Process for the preparation of alkoxyalkanoic acids

A process for the preparation of an alkoxyalkanoic acid by reacting the corresponding alkoxyalkanol with a stable free radical nitroxide in the presence of a NO.sub.x -generating compound, a solvent and an oxidant, at a temperature in the range of from about 0.degree. C. to about 100.degree. C. and thereafter separating out the alkoxyalkanoic acid.

FIELD OF THE INVENTION 
This invention relates to a process for the preparation of alkoxyalkanoic 
acids by the oxidation of the corresponding alkoxyalkanols in the presence 
of a stable free radical nitroxide, a NO.sub.x -generating compound, a 
solvent and an oxidant. 
BACKGROUND OF THE INVENTION 
Alkoxyalkanoic acids are useful as anionic detergents or emulsifying 
agents. These acids, being composed of only the elements C, H and O, do 
not pose the environmental problems that other detergents containing 
heteroatoms such as N, S and P pose. Commercially, the alkoxyalkanoic 
acids are prepared in a two-step process of first reacting an 
alkoxyalkanol with sodium and then reacting the resultant alkoxide with 
the sodium salt of chloroacetic acid. 
It is also known to convert alkoxyalkanols such as methyl carbitol to the 
corresponding carboxylic acids by oxidizing them with nitric acid. 
However, not all of the nitric acid can be separated by distillation, and 
the reaction product contains nitric acid, which is corrosive and 
therefore undesirable. In addition, cleavage of the ether linkages occurs 
to a large degree during this process. 
Japanese Patent No. 50-96519 issued on Jul. 31, 1975, discloses a process 
for the preparation of carboxylic acid salts by the liquid phase 
dehydrogenation of alcohols with caustic alkali in the presence of 
precious metal catalysts, including palladium. This process uses a 
relatively high temperature, 100.degree. C.-270.degree. C. These high 
temperatures can degrade the ether linkages, especially in the highly 
ethoxylated alcohols. 
It is known to use nitroxyl radicals/oxoammonium salts in the oxidation of 
primary alcohols to produce aldehydes and acids and secondary alcohols to 
ketones (Journal of Organic Chemistry, Vol. 52 (12), pp. 2559-2562, Pure 
and Applied Chemistry, Vol. 62(2), 1990, pp. 217-222, and Journal of 
Organic Chemistry, Vol. 55, 1990, pp. 462-466). The primary products 
produced in these processes are aldehydes and the stoichiometrically 
consumed oxidant is hypochlorite. 
It is generally more difficult to oxidize alkoxyalkanols than alkanols as 
it is difficult to oxidize alkoxyalkanols without splitting the molecular 
chain at the ether linkage and thereby produce a large proportion of 
undesired by-product. 
It has been found that alkoxyalkanoic acids can be produced in high yields 
and with high selectivities from alkoxyalkanols without producing large 
amounts of highly corrosive, difficult to separate, by-products. This can 
be accomplished by using catalytic amounts of a stable free radical 
nitroxide, a NO.sub.x -generating compound, a solvent, and an oxidant. 
