Ferric Hydroxamate Determination of Hydroxyl Groups After Acid -Cata lyzed Acetylation GEORGE GUTNIKOV AND GEORGE H. SCHENK Department of Chemistry, Wayne State University, Defroit 2, Mich.
b An investigation of the acid-catalyzed acetylation of organic hydroxyl groups in pyridine, followed by basecatalyzed hydroxamation of the acetate esters and color development as the ferric hydroxamate chelate, was undertaken. The method developed is suitable for the determination of 1 .O to 14 X lO-'M hydroxyl groups in the final solution. The use of a large excess of sodium hydroxide over hydroxylamine, in contrast to other hydroxamate methods, permits the rapid reproducible hydroxamation of acetates with a hindered secondary alkoxy1 group. A mechanism is proposed to account for this. Acetone is used to convert the excess hydroxylamine to acetone oxime to prevent reduction of Fe(lll) during color development. The method has been applied to the determination of hydroxyl groups in various commercial polyglycols.
drate or rearrange in the presence of the strong Lewis acid acetyl chloride during the ether evaporation. The existing ferric hydroxamate methods for esters (7, 8, 16) and the recent appearance of rapid but mild room temperature acid-catalyzed acetylation methods (6), supplementing lengthy reflux acetylation methods, suggested the convenient color method below. The alcohol-containing sample is first acetyhted under mild, buffered acidcatalyzed conditions in pyridine a t room temperature to an acetate, ROAc. The excess acetic anhydride is hydrolyzed with a minimum of water a t room temperature, the acetate is converted with NHzOH to the anion of the corresponding hydroxamic acid, CH3CO"0-, in basic solution, and finally ferric perchlorate is added to the acidified solution to form the purple ferric chelate, (CH,CONHO -),Fe+3-" (17 ) . EXPERIMENTAL
R
spectrophotometric methods for the estimation of hydroxyl groups have been limited to the 3,5-dinitrobenzoyl chloride method of Johnson and Critchfield ( f f ) , the 2,Pdinitrophenylhydrazine method for secondary alcohols by Critchfield and Hutchinson (4), the 5-(p-sulfaniylphenylazo) salicylic acid method of Pesez (14 ), and the vanadium oxinate method of Tanaka (20). Unfortunately, amines react preferentially with the first and third reagents above and may interfere in the vanadium oxinate method. Esters of 3,5dinitrobenzoyl chloride must then be extracted for subsequent color measurement. Morgan (13) determined methanol and ethanol indirectly after reaction with excess acetic anhydride by determining the excess anhydride via the ferric hydroxamate reaction. However, Hill (10 ) mentioned preliminary work on a more convenient ferric hydroxamate colorimetric determination of alcohols as esters after acetylation with acetyl chloride in ether, evaporation of the ether, and hydrolysis of the acetyl chloride. It is obvious that a few volatile esters might evaporate, but it should also be recognized that acidsensitive alcohols or esters might dehyELIABLE
1316
GENERAL
ANALYTICAL CHEMISTRY
Apparatus. A Beckman Model DU or D B spectrophotometer with 1-cm. corex cells was used throughout this investigation. The recommended wavelength is 524 mp. Acetylating Reagent. Pipet 1 ml. of 72% perchloric acid into a 50-ml. volumetric flask and immerse in ice water. Slowly pipet in 20 ml. of Fisher brand pyridine down the sides of the flask and then dilute to the mark with Eastman brand acetic anhydride. Allow to come to room temperature before making the final dilution to volume with acetic anhydride. Prepare fresh daily, or to retard rapid discoloration of the reagent, add 100 mg. of 2,6-di-tert-butyl-4methylphenol per 50 ml.; this is effective for ca. 3 days. Although Fisher brand pyridine was satisfactory, for careful work the pyridine should be distilled from barium oxide or purified by the method of Banick (2). Hydrolysis Reagent. Mix 4 volumes of water with 1 volume of pyridine. 1.4M Hydroxylammonium Perchlorate. Dissolve 195 grams of sodium perchlorate in 450 ml. of absolute methanol with heating. Dissolve 105 grams of reagent grade hydroxylamine hydrochloride in 550 ml. of absolute methanol with heating and stirring. Add the methanolic sodium perchlorate slowly to the
