Amine oxide process

Tert-amine oxides that are substantially free of nitrosamines are made by reacting a tert-amine, e.g., dodecyl dimethylamine, with aqueous hydrogen peroxide in the presence of the synergistic combination of (a) ascorbic acid and (b) a promoter formed from carbon dioxide.

Tertiary amine oxides are conventionally made by the reaction of an 
appropriate tert-amine with aqueous hydrogen peroxide. The reaction is 
generally conducted at 50.degree.-75.degree. C. and requires a long 
reaction period to obtain complete conversion of the amine. Several 
promoters for the reaction have been reported. Among the more effective is 
carbon dioxide. Murato et al. U.S. Pat. No. 4,247,480 report the reaction 
of N,N-diethyl 3,7-dimethyl-2,6-octadienylamine with 30 percent aqueous 
hydrogen peroxide at 55.degree.-65.degree. C. in the presence of carbon 
dioxide to give a 99 percent yield of the corresponding amine oxide. Also 
Japan Pat. application No. 56-83465 describes the use of ascorbic acid to 
stabilize shampoo and detergent compositions which contain an amine oxide 
from degradation leading to color formation and foul odor during storage. 
Nitrosamines are formed as minor by-products in the conventional 
preparation of tert-amine oxides using aqueous hydrogen peroxide. Although 
the amount of nitrosamine is very small, on the order of parts per billion 
(ppb), this small amount renders the amine oxide unsuitable in many 
applications that involve human contact. This is because nitrosamines are 
reported to be carsinogenic and/or mutagenic. Amine oxides have properties 
that would make them very useful in shampoo, hair conditioners, dish and 
laundry detergent, fabric softeners and the like. Hence a need exists for 
a method for making tert-amine oxides in high conversion and yield and at 
a fast reaction rate and at the same time producing a tert-amine oxide 
product that is substantially free of nitrosamines. The present invention 
provides such a process. 
SUMMARY 
It has now been discovered that tert-amine oxides that are substantially 
free from nitrosamines can be produced in high yield at a fast reaction 
rate by reacting a tert-amine with aqueous hydrogen peroxide in the 
presence of ascorbic acid and a promoter formed from carbon dioxide. This 
is quite unexpected because under the same reaction conditions, amine 
oxides made in the presence of either carbon dioxide or ascorbic acid 
individually contained significant quantities of nitrosamines.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the invention is a process for making a 
substantially nitrosamine-free amine oxide by reacting a tert-amine 
capable of forming an amine oxide with aqueous hydrogen peroxide in the 
presence of the synergistic combination of (a) ascorbic acid and (b) a 
promoter formed by adding carbon dioxide to the reaction mixture. 
The process is applicable to any tert-amine capable of forming an amine 
oxide. These are well known to organic chemists. They include amines which 
do not have a hydrogen atom bonded to the amine nitrogen atom. Such amines 
include trialkylamines; triarylamines; triarylalkylamines; mixed 
alkyl-aryl, alkyl-arylalkyl, aryl-arylalkyl or alkyl-aryl-arylalkylamines; 
tricycloalkylamines; alkyl-cycloalkylamines; aryl-cycloalkylamines; cyclic 
amines, e.g. N-methyl piperidine, N,N'-dimethyl piperazine, pyridine, 
2-methyl pyridine, N-methyl pyrrolidine, N-methyl pyrrolidone, N-ethyl 
morpholine and the like. 
In a more preferred embodiment the tert-amine has the formula R.sup.1 
R.sup.2 R.sup.3 N wherein R.sup.1 is an alkyl group containing 1-30 carbon 
atoms and R.sup.2 and R.sup.3 are alkyl groups containing 1-30 carbon 
atoms, cycloalkyl groups containing 5-12 carbon atoms, aryl groups 
containing 6-12 carbon atoms, aralkyl groups containing 7-12 carbon atoms 
or any two of the R groups can join to form a carbocyclic or hetrocyclic 
ring or all three of the R groups may participate to form a pyridine ring. 