SUMMARY OF THE INVENTION 
This invention relates to a process for the preparation of an 
alkoxyalkanoic acid of the formula RO(CH.sub.2 CHR'O).sub.n CH.sub.2 
CO.sub.2 H wherein R is an alkyl group of from about 1 to about 22 carbon 
atoms, R' is hydrogen or methyl or mixtures thereof (on the individual 
molecule) and n is an integer of from about 1 to about 500 which comprises 
reacting the corresponding alkoxyalkanol with a stable free radical 
nitroxide having the formula: 
##STR1## 
wherein (1) (a) each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is an alkyl, 
aryl or substituted alkyl group having 1 to about 15 carbon atoms, and (b) 
R.sub.5 and R.sub.6 (i) each is an alkyl group having 1 to about 15 carbon 
atoms provided that R.sub.1 -R.sub.6, are not all alkyl groups, or a 
substituted alkyl group having 1 to about 15 carbon atoms wherein the 
substituent is hydrogen, cyano, --CONH.sub.2, --OCOCH, OCOC.sub.2 H.sub.5, 
carbonyl, alkenyl wherein the double bond is not conjugated with the 
nitroxide moiety, or --COOR wherein R of the --COOR group is alkyl or 
aryl, or (ii) together form part of a ring that contains at least 3 carbon 
atoms and up to two heteroatoms of O or N, or (2) the 
##STR2## 
moiety and the 
##STR3## 
moiety individually are aryl, or (3) the 
##STR4## 
moiety and the 
##STR5## 
moiety together form a bicyclic ring with the proviso that the group 
directly adjacent to the N-O moiety is a bridgehead C-H, or a fully 
alkylated carbon, in the presence of a NO.sub.x -generating compound, a 
solvent and an oxidant, at a temperature in the range of from about 
0.degree. C. to about 100.degree. C. and thereafter separating out the 
alkoxyalkanoic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present process converts alkoxyalkanols of the formula RO(CH.sub.2 
CHR'O).sub.n CH.sub.2 CH.sub.2 OH (I) wherein R is an alkyl group having 
from about 1 to about 22 carbon atoms, preferably from about 11 to about 
18 carbon atoms,R' is hydrogen or methyl or mixtures thereof (on the 
individual molecule) and n represents the average number of oxyalkylene 
groups and is an integer of from about 1 to about 500, preferably from 
about 1 to about 50, more preferably from about 1 to about 12, and most 
preferably from about 2 to about 9, to the corresponding alkoxyalkanoic 
acids of the formula RO(CH.sub.2 CHR'O).sub.n CH.sub.2 CO.sub.2 H (II) by 
contacting the alkoxyalkanol with a stable free radical nitroxide in the 
presence of a NO.sub.x -generating compound, a solvent and an oxidant at a 
temperature in the range of from about 0.degree. C. to about 100.degree. 
C. and thereafter separating out the alkoxyalkanoic acid. The R group in 
the above formula I can be substituted with any substituent which does not 
interfere with the oxidation of the hydroxy group. Such substituents 
include --OR", --CH.sub.3, --COOH, CONH.sub.2 and COOR" wherein R' is an 
alkyl or aryl group. The process of the instant invention is particularly 
suited to detergent range ethoxylated, or propoxylated alcohols with alkyl 
chains (R) of about 8 to about 20, preferably about 11 to about 18 carbon 
atoms. The R' groups on an individual molecule can be hydrogen, methyl, or 
mixtures thereof. For example, straight ethoxylated, straight propoxylated 
and mixed ethoxylated-propoxylated detergent alcohols are commercially 
available. The number of such alkoxylate groups, CH.sub.2 CHR'O, ranges 
from about 1 to about 20. Commercially, detergent range ethoxylate 
alcohols are available with an average of 3, 7, 9 and 12 ethoxylate units 
per molecule. Others can be readily prepared. In a preferred embodiment, 
the alkoxyalkanol reactant is an ethoxylated alcohol which has had the 
unreacted alcohols and lower ethoxylates topped off in order to give an 
ethoxylate having about four ethylene oxide units per molecule. 
The term "stable free radical nitroxide" as used herein shall mean a free 
radical nitroxide that can be prepared by conventional chemical methods 
and will exist long enough to be used in a subsequent chemical reaction or 
examined in a static system by normal methods of spectroscopy. Generally, 
the stable free radical nitroxides of the present invention have a half 
life of at least one year. The term "stable free radical" shall also be 
understood to include the precursor to a stable free radical from which 
the stable free radical may be produced in situ. 
The stable free radical nitroxides, as used in the present process, are 
precursors to catalysts, i.e., oxoammonium salts, active for the oxidation 
of alkoxyalkanols to the corresponding acids. These catalysts are 
generated in situ by the oxidation of a stable free radical nitroxide to 
an oxoammonium salt with an oxygen-containing oxidant. The stable free 
radical nitroxide can be obtained by the oxidation of amines or 
hydroxylamines. 