latter solution with stirring. Add 50 ml. of benzene and allow to come to room temperature. Chill the solution and the precipitate for 1 hour in an ice bath, filter the sodium chloride on a sintered glass suction filter, and store the hydroxylammonium perchlorate solution in a polyethylene bottle. 2.5M Sodium Hydroxide. Prepare a liter, using absolute methanol. Stock Ferric Perchlorate Solution. Dissolve 50 grams of anhydrous ferric perchlorate in 400 ml. of absolute ethanol, add 40 ml. of 72% perchloric acid, and dilute t o exactly 500 ml. with ethanol. Ferric Perchlorate Reagent. To about 1.7 liters of absolute ethanol add 100 ml. of the stock ferric perchlorate solution. Slowly and with cooling, add 140 ml. of 72y0 perchloric acid in small portions. Allow to cool and dilute to 2 liters with absolute ethanol. The preparation of a large volume of this reagent as well as the hydroxylammonium perchlorate will eliminate variations in molar absorptivities that might occur if the reagent were prepared in small batches. PROCEDURE
Weigh a hydroxyl-containing sample into a 10-ml. or larger volumetric flask so that the solution contains 0.005 to 0.07 mmole of hydroxyl per ml. Dilute to the mark Kith the acetylating reagent, mix, and allow to stand 5 minutes, or longer for hindered compounds. The ratio of the sample e0 the total volume a t this point should be no more than 1: 20 or 0.5-ml. sample10-ml. total volume. Pipet exactly 1 ml. of this solution into a 50-ml. volumetric flask (for the blank, use 1 ml. of the acetylating reagent), add 0.5 ml. of the hydrolysis reagent, mix, and allow to stand at room temperature for 10 minutes. Pipet in 3 ml. of 1 . U hydroxylammonium perchlorate and then 10 ml. of 2.5M sodium hydroxide. After 20 minutes, add about 22 to 23 ml. of a ferric perchlorate-acetone mixture made by mixing, for each sample, 2.5 ml. of acetone with 35 ml. of the ferric perchlorate reagent. Allow to stand 5 minutes. Dilute to the mark with same ferric perchlorate-acetone mixture, mix thoroughly, and read the absorbance of the sample us, a blank treated in the same manner as above, a t 524 mp. Weigh samples containing low concentrations of hydroxyl groups directly into a 50-ml. volumetric flask. Use no
more than 2 ml. of sample. Add exactly 1 ml. of the acetylating reagent, allow to react 10 minutes or longer, and then proceed with 0.5 ml. of the hydrolysis reagent, other reagents as directed. Substances such as sugars which are insoluble in the acetylating reagent may first be dissolved in a minimum amount of water and then treated with the acetylating reagent as described above. DISCUSSION
Choice of Acetylation Conditions.
Because of the difference in mechanism, acid-catalyzed acetylation in pyridine (6) occurs quantitatively with even the most sensitive unhindered alcohol, and hence pyridine was chosen as solvent. The concentration of acetic anhydride was increased from 2M (6) to 5.5M to increase the rather slow rate of reaction. All primary and unhindered secondary alcohols are esterified within 5 minutes. The excess anhydride is sufficient to react with up to 2.5 meq. of most primary and secondary amines or up to 5 meq. of water. An extended acetylation time may be necessary in the presence of large amounts of amines or water. The pyridine in the reagent also catalyzes room temperature hydrolysis of the excess acetic anhydride and provides buffering action during removal of the excess hydroxylamine after hydroxamation. If an aprotic solvent such as ethyl acetate or tributyl phosphate were used instead of pyridine, acid-catalyzed acetylation in this type of solvent (6) would be more rapid but would tend to dehydrate or polymerize acid-sensitive alcohols. Pyridine would still have to be added for hydrolysis. Esters also obviously cannot be used as solvents, although trialkyl phosphates are suitable. High blanks were obtained with many brands of acetic anhydride and pyridine. Of the reagents tested, Eastman acetic anhydride and Fisher pyridine appeared to give the lowest blanks. I t is recommended that the absorbance of the blank us. water be determined prior to using these reagents; the absorbance will generally be about 0.3 but should not exceed 0.5, since this leads to poor precision. After the acetylation, the excess acetic anhydride is hydrolyzed with a pyridine-water solution so that the ratio of water to anhydride is greater than 5 to 1. Such a ratio has been shown (18) to hydrolyze acetic anhydride quantitatively and rapidly but does not result in a water concentration high enough to precipitate esters of high molecular weight alcohols and thus cause incomplete hydroxamation in the next step. Any unhydrolyzed acetic anhydride will react like any ester and cause erroneous results.
