Some e t the papers
Analysis via Functional Groups
prenrmted in the Symposiom on Analynis VIA Fonetionul Gronpn, Division of Annlyficnl Chemistry, 13Dth Meeting, Amerienn Chemical Society, St. Lonis, Me., Mnreh 1081. Other papers from thie symposiom will b e poblished in later issnes.
Determination of Alkoxy1 Groups by Nonaqueous Titration ROBERT H. CUNDIFF and P. C. MARKUNAS R. J . Reynolds Tobacco Co., Winston-Salem, N . C.
b A procedure was developed for the determination of alkoxyl groups, whereby the alkyl iodide i s absorbed in pyridine and titrated directly with tetrabutylammonium hydroxide. Since hydriodic acid, iodine, and sulfides do not interfere in the titration, which may b e followed either visually or potentiometrically, the scrubbing reagents can b e eliminated. This permits considerable simplification of the apparatus. Except for requiring a longer reaction time, the method can be: applied as readily to the determination of ethoxyl, propoxyl, butoxyl, and S-methyl as to methoxyl compounds.
T
methods for alkosyl group determination are basrd on the Zeisel (18) procedure, in which the alkoxy1 group reacts with hydriodic acid to form the alkyl iodide. The iodide is swept from the reaction mixture with carbon dioxide or nitrogen. An exeellent survey of the procedure is given by Elek @), who includes the pertinent references through 1951. Most means for determination of the liberatrd alkyl iodide are linked to the volumetric method of Viebock and Brecher ( I I ) , or the gravimetric procedure of Pregl (?’). In the former, the alkyl iodide is collected in a solution of bromine, sodium acetate, and acetic acid to form first iodine monobromide, then iodic acid. The iodine liberated from the iodate on addition of potassium iodide is titrated with standnrd sodium thiosulfate. I n the gravimetric version, the alkyl iodide is absorbed in silver nitrate, and the silver iodide formed HE
1028
ANALYTICAL CHEMISTRY
after acidifiration is weighed. In the volunirtric rnrthod of Kirpal and Buhn (6) the alkyl iodide is absorbed in pyridine, an aqilrous solriti’on of the resulting compound is mixed with potassium chromate, and the iodine liberated is titrahed with sodium thiosulfate. More recmt innovations include separation and determination by gas-liquid chromatography ( 6 ) , a new combustion method (4) in which the alkyl iodide is burned with platinum in an air stream to form iodine and carbon dioside, and a spectrophotometric technique (IO) in which the alkyl iodide is absorbed in pyridine and the pyridinium alkyl iodide measured at 366 mp. All these procedures require a scrubber to be incorporated into the apparatus to remove interfering hydriodic acid, iodine, sulfides, or a combination of the three from the alkyl iodide. At least 15 different scrubbing agents, solid and liquid, have been proposed for removal of these impurities. Most previous procedurcs recommend dissolving the sample in phenol and acetic or propionic anhydride, prior to adding the hydriodic arid. The concentration and purity of the hydriodic acid used in the reaction are a critical part of the procedures. In a study in these laboratories, as yet unpublished, it was observed that a number of alkyl halides, including the lower aliphatic alkyl iodides, stoichiometrically react with pyridine to form the corresponding alkyl pyridinium halides, which behave as very weak acids in this nonaqueous medium. This, plus the studies conducted on differentiation of acid mixtures (8,
indicated that t h c alkyl iodide could be swept directly into pyridine, and the resulting pyridinium alkyl iodide determinrd in the presence of any hydriodic acid carried along. There should be no interference from iodine or sulfides. Thus, the apparatus could be simplified by elimination of the scrubber, and the means of determining the alkyl iodides considerably shortened by a direct titration. PRINCIPLE OF METHOD
The alkyl iodide formed on reaction of the alkoxyl compound with hydriodic acid reacts with pyridine according to the following equation : R-I
+ CbHsN
-.c
[CrHsN-It] +I-