The process is applicable to any of a broad range of tert-amines such as 
butyldimethylamine, hexyl dimethylamine, isobutyl dimethylamine, 
2-ethylhexyl dimethylamine, octyl dimethylamine, decyl dimethylamine, 
dodecyl dimethylamine, tetradecyl dimethylamine, hexadecyl dimethylamine, 
eicosyl dimethylamine, docosyl dimethylamine, triacontyl dimethylamine, 
tributylamine, butyl diethylamine, isobutyl diethylamine, decyl butyl 
ethylamine, hexadecyl hexyl methylamine, eicosyl dibutylamine, 
trioctylamine, tridodecylamine, dieicosyl ethylamine, ditriacontyl 
methylamine, N,N,-dimethylaniline, N-methyl-N-dodecylaniline, cyclopentyl 
dimethylamine, cyclohexyl dimethylamine, dicyclohexyl methylamine, 
cyclododecyl dimethylamine, diphenyl butylamine, p-tolyl diethylamine, 
.alpha.-naphthylbutylmethylamine, benzyl butyl methylamine, 
.alpha.-methylbenzyl butyl methylamine, 4-butylbenzyl octyl methylamine, 
dibenzyl butylamine, 4-pentyl-benzyl dibutylamine, N-butylmorpholine, 
N-methylmorpholine, N-methylpiperidine, N-dodecylpiperidine, 
N-octadecylpiperidine, N-triacontylpiperidine, N-methylpiperazine, 
N-butylpiperazine, N-octylpiperazine, N-phenylpiperidine, 
N-benzylpiperidine, N-cyclohexylpiperidine, pyridine and the like. 
In a more preferred embodiment the tert-amine is a primary trialkylamine 
having the structure R.sup.1 R.sup.2 R.sup.3 N wherein R.sup.1, R.sup.2 
and R.sup.3 are primary alkyls having 1-30 carbon atoms. Representative 
examples of these include but are not limited to trimethylamine, 
tri-n-pentylamine, tri-n-dodecylamine, n-octadecyl di-(n-butyl)amine, 
n-eicosyl di-(n-decyl)amine, n-triacontyl, n-dodecyl methylamine and the 
like. 
In a still more preferred embodiment R.sup.1 is a primary alkyl group 
containing 6-22 carbon atoms and R.sup.2 and R.sup.3 are independently 
selected from methyl and ethyl groups. 
In a further preferred embodiment R.sup.1 is a mainly linear primary alkyl 
containing 8-20 carbon atoms and R.sup.2 and R.sup.3 are methyl groups. By 
"mainly linear" is meant that over 50 percent, more preferably 70 percent 
and most preferably 90 percent of the R.sup.1 groups are linear alkyls 
containing 8-20 carbon atoms. 
Examples of these preferred embodiments are octyl dimethylamine, decyl 
dimethylamine, dodecyl dimethylamine, tetradecyl dimethylamine, hexadecyl 
dimethylamine, octadecyl dimethylamine, eicosyl dimethylamine and mixtures 
thereof. 
In another more preferred embodiment of the invention, both R.sup.1 and 
R.sup.2 are independently selected from primary alkyls containing 6-22 
carbon atoms and R.sup.3 is a methyl or ethyl group. 
In a highly preferred embodiment R.sup.1 and R.sup.2 are independently 
selected from mainly linear primary alkyl groups containing 8-20 carbon 
atoms and R.sup.3 is methyl. Examples of this highly preferred embodiment 
are dioctyl methylamine, didecyl methylamine, didodecyl methylamine, 
ditetradecyl methylamine, dihexadecyl methylamine, dioctadecyl 
methylamine, dieicosyl methylamine, decyl octyl methylamine, dodecyl octyl 
methylamine, tetradecyl decyl methylamine, hexadecyl tetradecyl 
methylamine, octadecyl hexadecyl methylamine, eicosyl dodecyl methylamine 
and the like including mixtures thereof. 