The stable free radical nitroxides which are suitable for use in the 
instant invention have the formula: 
##STR6## 
wherein each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is an alkyl, aryl or 
substituted alkyl groups and no hydrogen is bound to the remaining 
valences on the carbon atoms bound to the nitrogen. As used herein, the 
term "alkyl" is meant to include cycloalkyl. The alkyl (or substituted) 
groups R.sub.1 -R.sub.4 may be the same or different, and preferably 
contain 1 to 15 carbon atoms. Preferably, R.sub.1 -R.sub.4 are methyl, 
ethyl, or propyl groups. In addition to hydrogen, the substituents may 
include, halogen, oxygen, nitrogen and the like. 
The remaining valences (R.sub.5 and R.sub.6) in formula III above may be 
satisfied by any atom or group except hydrogen which can bond covalently 
to carbon, although some groups may reduce the stabilizing power of the 
nitroxide and are undesirable. When R.sub.1, R.sub.2, R.sub.3 and R.sub.4 
are each alkyl groups, however, at least one of R.sub.5 and R.sub.6 must 
be an aryl group. Preferably, R.sub.5 and R.sub.6 are substituted alkyl 
groups having 1 to about 15 carbon atoms wherein the substituent is 
selected from halogen, cyano, --COOR, wherein R is alkyl or aryl, 
--CONH.sub.2, --OCOC.sub.2 H.sub.5, carbonyl, or alkenyl where the double 
bond is not conjugated with the nitroxide moiety, or alkyl groups of 1 to 
about 15 carbon atoms. 
The remaining valences (R.sub.5 and R.sub.6 ) in formula III above may also 
form a ring containing at least three carbon atoms and up to two 
heteroatoms, such as O or N. R.sub.5 and R.sub.6 can, for example, form a 
five-membered ring containing 3 carbon atoms and up to two heteroatoms, 
such as O or N, a five-membered ring containing 4 carbon atoms, a 
six-membered ring containing 5 carbon atoms, a seven-membered ring 
containing 6 carbon atoms, an eight-membered ring containing 7 carbon 
atoms, etc. For purposes of this invention, it is preferred that R.sub.5 
and R.sub.6 together form a five-membered ring, a six-membered ring, a 
seven-membered ring, or an eight-membered ring, although larger rings 
would also be suitable. Examples of suitable compounds having the 
structure above and in which R.sub.5 and R.sub.6 form part of the ring are 
2,2,6,6,-tetramethylpiperidine-1-oxyl, 2,2,5,5-tetramethyl 
pyrrolidin-1-oxyl, 2,2,7,7-tetramethylcycloheptan-1-oxyl, mixtures 
thereof, and the like. It is understood that these compounds may contain 
substituents which do not interfere with the reaction. 
The 
##STR7## 
and the 
##STR8## 
moieties in formula III above can individually be aryl, i.e., 
##STR9## 
Examples of suitable compounds having the structure above in which the 
##STR10## 
and the 
##STR11## 
moieties are individually aryl are diphenylamine-N-oxyl, phenyl 
tertiary-butylamine-N-oxyl, 3-methylphenyl phenylamine-N-oxyl, 
2-chlorophenyl phenylamine-N-oxyl and the like. These compounds may be 
substituted with an substituents which do not interfere with the reaction. 
The 
##STR12## 
and the 
##STR13## 
moieties in formula III above can also form a bicyclic ring wherein the 
group adjacent to the N-O moiety is either a bridgehead C-H or a 
quaternary carbon. As used herein, the term "bridgehead C-H" refers to a 
tertiary carbon which is common to both rings of the bicyclic ring system. 
