Stabilization of aldehydes

Aldehydes having 3 to 14 carbon atoms are stabilized against polymerization and autocondensation by the addition thereto of triethanolamine or dimethylethanolamine in an amount at least 10 ppm (based on the amount of aldehyde) and preferably 20 to 100 ppm.

This application claims the priority of German application Nos. P 29 05 
267.3 and P 29 17 789.7 filed Feb. 12, 1979 and May 3, 1979, respectively. 
The invention relates to the stabilization of aldehydes having 3 to 14 
carbon atoms against polymerization and autocondensation. 
Such aldehydes have a tendency to spontaneously undergo polymerization and 
autocondensation. They have a tendency to form a cyclic trimeraldehyde 
(trialkyltrioxane) of the general formula 
##STR1## 
(where RCHO is the formula of the initial aldehyde and R represents an 
alkyl radical), and to form aldol condensation products by 
autocondensation. The ring-forming polymerization of isobutyraldehyde 
mainly produces 2,4,6-triisopropyl-1,3,5-trioxane. The autocondensation of 
isobutyraldehyde leads to the aldol condensation product 
2,6-diisopropyl-5,5-dimethyl-1,3-dioxan-4-ol. The ring-forming 
polymerization of aldehydes is catalyzed by various acid reagents such as 
sulphuric acid, hydrochloric acid, hydrogen fluoride, boron trifluoride 
and aluminum chloride. Under the action of such acidic compounds the 
polymerization of the aldehydes occurs spontaneously and leads within a 
few minutes to crystalline trimeraldehydes. Even the small amounts of 
carboxylic acids formed by the oxidation of the aldehydes in the presence 
of atmospheric oxygen are sufficient for catalyzing the trimer formation. 
The trimerization of isobutyraldehyde is catalyzed particularly by 
chlorine, bromine, phosphorus pentoxide, hydrogen chloride, oxygen and 
zinc chloride. Trimerization of isobutyraldehyde also occurs under the 
action of UV light. Also in the presence of alkali, derived for example 
from the containers in which the isobutyraldehyde is stored, 
autocondensation of isobutyraldehyde to form 
2,6-diisopropyl-5,5-dimethyl-1,3-dioxan-4-ol occurs even at 5.degree. to 
10.degree. C. In addition, low temperatures, i.e. temperatures of about 
0.degree. C. and below, promote the polymerization of aliphatic aldehydes. 
On account of its conversion into higher molecular weight compounds, in 
particular under the action of oxygen or UV light or alkaline-reacting 
substances, isobutyraldehyde cannot be stored for an indefinite period of 
time. Although in fact the polymerization and autocondensation products of 
isobutyraldehyde decompose further at elevated temperature, their 
formation nevertheless prevents the unlimited technical use of the 
aldehyde. 
Trimerization and autocondensation of aldehydes can be prevented for a 
limited period of time if the aldehydes are highly pure. The purification 
operations required are however so costly that they are not practicable 
for the commercial preparation of aldehydes. Also, addition of a solution 
of diphenylamine in ethanol to aldehydes suppresses polymerization but is 
not reliable for this purpose over a prolonged period of time. 
Attempts have been made to prevent the formation of higher molecular weight 
products from isobutyraldehyde. Polymerization and autocondensation 
reactions can be prevented by the addition of suitable substances. In 
practice these substances are subject to a whole range of requirements 
that have to be satisfied if the aldehyde is to be used without any 
restriction in a very wide variety of applications. One such requirement 
is that the substance in question must be effective for a long period of 
time in low concentrations and furthermore must not interfere in the 
processing of the aldehyde by chemical conversions. 
Mercaptobenzimidazole and 2,2-methylene-di-(4-methyl-6-tert.-butylphenol) 
have been described as stabilizers for isobutyraldehyde. It has been 
found, however, that both of these stabilizers are not active for a 
sufficient length of time. 
An object of the invention is to provide stabilizers that prevent, even in 
low concentration, polymerization and autocondensation reactions of 
aldehydes for as long a period of time as possible. 
It has now surprisingly been found that aldehydes with up to 14 carbon 
atoms can be effectively stabilized against polymerization and 
autocondensation by adding triethanolamine or dimethylethanolamine to 
them. Accordingly, the present invention provides a process for 
stabilizing an aldehyde having 3 to 14 carbon atoms against polymerization 
and autocondensation comprising adding triethanolamine or 
dimethylethanolamine to the aldehyde. 
The invention also provides a composition comprising an aldehyde having 3 
to 14 carbon atoms and triethanolamine or dimethylethanolamine, the 
aldehyde being stabilized against polymerization and autocondensation by 
the triethanolamine or dimethylethanolamine. 
Examples of aldehydes of the aforementioned molecular size are: propanal, 
isobutyraldehyde, n-pentanal, and dimethylhexanal. Further examples are, 
in particular, higher aldehydes containing 8 to 14 carbon atoms, such as 
n-octylaldehyde, n-nonyl-aldehyde, n-decylaldehyde, undecylaldehyde, 
lauraldehyde, methylnonylacetaldehyde (MNA), tridecylaldehyde, and 
myristylaldehyde, which are used on a large scale in the preparation of 
synthetic perfumes and fragrances. In this connection, it is therefore 
important that the substances added to prevent polymerization are not only 
highly effective, but also do not have any negative effect on the nature 
or character of the perfumes prepared therefrom. It has surprisingly been 
found that triethanolamine and dimethylethanolamine are highly effective 
and meet these requirements in an outstanding manner. Neither amine 
interferes in the further processing of the aldehydes to form secondary 
products. It should be noted in particular that triethanolamine and 
dimethylethanolamine, although they react in an alkaline manner, do not 
catalyze the aldol condensation of the aldehydes. 
These stabilizers are effective even in very low concentrations. As little 
as 10 ppm (based on the aldehyde) of the amines prevent the formation of 
high molecular weight compounds over a period of at least several weeks, 
e.g., in the case of isobutyraldehyde under the action of oxygen, for a 
period of 30 weeks. In general, it is preferred that the stabilizers be 
used in amounts of 20 to 100 ppm (based on the aldehyde). In such a 
concentration, the stabilizers prevent the formation of the trimer or 
autocondensation aldol product during storage of the aldehyde, even at low 
temperatures, without any further measures or precautions and for a period 
of at least several months, depending on the aldehyde, and one year in the 
case of isobutyraldehyde.