Table I contains data on acetylation and hydroxamation of alcohols of varying degrees of steric hindrance. Although tert-butyl alcohol dehydrates and perhaps polymerizes, it is acetylated to a constant 80y0 and can thus be determined by use of a calibration curve prepared under the same conditions. Hydroxamation Conditions. The hydroxamation of esters involves the displacement of the alkoxide anion by a basic hydroxylamine reagent. This rather unstable reagent is generally prepared (7) by neutralizing alcoholic hydroxylamine hydrochloride and filtering off sodium chloride. To avoid this and the unstable reagent, a stock solution of methanolic hydroxylammonium perchlorate is prepared which forms alcohol-soluble sodium perchlorate after neutralization with sodium hydroxide; the stock solution does not deteriorate significantly for 6 months. The rate of hydroxamic acid formation of an ester of a primary alcohol a t pH 7 to 10 is measurably slow and has been shown to be proportional to ester, hydroxylamine, and base coneentrations (9). It is also known that the reaction rate is altered significantly by changing the acid moiety's steric or inductive properties (8). In contrast, the effect on the rate of hydroxamation by changing to a hindered secondary alcohol moiety has apparently not been studied. To investigate this, Pheptanol was treated by the above procedure, but with varying concentrations of base and hydroxylamine. The data are summarized in Tables I1 and 111. The reaction velocity shows a more pronounced change with a change in sodium hydroxide concentration than with hydroxylamine. These results differ somewhat from those obtained a t pH 7 to 10 (9), but the change from a primary alcohol moietv to a hindered secondary alcohol moiety may alter the relative rates of the reaction steps or may alter the mechanism itself. The data in Tables I1 and I11 demonstrate that high concentrations of both sodium hydroxide and hydroxylamine make the procedure applicable to the determination of even hindered alcohols. Development of Color. After hydroxamation, the solution containing excess hydroxylamine and base is acidified and ferric ion is added t o form the purple ferric hydroxamate chelate. Although a large excess of hydroxylamine is necessary for rapid hydroxamation, it reduces Fe(II1) rapidly in acid with a resulting decrease in absorbance ( 1 ) . To avoid fading, nitric acid or hydrogen peroxide (IO)or 0.1M acid solutions (7) have been used. However, the sodium nitrate formed has low alcohol solubility. The addition of hydrogen peroxide seemed to result in destruction rather than stabilization of color, and
Table 1. Acetylation and Hydroxamation Times for 1 OOyo Reaction (0.03-0.4 mmoles of hydroxyl, per 1 ml. acetylating reagent)
HYdroxaAcetylation mation Time, Time, Min. Min. 5 5 5 20 60 10
Alcohol Cyclohexanol 2-Butanol 4-Heptanol 2,6-Dimethyl-4hept,anol 10 90 2,2,4Trimethyl1 3-pentanediol(6' OH) 10 (hr.) 10 tert-Butyl alcohol 20 (hr.)o 180 Acetylates to a constant 80%; apparently simultaneous dehydration, etc.
Table II. Effect of Hydroxide Concentration on Hydroxamation Rate (Hydroxylamine = 0.31 6M; 0.00227mM)
NaOH, M 0.062 0,235
0.407 0.579 0.768 0.958
4-heptanol =
Reaction, % 15 min. 30 min. 60 min. 6 ... ... 27 52 TO
85 92
...
... 89 97 98
...
...
100
...
...