Any aqueous hydrogen peroxide can be used including those containing 3-100 
percent H.sub.2 O.sub.2. Preferably the hydrogen peroxide is 20-70 weight 
percent active H.sub.2 O.sub.2. When the tertamine is linear C.sub.8-20 
alkyl dimethylamine, it is preferred that the aqueous hydrogen peroxide be 
about 20-40 weight percent H.sub.2 O.sub.2 to avoid gel formation. 
Alternatively, more concentrated hydrogen peroxide can be used and 
additional water co-fed to maintain a stirrable reaction mixture. 
Likewise, co-solvents such as lower alcohol, e.g., isopropanol, isobutanol 
and the like, can be used to avoid gelation. 
The amount of hydrogen peroxide should be at least a stoichiometric amount. 
A useful range is about 1-5 moles of H.sub.2 O.sub.2 and more preferably 
1-1.5 mole of H.sub.2 O.sub.2 per mole of tert-amine. A highly preferred 
amount is about 1.05-1.3 moles of H.sub.2 O.sub.2 and especially about 
1.1-1.2 moles of H.sub.2 O.sub.2 per mole of tert-amine. Any excess 
H.sub.2 O.sub.2 remaining after the reaction can be destroyed by the 
addition of a reducing agent or a peroxide decomposition catalyst such as 
manganese dioxide. 
When the process is conducted using a di-linear alkyl methylamine, the 
process can be carried out using more concentrated aqueous hydrogen 
peroxide such as about 45-70 weight percent hydrogen peroxide. When the 
di-linear alkyls contain up to about 6-12 carbon atoms each, the reaction 
mixture will remain substantially gel free. When the di-linear alkyls 
contain 14 or more carbon atoms the reaction mixture will set up to a dry 
flakeable solid on cooling. 
The reaction can be conducted over a wide temperature range. The 
temperature should be high enough to cause the reaction to proceed at a 
reasonable rate but not so high as to lead to decomposition of the 
reactants or products. A useful temperature range is from about 
0.degree.-100.degree. C. A more preferred temperature range is about 
30.degree.-90.degree. C. A still more preferred temperature range is about 
40.degree.-75.degree. C. Most preferably the reaction is conducted at 
about 50.degree.-75.degree. C. In this temperature range the reaction is 
quite rapid and normally, without the presence of both carbon dioxide and 
ascorbic acid, would produce significant quantities of nitrosamines. 
Excellent results have been achieved at about 65.degree. C. 
The amount of carbon dioxide can vary over a wide range. It is required 
that the amount of carbon dioxide in the reaction mixture in whatever form 
it exists be an amount which causes the reaction to proceed at a faster 
rate than the rate achieved without the addition of carbon dioxide. In 
other words there should be at least a promoter amount of carbon dioxide. 
The upper limit of carbon dioxide is not critical and is determined by the 
solubility limit of carbon dioxide in the reaction mixture. A useful 
concentration is about 0.05-5.0 weight percent based on the weight of the 
initial tert-amine. Even more carbon dioxide can be used, e.g. 10 weight 
percent or more but this generally requires the reaction to be conducted 
under carbon dioxide pressure. A useful pressure range when carbon dioxide 
pressure is used is about 1-500 psig and preferably about 5-100 psig and 
more preferably about 10-50 psig. 
Another way to add the carbon dioxide is to dissolve the carbon dioxide in 
the tert-amine prior to adding the amine to the reaction vessel. In 
another mode of operation the carbon dioxide can be dissolved in the 
aqueous hydrogen peroxide prior to adding the aqueous hydrogen peroxide to 
the reaction vessel. Alternatively, the carbon dioxide can be dissolved in 
any water or other solvent (e.g. alcohol) added to the reaction vessel. 
Carbon dioxide is fairly reactive and will form other species when added 
to the reaction system. For example carbon dioxide dissolves in water to 
form carbonic acid which in this reaction is considered the equivalent of 
carbon dioxide. Likewise carbon dioxide may react with the tert-amine to 
form other catalytic species which in the present process are considered 
the equivalent of carbon dioxide. 