As used herein, "a quaternary carbon" refers to a fully substituted carbon 
atom having alkyl, aryl or substituted alkyl groups having 1 to about 18 
carbon atoms as substituents. Examples of suitable compounds having the 
structure above in which the 
##STR14## 
and the 
##STR15## 
moieties form a bicyclic ring are 2-azabicyclo-[2.2.1]heptan-2-oxyl, 
2-azabicyclo[2.2.2]-3,3-dimethyloctan-2-oxyl, 3-azabicyclo[3.2.2]-2,2, 
4,4-tetramethylnonan-3-oxyl and the like. These compounds may be 
substituted with any substituents which do not interfere with the 
reaction. 
In a preferred embodiment, the stable free radical nitroxide is a 
piperidine-1-oxyl having the formula: 
##STR16## 
wherein each of R.sub.7, R.sub.8, R.sub.9 and R.sub.10 is an alkyl, aryl 
or substituted alkyl group having 1 to about 15 carbon atoms and no 
hydrogen is bound to the remaining valences on the carbon atoms bound to 
the nitrogen, and each of R.sub.11 and R.sub.12 is alkyl, hydrogen, aryl 
or a substituted heteroatom. As used herein, the term "alkyl" is meant to 
include cycloalkyl. The alkyl (or substituted) groups R.sub.7 -R.sub.10 
may be the same or different, and preferably contain 1 to 15 carbon atoms. 
Preferably, R.sub.7 -R.sub.10 are methyl, ethyl, or propyl groups. In 
addition to hydrogen, the substituents may include, halogen, oxygen, 
nitrogen and the like. Typically, one of R.sub.11 and R.sub.12 is 
hydrogen, with the other one being a substituted heteroatom which does not 
interfere with the reaction. Suitable substituted heteroatoms include 
--OR, 
##STR17## 
--.sup.+ NMe.sub.3 Cl--, --O--SO.sub.3 H, --O--polymer and the like. 
In a particularly preferred embodiment, the nitroxide is selected from the 
group consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-hydroxy-2,2, 
6,6-tetramethylpiperidine-1-oxyl, 
2,2,6,6-tetramethylpiperidine-1-oxyl-4-sulfate, 
4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-ethoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-carbamoyl-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-benzoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-pivaloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-dodecyloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-dodecanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 
4-octanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and mixtures thereof, 
with 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetylamino-2,2, 
6,6-tetramethylpiperidine-1-oxyl, and 
4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl being especially preferred. 
The NO.sub.x -generating compound in the present process is typically 
nitric acid. However, any compound which serves to generate NO.sub.x 
during the course of the reaction, such as, for example, an alkali metal 
nitrosodisulfonate, and which does not interfere with the reaction would 
be suitable. While not wishing to be bound by any particular theory, it is 
believed that nitrogen oxides (NO.sub.x) are generated in the reaction and 
are the active species in the reaction. 
As used herein, the term "nitric acid" refers to nitric acid, fuming nitric 
acid or nitrous acid generated by contacting a nitrate or nitrite salt 
such as, for example, an alkali metal salt, a tetraalkylammonium salt, an 
alkaline earth salt or a rare earth salt, with a strong acid such as, for 
example, a mineral acid. The nitric acid suitable for use as a NO.sub.x 
-generating compound in the present invention typically has a 
concentration in the range of from about 50 percent to about 100 percent, 
preferably about 70 percent. Generally, an amount of nitric acid in the 
range of from about 5 mole percent to about 200 mole percent, basis the 
moles of starting alkoxyalkanol is utilized. The nitric acid is typically 
added to the reaction mixture after all of the other reactants have been 
added. 
The reaction of the present invention is carried out in the presence of a 
solvent. The solvent is generally one in which the alkoxyalkanol is 
readily soluble. Solvents which are most suitable are those which are 
inert in the reaction. The solvent may be added to the reaction mixture, 
or alternatively, the nitroxide may be dissolved in the solvent prior to 
addition of the nitroxide to the reaction medium. The solvent is typically 
selected from the group consisting of alkyl ethoxycarboxylates, alkyl 
carboxylates, chlorinated hydrocarbons, chorofluorohydrocarbons, 
hydrocarbons, polyethers, dipolar aprotic solvents and mixtures thereof. 