Table 111. Effect of Hydroxylamine Concentration on Hydroxamation Rate (Sodium hydroxide = 0.295M; 4-heptanol = 0.00203mM)]
NHiOH Molarity 0.093 0.185 0.370 0.463
Reaction, 7015 min. 30 min. 21 35 25 40 33 51 36 60
this has been confirmed recently by the work of Meloan and Brandt (IS). Finally, a t the high concentrations of hydroxylamine used, the color continued to fade rapidly even if the final acidity were maintained a t 0.1714or less. The possibility of removing the excess hydroxylamine through oxime formation with an aldehyde or ketone appeared promising, since the alcoholic medium as well as the pyridine and acetic acid formed during hydrolysis provided excellent conditions for oximation (19). Attempts to use formaldehyde failed, but acetone stabilized the colored chelate. A sample treated by the procedure, to which acetone had been added, did not change appreciably in ita absorbance of 0.688 from 12 minutes after color development to 3 hours after. The absorbance of a solution to which \rOL 34, NO. 10, SEPTEMBER 1962
1317
n o acetone was added faded from '0.560 a t 12 minutes to 0.507 at 42 minutes and to 0.411 a t 3 hours. Values for the wavelength of maximum absorbance of the ferric acethydroxamate chelate have been reported, ranging from 490 to 540 mp. This large variation has been attributed to
Table IV. Molar Absorptivities of Hydroxyl Compounds E
Hydroxyl Compound
(average of 2 to 6 detn.)
PRIMARY HYDROXYL 1340 Methanol 1270 Ethanol 1180 1-Propanol 1170 1-Butanol 2-Methyl-1-butanol 1160 1-Decanol 1160 1210 1220 EGge,nebthanol 1210 Benzyl alcohol Ethylene glycol 2350
SECONDARY HYDROXYL 2-Pronanol _..
2-Butanol 4Heptanol 2,6-Dimethyl-4-heptanol c clohexanol dandelic acid Cholesterol
1180 _ ~ _
1170 1150 1140 1190 1200" 1130"
OTHER Sucrose 2,2,4Trimethvl-l,3-~entanediol Methoxyphenol Ascorbic acid 1-Heptanethiol '
'
r-
93100 2 1400
1210 5150" 1130°
Uncorrected in that the purity of the compound waa not checked.
~
the type of solvent and the composition of the chelate ( 1 7 ) . Under the conditions employed, the absorbance peak occurs a t 524 mMl although between 520 and 526 mp the absorbance does not change more than 0.1% from its maximum value. Maximum color development also depends to a large extent on the final acidity of the solution. The optimum final acidity of 0.07 to 0.08M perchloric acid found in this study is in good agreement with the recommended final acidity (7). Deviation from this final acidity range may result in diminished color intensity or in an inconstant molar absorptivity. It has also been suggested (15) that the solvent used affects the concentration of Fe(I1I) required for maximum color intensity. Qualitative tests indicated that incremental addition of water to the predominately nonaqueous solution causes a rapid increase in the concentration of the ferric ion required for maximum color development. As the water content decreases, color intensity increases; it approaches its maximum as the solvent composition approaches 1 0 0 ~ o nonaqueous conditions.
RESULTS
Despite the fact that the same compound, acethydroxamic acid, is formed from the hydroxamation of all the acetate esters, the molar absorptivities of the purified alcohols differed somewhat. The literature reports that molar absorptivities differ somewhat (7) and and also reports that they are the same (16).
Table V.
Determination of Alcohols in Mixtures
Mixture 2-Propanol in 3-isopropoxy-n-propylamine Methanol in N-methvlaniline #
-
Ethanol in water Sucrose in water 1-Butanol in tributyl phosphate Ethanol in chloroform
1318
Amine or Water, Wt. % Alcohol Meq. Added Found
0.66 2.64 4.49
0.84 0.34
0.22
0.84 0.33 0.16
1.58 1.64
0.22 0.082
0.20 0.076
3.64 7.75
1.33 0.30
1.31 0.31
2.85 8.46
0.81 0.081
0.80 0.075
.. .
0.136
0.138
...