The alkyl pyridinium iodide thus formed and the hydriodic acid carried along are resolved by a differentiating titration with tetrabutylammonium hydroxide. A typical potentiometric curve, obtained from titration of a hydriodic acid-n-butyl iodide mixture, is shown in Figure 1. The first inflection represents neutralization of the hydriodic acid; the second represents neutralization of the iodide. The volume difference between the two end points is a measure of the alkosyl content. The alkyl iodides do not behave as acids in neutral solvents such &s acetone, methyl isobutyl ketone, or acetonitrile. The first equivalence point in the titration corresponded to the visual change of azo violet indicator from orange to red, and thc sccond equivalence point corresponded to the
the surface of the solvent. I’ass nitrogen through the system, adjusting the initial rate to one bubble pcr second. Apply heat with a heating mantle, and adjust the heat so that the, top of the condensate is just above bulb A . Continue this nitrogen rate for 20 minutes, then increase the rate to 2 to 3 bubbles per second for the remainder of the reaction period. Allow a minimum amount of condensate to pass into the receiver, and if fuming is observed in the receiver, lower the nitrogen rate and adjust the heat so that fuming is minimized. Continue the reaction a n additional 25 minutes for methoxyl determination, an additional 40 minutes for ethoxyl determination, an additional 100 minutes for propox 1 and butoxyl determination, and a n aJditional 160 minutes for Smethyl determination. Disconnect the Erlenmeyer flask and rinse the esit
t 900
+ 800
+700
+*oo
ul
3
Z+¶OO
-a +400
+so0
4200
Table I, Determination of Alkoxyl Content in Organic Compounds +IO0
I
I O Y L
I
Figure 1. Titration of acid-n-butyl iodide mixture
hydriodic
Lisual change from red t o violet. Thus the titration may be performed visually using a single indicator. PROCEDURE
.4lkoxyl Apparatus. A d i a g r a m of this apparatus ia shown in Figure 2. Any conventional alkoxyl apparatus may also b e used, although t h e scrubber is superfluous. Tetrabutylammonium h droxide, 0.02~. Pre are 0 . 1 ~tetratutylammonium hy&oxide as described (1). Add 20 ml. of methanol to 200 ml. of this solution and dilute t o 1 liter with benzene. P ridine. Flashdistill technical gradie pyridine from barium oxide using an upright condenser, discarding the first and last 10% of the distillate. Hydriodic Acid, Merck & Co., Inc., reagent grade, 55 to 58% HI, specific gravity 1.7. No additional purification is necessary. Azo Violet Indicator Solution. Dissolve 0.5 gram of p-nitrobenzeneazoresorcinol in 100 ml. of pyridine. Xylene, analytical reagent grade. Method. Accurately weigh 10 t o 15 mg. of the solid alkoxyl compound and transfer t o t h e reaction flask. Weigh volatile samples in gelatin capsules. Add 0.5 ml. of xylene t o t h e flask a n d dissolve t h e solid samples, heating if necessary. Add 5.0 ml. of hydriodic acid and a few boiling stones. Lightly grease t h e standard-taper joints and connect the flask t o the apparatus. Place 50 ml. of pyridine in an Erlenmcyer flask and allow the delivery tip to &end below Apparatus and Reagents.
Compound CHsO3-Methoxy-4-hydroxybenzaldehyde p,p’-Dimet hoxybenzophenone m-Methoxyphenol p M e t hoxyphenol 4-Met hoxy-2-nitroaniline 4’-Methoxy-Z( pmethaxyphenyl) acetophenone 1-(pMethoxypheny1)-1propvl palmitate Methyl aminobenzoate 1-Naphtxil methyl ether %Naphthyl methyl ether pMethylanisole m-Methox anisole 2,4-Dimetiylanisole 3,CDimethylanisole 3,bDimethylanisole 1,3-Dimethoxy-5-methylbenzene 1,2-Dimethoxy-4-allylbenzene Methocel, Dow CsHbO3-Ethox Chydroxybenaa&ehy de p-Diethoxybenxene p-Et hoxyacetanilide ZEthoxynaphthalene EEthoxybenzoic acid thy1 p-aminobenaoate Ethocel, Dow CIH~On-Propylcellulose, Dow
% Alkoxy1 ~Theory Found
20.40 20.47 25.64 25.00 25.00 18.46
25.58 25.24 25.37 18.40
24.22 24.39 7.67 20.53 19.62 19.62 25.40 44.92 22.79 22.79 22.79
7.75 20.74 20.00 20.03 25.98 44.90 22.80 23.11 22.95
40.78 40.41 34.63 35.07 30.0“ 30.06
27.12 54.22 25.14 26.16 27.12 27.28 46.3“
27.07 54.61 25.11 25.97 27.63 27.47 46.04
51 00 50.77
CdHoODibu toxybenzene 65.77 66.10 utyl paminobenzoate 37.83 37.99
b-
CHaSMethionine 31.57 31 49 a Standard rellulose ether samples suplied by The Dow Chemical Co. and anayzed by Samsel and McHnrd method
P
(8).