Tertiary alcohols may also be utilized as solvents in the process of the 
present invention, however, reaction temperatures in the range of about 
40.degree. C. to about 60.degree. C. are required in order for the 
reaction to proceed. In a preferred embodiment, the solvent is selected 
from the group consisting of alkyl ethoxycarboxylate, dichloromethane, 
carbon tetrachloride, proprionic acid, heptane, diglyme, tertiary amyl 
alcohol, freon and mixtures thereof. The amount of solvent utilized in the 
process is typically in the range of from about 0.5:1 to about 100:1, 
preferably from about 1:1 to about 10:1, basis the weight of the starting 
alkoxyalkanol. 
The process of the present invention is also carried out in the presence of 
an oxidant. The oxidants suitable for use in the instant invention are 
those compounds which are capable, in the presence of a NO.sub.x 
-generating compound, of oxidizing the stable free radical nitroxide to 
the oxoammonium salt. Suitable oxidants include oxygen-containing gases 
such as pure oxygen and oxygen in air. Whereas pure oxygen can is 
preferred to accomplish the desired conversion, the oxygen can also be 
diluted with an inert gas such as nitrogen, helium, argon, or other 
similar gas. For purposes of increasing the reaction rate, higher O.sub.2 
pressures such as, for example, up to about 2000 psig can be utilized. In 
a preferred embodiment, pure oxygen is used as the oxidant and it is 
bubbled into the reaction solution. In another embodiment, the reaction 
solution can be exposed to compressed air under pressure. 
The amounts and concentrations of the reactants utilized in the process of 
the instant invention can vary within wide ranges. The amount of stable 
free radical nitroxide is typically in the range of from about 0.01 mole 
percent to about 200 mole percent, preferably from about 5 mole percent to 
about 20 mole percent, basis the number of moles starting alkoxyalkanol. 
Generally, the amount of NO.sub.x -generating compound used is in the 
range of from about 5 mole percent to about 200 mole percent, basis the 
number of moles of alkoxyalkanol. 
The process of the present invention is typically conducted under mild 
conditions, with good results being obtained using a temperature in the 
range of from about 0.degree. C. to about 100.degree. C., preferably about 
20.degree. C. to about 70.degree. C., and most preferably, about 
35.degree. C. to about 65.degree. C. Higher pressures can result in 
increased reaction rates. Pressures in the range of from about atmospheric 
pressure up to about 2000 psig can be employed with good results. 
The process of the instant invention can be carried out either batchwise or 
continuously, using a stirrer equipped reactor or other well known 
contacting technique to achieve adequate mixing. Preferred reaction 
conditions, e.g., temperature, pressure, flow rates, etc., vary somewhat 
depending on the specific nitroxide utilized and on the concentration of 
the nitroxide. 
The process of the instant invention can be carried out in a variety of 
ways. For example, 0.03 moles of alkoxyalkanol, and 0.003 moles of the 
nitroxide, and solvent may be added to the reaction vessel, followed by 
the addition of 0.011 moles of 70 percent nitric acid and bubbling oxygen 
through the reaction mixture. Following the reaction, the product may be 
separated from the reaction mixture using conventional procedures such as 
extraction using a suitable extraction solvent such as, for example, ethyl 
acetate; evaporation wherein the solvent is stripped from the reaction 
mixture by using heat or vacuum. Phase separation of the final product 
mixture can take place at 100.degree. C. with water. The reaction product 
can be purified by a number of conventional means such as high temperature 
water washing or distillation. 
Depending upon process conditions and the nitroxide used, the yields of 
alkoxyalkanoic acid obtained by this invention can be very high, with 
greater than about 98% of starting material being converted. The products 
produced by the instant process can be used in a variety of detergent 
applications or emulsifying agents. For example, these products can be 
used in light duty dishwashing liquids, shampoos, heavy duty laundry 
liquids and heavy duty powders. 
