of the materials used in the liquid phase in the reactor is equal to 
the ratio in the mixture to be fed in. In the case that one of the 
materials used, because of its greater volatility, is under-represented in 
the reaction zone, it can, however, also be fed in, wholly or in part, 
below the reaction zone or in the lower part of the reaction zone and 
thus, wholly or in part, be conveyed towards the other reactant in 
counter-current flow. 
If a separate azeotropic agent is used, it can also be fed in separately 
from the starting materials; in a preferred embodiment, however, it is fed 
in together with the mixture of the starting materials or at least mixed 
with one of the starting materials. 
The same applies to the catalyst or the catalyst mixture which is 
preferably metered in in a mixture with the starting materials, but which 
may also, if required, be conveyed towards the starting materials from the 
lower part of the apparatus, in which case care must be taken that little 
or no catalyst reaches the lowermost region of the column. 
The azomethine as the reaction product is discharged at the lower reactor 
end in the high-temperature region, for example from the bottom 
circulation. The product is normally very pure and can be used 
subsequently without further purification. In the case that by-products of 
low volatility are to be separated, the desired azomethine may, in a way 
known in principle to the process engineer, be removed from the 
column-type reactor at a location above the discharge of such 
low-volatility by-products, which location is chosen such that maximum 
purity is ensured. 
In a preferred embodiment, in counterflow to the azomethine withdrawn in 
the high-temperature region, an inert gas stream is fed in, also in the 
high-temperature region, i.e. in any case below the reaction zone. Inert 
gases to be mentioned are, for example: air, nitrogen, argon or methane, 
which are all preferably used in dried form. 
At the upper end of the column-type reactor, that is in the low-temperature 
region, water of reaction is removed as an azeotrope and in a separating 
vessel is separated into an aqueous and an organic phase of the azeotropic 
agent. The organic phase, in a preferred embodiment, is recycled to the 
reactor, in particular if one of the reactants is also the azeotropic 
agent; this can be done at one of the abovementioned locations. 
By means of this circular flow, a not inconsiderable amount of acid 
catalyst is returned to the reaction zone. This amount must be taken into 
account when metering the acid. 
The acid content of an apparatus having circular flow is assessed most 
easily by measuring the pH of the aqueous phase of the distillate and 
keeping said pH constant by means of the catalyst infeed. 
The advantage of the method according to the invention compared to the 
prior art consists in the possibility of using nearly equimolar amounts of 
the starting materials which can then be reacted completely in a single 
pass. This achieves a quantitative yield while avoiding additional 
working-up stages. The method to be used is notable for great simplicity 
and for optimum utilisation of the energy employed. 
The azeomethines which can be prepared according to the invention are 
starting materials for the hydrogenation to give 
cyclohexyl-cycloalkyl-amines and phenyl-cycloalkyl-amines and, in the case 
that the cycloalkanone is a cyclohexanone, for the dehydrogenation to give 
optionally substituted diphenylamines.

EXAMPLE 1 
The apparatus, from bottom to top, comprises: glass flask (250 ml) with 
regulated nitrogen inlet for receiving the bottom product (from this 
flask, the azomethine is obtained by being removed, from time to time, 
with suction); oil-thermostated coil evaporator (coil tube length approx. 
1.50 m, internal diameter of tube approx. 1.5 cm); vacuum-mirrored packed 
column, filled with porcelain saddle packings 0.5 cm in size (internal 
diameter of column approx. 2.7 cm, height of the column approx. 35 cm); 
oil-thermostated column part, filled with porcelain saddle packings 
(diameter approx. 2.7 cm, height approx. 35 cm); oil-thermostated sieve 
plate column (internal diameter approx. 5 cm, height approx. 60 cm, 10 
plates having in total a liquid volume of approx. 60 ml: the oil jacket 
was thermostated to a temperature which equalled the internal temperature 
of the column as closely as possible at all locations); infeed pipe 
(internal diameter approx. 2.7 cm, height approx. 5 cm); oil-thermostated 
column part with porcelain saddle packings (diameter (internal) approx. 
2.7 cm, height approx. 15 cm); vacuum-mirrored packed column with 
porcelain saddle packings (internal diameter approx. 2.7 cm, height 
approx. 25 cm); air bridge to the down-flow condensing part, comprising 
two intensive condensers, Anschutz head, vacuum off-take connection and 
gas cap with vacuum tubing. 
The vacuum in the apparatus was maintained by a diaphragm pump; evacuation 
was controlled by a vacuum controller having a regulatable ball valve 
(.+-.0.5 mbar). 
Starting materials are metered in as mixtures by a vacuum-tight metering 
pump (TELAP, PTFE minidoser). The four thermostated column regions were 
heated, cooled or insulated by means of oil thermostats. 
The pressure was set to 60 mbar, and an Nz flow of 0.1 1/h S.T.P. was 
introduced into the lower part of the apparatus. After the oil thermostats 
had reached their required temperature (oil temperature of the lower 
evaporator approx. 195.degree. C., oil temperature of the middle 
evaporator approx. 190.degree. C., oil temperature of the sieve plate 
column approx. 80.degree. C., oil temperature of the upper condenser 
approx. 20.degree. C.), metering of the starting materials was initiated 
(330 ml/h). The starting materials consisted of 1.2 mol of cyclohexanone, 
1 mol of aniline and 1 mol % (based on the two components) of acetic acid 
(117.6 g, 93 g, 1.32 g). 
After a run time of about 1 to 2 hours, the apparatus was in equilibrium, 
and at the foot of the column, approx. 264 g of cyclohexylideneaniline 
having a purity of more than 99.9% were discharged from the coil 
evaporator per hour. The acetic acid content of the azomethine (anil) was 
below 10 ppm (determined by ion chromatography). 
At the head of the columns, approx. 59 g of a cyclohexanone/water mixture 
condensed which separated, at a volume ratio of 1.1 to 1, into an organic 
and an aqueous phase. This mixture contained the catalyst. 
EXAMPLE 2 
The procedure of Example 1 was followed, but 0.2 mol % (264 mg) of acetic 
acid and 0.0004 mol % (0.65 mg) of propionic acid were added to the 
starting material mixture of 1.2 mol of cyclohexanone and 1 mol of 
aniline. The experiment proceeded as described in Example 1, but the 
equilibrium was only achieved completely after a somewhat longer run time. 
The cyclohexylideneaniline (purity better than 99.9%) similarly contained 
less than 10 ppm of acetic acid and less than 0.5 ppm (lower detection 
limit) of propionic acid. 
EXAMPLE 3 
If the catalyst content of Example 2 was further reduced, the establishment 
of the chemical equilibrium for the loading described proceeded too 
slowly, and as a result the major part of the starting material mixture 
fed in distilled off at the top. 
EXAMPLE 4 
As Example 2 but without propionic acid. Outcome similar to Example 3. 
EXAMPLE 5 
As Example 1, but 1 mol % of anhydrous HCl (803 mg) was admixed to the 
starting material mixture of 1.2 mol of cyclohexanone and 1 mol of 
aniline. 
The course in this Example largely corresponded to that of Example 1, 
albeit with the difference that the temperature in the lower column part 
was 20.degree. to 30.degree. C. below that of Example 1 and, as the 
product, a mixture of 5% of unidentified high-boiling products, 48% of 
cyclohexenyl-cyclohexylideneaniline, 25% of cyclohexylideneaniline, 19% of 
aniline and 3% cyclohexanone was obtained.