Figure 2.
Alkoxyl apparatus
tube with pyridine, adding the washings to the receiving flask. Gently boil the pyridine solution 2 minutes, cool, then titrate. The titration may be performed potentiometrically or as follows: Add 2 drops of azo violet indicator solution to the pyridine solution and titrate under nitrogen to a red end point, record the volume, then titrate to a violet end point. The volume difference between the red and violet end points is a measure of the alkoxyl content. Perform blank analyses with each change of reagents, and subtract the volume difference between the red and violet end points from that obtained in the sample analysis. EXPERIMENTAL
Alkoxyl Compounds Analyzed. A series of alkoxyl compounds was analyzed by t h e described procedure. If t h e compound tested was less than 98% pure, i t was recrystallized or chromatographed until t h e purity was greater than 98%. Solid samples were weighed directly into t h e reaction flask; liquid samples were weighed in gelatin capsules. Results of these analyses (Table I ) are the average of a minimum of two determinat’ms. Attc qted Resolution of MethoxylEthox) J - ‘xtures. It is necessary t o boil p>-i r -elutions of ethyl, propyl, and but I t o obtain stoichiom r t i i r co I 1 7 the corresponding ’0
VOL. 33,
I
F, .JLY
1961
1029
alkyl pyridinium iodide. Methyl iodide reacts quantitatively at room temperature. This indicated t h a t i t might be possible to resolve methoxyl and ethoxyl mixtures. I n this investigation, the receiving Basks wcrc chilled during the reaction. One pyridinc solution was titrated while still a t a low temperature; a second solution was boiled, cooled, then titrated. The first titration indicated the methoxyl content; the second, the total alkoxyl content, and the ethoxyl content was obtained by difference. Imprecise results were noted in all variations of the technique tried. The temperature of the receiving solution is the critical factor. If the temperature is held too low, reaction of the methyl iodide is incomplete; if held too high, small amounts of ethyl iodide react. It seems entirely possible t h a t sufficiently sensitive conditions could be established so t h a t precise quantitative results could be obtained. Evcn so, qualitative distinction between methoxyl and higher alkoxyl groups can easily be realized by this means. DISCUSSION
T h a t iodine and sulfides do not intcrfere was established by adding iodine crystals to one reaction misture, iodine and ferrous sulfide to a second reaction mixture. The blank value from each of these reactions was the same as t h a t obtained with these additives absent. This was tested further by adding ferrous sulfide and iodine to a reaction mixture in the analysis of methylcellulose. The result for methoxy1 content was identical to that obtained with the interferents absent and corresponded to the theoretical value. Phenol and acetic anhydride or
propionic anhydride were tried in conjunction with hydriodic acid in the hydrolysis of the alkoxyl group. Phenol could not be used, as a n y phenol entering the receiver titrated simultaneously with the alkyl pyridinium iodide. The anhydrides could be used without the phenol, as the acid formed and carried into the receiver titrated separately from both the hydriodic acid and the iodide. However, no particular increase in efficiency was noted when the anhydrides were used. Xylene not only dissolves the test compound, but also boils at a sufficiently high temperature to aid in carrying the alkyl iodide t o the receiver. The presmce of xylene in the mixture also minimizes the fuming of hydriodic acid. It was not necessary to purify further the hydriodic acid as obtained commercially or to