Microdetermination of Methyl Groups Attached to Carbon VAZKEN H. TASHlNlAN and MARY JANE BAKER Department of Chemistry and Chemical Engineering, University of California, Berkeley, Calif.
CHARLES W. KOCH Department of Chemistry and Radiation Laboratory, University of California, Berkeley, Calif.
This study was undertaken as a first step toward a better understanding of the dichromate oxidation of aliphatic compounds containing methyl groups. Samples were digested in sealed tubes in a rocking furnace. A jacketed distillation apparatus, used in the acetic acid distillation step, eliminated carry-over of sulfuric acid spray and reduced the distillation time when the jacketing substance boiled in the range 110" to 118" C. Acetic acid is produced quantitatively from several low boiling aliphatic compounds, but not from terminal methyl groups attached to aliphatic amines of low molecular weight. Three complex substances were investigated, employing the Kuhn-Roth oxidation mixture and the increased amount of sulfuric acid recommended by Ginger. The results do not show one reaction mixture to be clearly superior, but they illustrate that the conditions of this determination cannot be as rigorous as has been previously employed.
T
Because of the uncertainty associated with volatilization from the reaction mixture of organic compounds of l o r molecular weight during the refluxing step, all samples in the present work were digested in sealed tubes. Also, to ensure continuous mixing of a sparingly soluble solid, an immiscible liquid, or a gaseous organic compound with the sulfuric acid-chromic acid solution, digestions were carried out in a rocking furnace.
HE Kuhn-Roth method (3) for the determination of terminal methyl groups is based upon the oxidation of hydrocarbon
/
chains, CHI-C-,
with a chromic acid-sulfuric acid mixture.
\
Under the conditions employed, the oxidation of acetic acid by the reaction mixture is relatively slow as compared to the oxidation of the sample to acetic acid. Ginger ( 9 ) reported that the amount of sulfuric acid employed in the Kuhn-Roth procedure was not sufficient to dissolve fatty acids. He found that increasing the amount of sulfuric acid from 1 ml. to 2 ml. resulted in complete solubility of the fatty acid and a marked increase in the rate of digestion. The higher results obtained by Ginger suggest that a re-examination of the procedure and apparatus might lead to further improvements in this functional group determination, provided the rate of oxidation of acetic acid is not markedly increased. Campbell and Morton ( 1 ) showed that they could not obtain quantitative yields of acetic acid when gem-dimethyl or tert-butyl groups were present in the compound studied. They also stated that lo^ results could not be attributed to the formation of acetone in the oxidation step, because samples of acetone and mesityl oxide each produced approximately three fourths of the theoretically expected amount of acetic acid. Although Campbell and Morton demonstrated that the major portion of each of these samples r a s oxidized to acetic acid, the missing one fourth in each instance is unexplained. For acetone, the loss must be due either to volatility or to stability of the molecule itself. I n the case of mesityl oxide the problem is complicated further by the presence of a (CH,)z-C
/
group, for n-hich Campbell and Morton obtained
\
recoveries of the order of 75 to 85%. Hon-ever, loss of sample by volatility should have been reduced by use of this higher boiling substance. The fact that the same relative amount of acetic acid is produced in the oxidation of mesityl oxide as of acetone tends to support the argument for the stability of some nonacidic fragment in the reaction mixture.
SECTION
A-A
Figure 1. Rocking furnace for digestion of samples
The conventional apparatus of Kuhn and Roth for the distillation of acetic acid was not employed, for three reasons. First, too frequently, the distillate contains appreciable amounts of sulfate ion. Secondly, it is desirable to shorten the distillation time, and this is accomplished by jacketing the distilling flask a t a temperature that approximates the boiling point of acetic acid. Lastly, the nuisance of collecting 5-ml. portions of distillate as well as the cumulative errors resulting from their successive titrations is avoided. APPARATUS
Rocking Furnace. The furnace shown in Figure 1 is-constructed so that it rocks through an angle of 45". It is driven by a Bodine Electric Go. '/e-hp., Type NSI-55RH motor which revolves a t 38 revolutions per minute. The furnace is constructed from a 12-inch length of 4-inch diameter Dural. Four 10-inch holes, each l l / s inches in diameter, are bored in the block. The cover is drilled so that each compartment has an outlet to the air, and a baffle is placed between the cover and the handle to protect the operator, should one of the tubes break. A '/(-inch pin on the face of the furnace, which fits into a hole in the cover, centers each vent over a compartment. The opposite end of the Dural block is drilled to contain a Fenwal thermosnritch which can be controlled over the temperature range 110" to 205" C. within &0.5" C. Two rings made from 0.5-inch Transite are used to center the Dural block in an outer steel tube. The base rjng is countersunk as shown in the figure. The two rings are slipped onto the Dural block and in each case held in place by means of three countersunk screws.