The amount of ascorbic acid should be an amount which when used in 
combination with the carbon dioxide promoter yields a tert-amine oxide 
that is substantially free of nitrosamines. In other words, it should be a 
nitrosamine inhibiting amount. A useful concentration of ascorbic acid is 
about 0.005-10 weight percent based on the tert-amine reactant. A 
preferred concentration of ascorbic acid is about 0.05-5 weight percent. A 
more preferred concentration is about 0.1-2 weight percent and a most 
preferred concentration of ascorbic acid is 0.2-1 weight percent. 
Instead of ascorbic acid, salts of ascorbic acid can be used such as 
ammonium or alkaline metal salts. Likewise isomeric forms of ascorbic acid 
are included in the scope of the invention. 
A series of reactions was conducted which revealed the synergistic effect 
of the combination of carbon dioxide and ascorbic acid. The following 
examples describe this series of experiments. 
EXAMPLE 1 
Reaction with Carbon Dioxide Promoter Only 
In a reaction vessel was placed 250 grams of dodecyl dimethylamine. The 
vapor space above the liquid was purged with carbon dioxide and a carbon 
dioxide sweep through the vapor space was continued during the reaction 
permitting carbon dioxide absorption up to its solubility limit in the 
liquid phase. While stirring at 65.degree. C., 86 grams of 50 weight 
percent aqueous hydrogen peroxide and 584 ml water which was sufficient to 
maintain a fluid reaction mixture and give a final amine oxide 
concentration of about 30 weight percent, were concurrently added over a 
30 minute period. Stirring was continued for 150 minutes at 65.degree. C. 
The reaction mixture was cooled to room temperature and analyzed, by 
spectroscopic and wet chemical methods, as follows: 
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dodecyl dimethylamine oxide 
30 percent 
dodecyl dimethylamine 0.2 percent 
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EXAMPLE 2 
Reaction with Ascorbic Acid Inhibitor Only 
This experiment was conducted in the same manner as Example 1 except that 
the carbon dioxide purge (and sweep) was replaced with a nitrogen pad and 
0.5 weight percent ascorbic acid, based on the tert-amine, was added at 
the start of the reaction. After the concurrent addition of 84 g of 
hydrogen peroxide and 584 ml water over 0.5 hour, the reaction mixture was 
stirred for an additional 10 hours. The reaction mixture analyzed as 
follows: 
______________________________________ 
dodecyl dimethylamine oxide 
29 percent 
dodecyl dimethylamine 0.5 percent 
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EXAMPLE 3 
The Synergistic Combination 
This experiment was conducted in the same manner as Example 1 including the 
carbon dioxide purge and sweep. In addition, 0.5 weight percent ascorbic 
acid was added at the start of the reaction followed by the concurrent 
addition of 84 g of hydrogen peroxide and 584 ml water. The final reaction 
mixture analyzed as follows: 
______________________________________ 
dodecyl dimethylamine oxide 
30 percent 
dodecyl dimethylamine 0.02 percent 
______________________________________ 
Each reaction mixture from Examples 1, 2 and 3 was analyzed for 
nitrosamines using a Thermal Energy Analyzer by an adaptation of the 
method described in Krull, I.S., et al., Anal. Chem 51, 1706 (1979). The 
nitrosamine results obtained were as follows: 
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Example 
1 2 3 
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N,N-dimethylnitrosamine 
96 ppb 37 ppb N.D. .sup.1 
N-methyl-N-dodecyl nitrosamine 
N.D. 216 ppb N.D. 
______________________________________ 
.sup.1 Not detected. The limits of detection by this method are 
N,Ndimethyl nitrosamine 10 ppb and Nmethyl-N-dodecyl nitrosamine 80 ppb. 
The above results show that the use of carbon dioxide alone (Example 1) 
does not eliminate the formation of nitrosamines under the above reaction 
conditions. Likewise the experiments show that ascorbic acid alone 
(Example 2) leads to the formation of substantial quantities of 
nitrosamines. Surprisingly, it was discovered that the combination of 
carbon dioxide and ascorbic acid (Example 3) reduced the amount of 
nitrosamines below the limit of detection.