use special storage precautions. The more dilute commercial hydriodic acid solutions, with and without preservative, can also be used, although the reaction time must be increased. The procedure was not evaluated with respect to the highly volatile compounds such as diethyl ether, although it is believed that if they are weighed in gelatin capsules and xylene is used in the reaction mixture, additional precautionary steps are unnecessary. If this should prove unsuitable, substitution of a similar apparatus with a water condenser, for use during the hydrolysis, should ensure quantitative results with this type of compound. One distinct advantage of this procedure is that only 2 hours are required for determination of the propoxyl and butoxgl groups. The efficient pro-
cedure of Shaw @), for example, requires a minimum of 3 hours for quantitative conversion of the butoxyl group; whereas other procedures require longer reaction periods and even more extensive modification of the apparatus. ACKNOWLEDGMENT
The authors thank The Dow Chemical Co. for supplying the cellulose ether samples and the analyses on these samples. They also thank A, J. Sensabaugh for preparation of t h e figures, and Norman VanHoy for technical assistance. LITERATURE CITED
(1) C u n d 8 R. H., Markunas, P. C., ANAL. &EM. 30, 1450 (1958). (2) Cundiff, R. H., Markunas, P. C., Anal. Chtm. Acta 20,506 (1959). (3) Elek, A., “Organic Analysis,” Vol. I, pp. 67-125, Interscience, New York,
1953. (4) Fukuda, M., Milcrochim. Acta 1960, 448. (5) Ki al, A., Buhn, T., Monatsh. 36,853 (1918 (6) Kratk, K.,Gruber, K.,Ibid., 89, 618 (1958). ( 7 ) Pregl, F. Grant, J. “Quantitative Organic Ivficroanalysis,’” 5th English ed., pp. 182-94, Blakiston, Philadelphia, 1951.
(S,-Samsel, E. P., McHard, J. A,, IND. ENO.CHEM., ANAL. ED.14, 750 (1942). (9) Shaw, B. M., J. SOC.Chem. Znd. 66, 147 (1947). (10) Sobue, H., Halano, A., Tosliaki, A. J. SOC. Textile Cellulose Ind. ( J a p a n ) 1 5 . 21 (1959). (1:i, Viebock-’F., Brecher, G., Ber. 63B, 3207 (19301. (1:2) Zeisel, S., Monatsh. 6,989 (1885). I
~~
\ -
RECEIVEDfor review January 11, 1961. Accepted March 20, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
Rapid Determination of Organic Hydroxyl Groups with 3,5-Dinitrobenzoyl Chloride W. T. ROBINSON, Jr., R. H. CUNDIFF, and P. C. MARKUNAS R . 1. Reynolds Tobacco Co., Winsfon-Salem, b 3,5-Dinitrobenzoyl chloride, commonly used for the identification of alcohols, reacts rapidly and quantitatively with organic hydroxyl groups in pyridine. This reaction, followed b y a visual or potentiometric titration of the reaction products, provides another simple procedure for the determinotion of primary and secondary alcohols. Polyols, sugars, phenols, primary and secondary amines, and some oximes may also b e determined. By employing a longer reaction period, a
1030
ANALYTICAL CHEMISTRY
N. C.
much wider range of tertiary alcohols may b e determined than b y existing procedures. Ketones do not interfere nor do aldehydes unless they exceed 40% of the alcohol being determined.
M
(8) has reviewed tlic literature through 1952 on the determination of hydroxyl groups by the most commonly used methods. Among the methods discussed are: acetylation, phthalization, formylation, EHLENBACHER
bromination, periodate osidation, coupling, and determination of active hydrogen. The major objections t o t h e acetylation and phthalization procedures are the length of time necessary for quantitative reaction, interference from aldehydes and ketones, and their applicability only to primary a n d secondary hydroxyl groups. Recently, Fritz and Schenk (7) reported a highly improved procedure for the determination of organic hydroxyl groups. The method is rapid