The ranges and limitations provided in the instant specification and claims 
are those which are believed to particularly point out and distinctly 
claim the present invention. It is, however, understood that other ranges 
and limitations which perform substantially the same function in the same 
or substantially the same manner to obtain the same or substantially the 
same result are intended to be within the scope of the instant invention 
as defined by the instant specification and claims. 
The process of this invention will be further described by the following 
embodiments which are provided for illustration and are not to be 
construed as limiting the invention. 
ILLUSTRATIVE EMBODIMENTS 
In the following examples, the starting alkoxyalkanol was a 
NEODOL.RTM.Ethoxylate 23-3T alcohol which was prepared by ethoxylating a 
mixture of C.sub.12 and C.sub.13 substantially straight chain alcohols 
(C.sub.12 :C.sub.13 .about.40:60) to form an ethoxylate alcohol having 
about 3 ethylene oxide units per molecule and then topping off the 
unreacted alcohols and lower ethoxylates so that the final product has 
less than about 5 percent by weight of the starting alkanol. 
EXAMPLE 1 
12 Grams of NEODOL.RTM. Ethoxylate 23-3T, 0.5 grams of 
2,2,6,6-tetramethylpiperidine-1-oxyl, 1 gram of 70 percent nitric acid and 
25 milliliters of triglyme were charged to a 100 milliliter round bottomed 
flask. O.sub.2 was bubbled through this mixture at a rate of 35 
milliliters/minute at ambient pressure. The reaction temperature was held 
at 35.degree. C. over a 4-hour period. The results are presented in Table 
I. 
EXAMPLE 2 
12 Grams of NEODOL.RTM. Ethoxylate 23-3T, 0.5 grams of 
2,2,6,6-tetramethylpiperidine-1-oxyl, 1 gram of 70 percent nitric acid and 
25 milliliters of an alkyl ethoxycarboxylate (a mixture of C.sub.12 and 
C.sub.13 alcohols having about 4 ethylene oxide units per molecule) were 
charged to a 100 milliliter round bottomed flask. O.sub.2 was bubbled 
through this mixture at a rate of 35 milliliters/minute at ambient 
pressure. The reaction was held at 45.degree. C. over a 5-hour period. The 
results are presented in Table I. 
EXAMPLE 3 
12 Grams of NEODOL.RTM. Ethoxylate 23-3T, 0.5 gram of 
2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acid 
and 25 milliliters of dichloromethane were charged to a 100 milliliter 
round bottomed flask. The reaction mixture was sparged with air at a rate 
of 0.7 liters/minute at ambient and held at 20.degree. C. over a 6-hour 
period. The results are presented in Table I. 
EXAMPLE 4 
12 Grams of NEODOL.RTM. Ethoxylate 23-3T, 1 gram of 
2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acid, 
and 25 milliliters of n-heptane were charged to a 100 milliliter round 
bottomed flask. O.sub.2 was bubbled through this mixture at a rate of 35 
milliliters/minute at ambient pressure. The reaction temperature was held 
at 35.degree. C. over a 3-hour period. The results are presented in Table 
I. 
COMATIVE EXAMPLE A 
Comparative Example A was carried out in a manner similar to Example 1 
except that no nitroxide was used. The results are presented in Table I. 
COMATIVE EXAMPLE B 
Comparative Example B was carried out in a manner similar to Example 1 
except that no nitric acid was used. The results are presented in Table I. 
As can be seen in Table I, nitroxide and nitric acid are necessary for the 
oxidation of the alkoxyalkanol to proceed. 
TABLE I 
______________________________________ 
Oxidation of Alkoxyalkanols to Alkoxyalkanoic acids 
% Selectivity 
% Conversion 
Alkoxyalkanoic Acids 
______________________________________ 
Example 1 &gt;99 &gt;99 
Example 2 95 93 
Example 3 &gt;99 95 
Example 4 97 85 
Comparative &lt;2 0 
Example A 
Comparative 0 0 
Example B 
______________________________________