Triethanolamine and dimethylethanolamine are soluble in the aforementioned 
aldehydes and can thus be used without solvents. The amines are preferably 
added in specified amounts while stirring and at such a rate that no local 
overheating occurs. The following examples are intended to illustrate the 
present invention in greater detail. 
EXAMPLE 1 
Tests on the stabilization of isobutyraldehyde were carried out in 
polyethylene-lined containers. The containers were stored in the open and 
thus subjected to variable temperatures. As stabilizer, either 
triethanolamine or dimethylethanolamine in a concentration of 20 to 100 
ppm was used. The polymer content was determined by gas chromatography 
every 4 weeks. Errors were excluded by a system of double determinations. 
The results are given in the Table 1 below. 
TABLE 1 
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Trimer content (%) as a function of the 
storage time under comparable conditions 
Amount of 
5 weeks 
10 weeks 
20 weeks 
30 weeks 
40 weeks 
50 weeks 
60 weeks 
Stabilizer material 
storage 
storage 
storage 
storage 
storage 
storage 
storage 
(concentration) 
used time time time time time time time 
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Mercaptobenzimidazol 
(100 ppm) 0.02 0.36 0.59 -- -- -- -- -- 
Dimethylethanolamine 
(100 ppm) 0.02 0.02 0.02 0.02 0.03 0.04 0.07 0.10 
Dimethylethanolamine 
(20 ppm) 0.02 0.02 0.02 0.02 0.04 0.06 0.10 0.17 
Triethanolamine 
(100 ppm) 0.02 0.02 0.02 0.02 0.02 0.02 0.05 0.09 
Triethanolamine 
(20 ppm) 0.02 0.02 0.02 0.02 0.02 0.02 0.06 0.12 
No stabiliser 
0.02 0.86 2.60 4.80 7.00 7.60 7.70 9.00 
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EXAMPLE 2 
20 ppm (based on the aldehyde) of triethanolamine or diphenylamine was 
added to n-dodecanal (lauraldehyde) and the aldehyde was stored in 
aluminum containers some of which were kept at 0.degree. C. and some of 
which were kept at 20.degree. C. 
The aldehyde content was monitored by determining the carbonyl number (CON) 
after 1, 3, 6, 9 and 14 weeks. Errors were eliminated by a system of 
double determinations. The results are given in Table 2. 
TABLE 2 
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CON 
Start 
of After 1 After 3 
After 6 
After 9 
After 14 
Stabilizer 
Test weeks weeks weeks weeks weeks 
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Temperature: 
0.degree. C. 
None 288 198 121 73 50 33 
Diphenyl- 
amine in 
ethanol 288 282 270 230 148 100 
Tri- 
ethanolamine 
288 287 286 286 284 273 
Temperature: 
20.degree. C. 
None 288 286 284 275 275 270 
Diphenyl- 
amine in 
ethanol 288 287 283 282 282 274 
Tri- 
ethanolamine 
288 287 287 286 283 281 
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