0.0042 0.0047 0.52 0.52
ANALYTICAL CHEMISTRY
The majority of the alcohols for which the molar absorptivity was determined were purified by drying and vacuum distilling or subliming. In most cases the purity was then also checked by acetylation (6). The data indicate that the majority of the alcohols have about the same molar absorptivity and thus mixtures of alcohols may be assayed for a total molarity of hydroxyl groups. Sterically hindered secondary alcohols have a somewhat lower molar absorptivity than the more reactive alcohols; methanol, in contrast, has a significantly higher molar absorptivity, as seen in Table IV. These inconsistencies may be explained by postulating a simultaneous saponification which can compete more successfully with hydroxamation, as the alkoxy1group increases in steric bulk. Table V contains data on the determination of alcohols in the presence of more than a hundred fold excess of an amine or water, or in the presence of chloroform or tributyl phosphate. Beer's law is obeyed over the concentration range of 1.0 to 14 X 10-4M as
the hydroxyl group in the final solution to be measured. The spectrophotometric method is applicable to primary and secondary alcohols as well as to tert-butyl alcohol, polyhydroxy1 compounds as well as sugars, mercaptans, and unhindered phenols. Phenols display a color enhancement probably due to the colored ferric phenolate complex; this renders the color less reproducible than the color for a given alcohol. The analysis for phenols is currently under further study to develop the method more fully. The method has also been applied to the determination of hydroxyl groups in various commercial polyglycols. Interferences. Primary and secondary amines which are not sterically hindered consume acetic anhydride preferentially t o form substituted amides which react much more slowly than esters with the alkaline hydroxylamine reagent. Corrections are usually unnecessary for mixtures containing more than 10 meq. yo of alcohol. Should a correction prove necessary, it can be established by running the procedure on the pure amine. Where this is impractical, the absorbance on three separate samples can be determined after 20, 30, and 40 minutes of hydroxamation, and the absorbance resulting from the ester determined by extrapolation of a plot of absorbance us. time to zero time. The results in Table V are so corrected and indicate the limitations therein. The extrapolation should also be valid to correct for a similar slow hydroxamation reaction of nitriles. Water also consumes acetic anhydride but, as shown in Table V, the reaction of aqueous ethanol is quantitative even though more water (7.7 mmole) is present than acetic anhydride (5.5 mmole). To determine alcohols which are poorer nucleophiles than water, an excess of anhydride over water should be maintained. The alcohol content of an alcoholester mixture, in which the concentrations of each are of the same order, should readily be determined by a differential procedure. The sum of ester plus alcohol is first obtained by the regular procedure. The ester content may then be determined by premixing the 0.5 ml. of hydrolysis reagent with 1.0 ml. of the acetylating reagent before adding the sample. The alcohol is determined by difference. The fact that carbamic acid esters do not form hydroxamic acids ( 5 ) also makes possible the determination of hydroxyl groups in certain urethane prepolymer mixtures. Aldehydes and ketones a t the same concentration as hydroxyl groups will interfere' probably because of acety-
lation of their enolate forms. Fivemembered lactones react like esters ( 7 ) ; lactones which undergo ring openings may not interfere provided they are not acetylated. Mechanism of Hydroxamation. The rate of hydroxamation of hindered acetates such as 4-heptyl acetate (Tables I1 and 111) is enhanced more by a n increase in hydroxide concentration than by a similar increase in hydroxylamine concentration. To account for this, one may postulate mechanisms similar to mechanism A proposed by Bruice and Bruno (3) for aqueous hydroxamation of lactones : NHzOH
+ OH-;=” 0
CH,COR
i“
HZSO- H--NOH u H
+ Hz0
“20-
(1)
0
#I
+
CH,CSHOH SH20- (2) ROH
++
Khere hydroxylamine is present a t higher concentrations than sodium hydroxide (Table 111), most of the hydroxide is consumed by reaction 1 in the nearly nonaqueous medium ( 3 7 , water), and mechanism 2 predominates. This is probably the case for standard hydrovamation methods ( 7 , 8) where heat is used to increase the rate of the reaction, which is probably kinetically fourth order over-all and seconil order in hydrouylamine (3). In the procedure, h y d r o d e is present
in such substantial excess over hydrosylamine that the hydroxide consumed in reaction 1 becomes less important, yet the rate increases with increasing hydroxide concentration. I n this case, the more rapid mechanism 3 may compete with mechanism 2 and probably predominates :
LITERATURE CITED
(1) Aksnes, G., Acta Chem. Scand. 11,
710 (1957). (2) Banick, W.M., . ~ N A L . CHEM.34, 296 11962’1. (3j Bruice, T. C.,Bruno, J. J., J. Am. Cheni. SOC.83, 3494 (1961). (4) Critchfield, F. E.,Hutchinson, J. A., ANAL. CHEY.32, 863 (1960). (5) . , Davidson. D.. J . Chem. Ed. 17, 81 (1940). (6) Fritz, J. S., Schenk, G. H., ANAL. CHEM.31, 1808 (1959). ( 7 ) Goddu, R. F., LeBlanc, N. F., Wright, C.M.,Ibid., 27, 1251 (1955). (8) Goldenberg, V., Spoerri, P. E., Ibid., 30, 1327 (1958). (9) Hestrin. S..J . Biol. Chem. 180, 249 ( 1949). (10)Hill, U., Ind. Eng. Chein., 9naI. Ed. 18, 317 (1946). (11)Johnson, D. P.,Critchfield, F. E., AXAL. CHEW32, 865 (1960). 112) hleloan. C. D..Brandt. W.W., Zbzd., 33, 1’ (13’1hl I
CH,&oR ----+
i*
HO- H--NOH u H
I1 CH,CSH~H R O H + O H - (3)
+
k t very high hydroxide concentrations, this third order reaction n ill become pseudo second order (only first and is also thus more order in “,OH) rapid than the fourth order reaction in mechanism 2. This is then the actual nucleophilic attack of -”OH on the carbonyl group. In the latter case or in mechanism 3, it should be understood that equilibrium forms of -“OH, XH20H,or the hydroxamic acid may be involved. ACKNOWLEDGMENT
The authors express their appreciation to Sorman A. LeBel for helpful discussion and to the Kyandotte Chemical Corporation for permission to publish a portion of their n-ork and for use of their research facilities for part of the research.
,
‘ (ig58). (16) Pilz, W.,Ibid., 166, 189 (1959). (17)Reuther, K. H., Bayer, E., Ber. 89, 2.541 - _- (1956). (18)5Schenk, G. H., AYAL. CHEM.33, 299 (1961). (19)Siggia, S., “Quantitative Organic Analysis via Functional Groups,” p, 28. Wilev. New York. 1954. (2O)’Tanakk,M.,Talanta 5, 162 (1960). RECEIVEDfor review April 27, 1962. Accepted July 16, 1962. JVork sup orted by Public Health Research Grant RE-7760 from the National Institutes of Health,
Public Health Service.
ChemicaI Qua Iitative a nd Qua nt it a t ive An a lysis of Some Epoxy Coating Materials M.
H. SWANN
and M. L. ADAMS
Coating and Chemical laboratory, Aberdeen Proving Ground, Md.
b Two related methods of analysis for epoxy resins of the bisphenolepichlorohydrin type are described that allow fast, dependable detection of epoxy resin in painted surfaces and a means of estimating the epoxy portion of most modifications including amine-, polyamide-, and phenol-catalyzed enamels, fatty acid esters, and simple mixtures.
E
of the type formed by the condensation of epichlorohydrin with bisphenol (p,p’-isopropylidenediphenol) may be used in coatings in three important ways: as epoxy resin esters or styrenated epoxy esters; in combination with other resins such as urea, melamine, or phenolics; and in combination with eo-reactants or POXY RESINS
curing agents such as polyamine or polysulfide resins. In the first case, the fatty acid esters, the epoxy content usually ranges around 50%; in the second, it may vary from very low to very high quantities; in the third case, the concentration usually runs from iOYGto loo%, either in the finished coating or as one part of a two-part system. Lesser amounts of other materials such as flow control agents-Le., silicone resin or ethyl cellulose-may be present but do not usually exceed 1% and are not an analytical problem; neither are such quantities sufficient to interfere seriously with other analyses. Sornially in analyzing polymeric materials, it is easier and more accurate to determine the quantities of the minor constituents present in the system, but in the great
variety of coating materials that may be encountered which are based on epoxy resins, all of the additives may not be identifiable and, in addition, analytical procedures may not be available. An analytical method for measuring the epoxy resin in various coating systems is particulsrly useful for specification control purposes. The two tests described here for qualitative and quantitative determination of epoxy resins in coatings are based on reaction of the resins with fuming nitric acid. Khen the reaction product is diluted with water, a filterable yield of 165% is obtained with epoxy resins of all molecular weights that allows gravimetric measure of the epoxy portion of coating systems. The rapid qualitative technique is based on the red to violet color obtained by VOL. 34, NO. 10, SEPTEMBER 1962
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