1304
1305
V O L U M E 28, NO. 8, A U G U S T 1 9 5 6 The walls of the Dural block are covered vith asbestos paper and wrapped with 30 feet of 22-gage Chromel wire. The wire is fastened sccurely to two l/c-inch Transite pins mounted in the side of the Dural block, as shown in the figure. The Chromel wire then is covered with asbestos rope and the unit is inserted into the steel tube, which is 43/r inches in outside diameter and has a l/le-inch wall thickness. The steel tube is fastened to each Transite ring n i t h screws. The furnace is attached to a rocking arm and mounted on a stand by means of a bearing as shown in Figure 1, A-A. Sealed Tube Container. The sealed tube container shown in Figure 2 is made of steel tubing 1 inch in diameter, having a 3/a~-inch wall thickness, and is approximately 10 inches long. Its length is adjusted to fit into the furnace compartment, so that it ail1 not slide when the furnace is rocking. The cap contains a baffle, as indicated in Figure 1, as a protection to the operator if a sealed tube should break. Coils of wire are inserted before and after the sealed tube to prevent breakage while the furnace is rocking and also to allow variation in the length of the tube. The sealed tubes are made from borosilicate glass tubing 18 mm. in outer diameter and may vary from 6 to 9 inches in length. Distillation Apparatus. The distillation apparatus is shown in Figure 3. Its use coinpletely eliminates carry-over of sulfuric acid spray during distillation and decreases the time necessary to obtain quantitative distillation of acetic acid. This apparatus permits jacketing the distilling flask a t a temperature above that of the entering steam, thus increasing the rate of distillation of acetic acid. Figure 2. Steel casing PROCEDURE and cover containing The oxidation mixtures all contained 4 ml. sealed tube of 5N chromic acid. The hydrogen ion conto be placed centration was varied, however, and, in each in rocking set of experiments, the amount of acid emfurnace ployed is specified, together with the results. A sample sufficiently large to yield 0.05 to 0.08 mmoleof acetic acid was emaloved for these d e terminations. K h e n the ;ample to be analyzed was a solid, it was weighed into the borosilicate glass tube, the oxidation mixture added, and the tube sealed. When the sample was a liquid having a low partial pressure, so that it could be handled in air, it was weighed into a boat. The boat and sample were transferred to the oxidation mixture in the borosilicate glass tube, and the tube was sealed. If the sample was a liquid with a high vapor pressure, it was weighed in a capillary tube, the tube was dropped into the reaction mixture in the borosilicate tube, and the tube was sealed. If the boiling point of the sample was greater than 100" C., i t was expelled from the capillary prior to the digestion period. The sealed tube was carefully tilted to leave the capillary suspended on the wall of the vessel free of the oxidation mixture and the tube was warmed gently in the immediate vicinity of the sample until the liquid was expelled. The tube was righted rapidly to draw some of the oxidation mixture into the capillary. The sealed tube was then placed in the steel container shown in Figure 2 which, in turn, was inserted into the rocking furnace a t the desired temperature and the sample was heated for the required period of time. Prior to opening the sealed tube, the tip was heated to drive the liquid from it and the contents were cooled to room temperature. Finally the tube was opened, using an oxygen torch. The contents of the tube were transferred into the distilling flask (the inner flask of Figure 3) with the aid of three small dis. tilled water rinses. A small quantity of lubricant was applied to the ground-glass area. The outer flask shown in Figure 3 was detached from the apparatus, and the distilling flask n-as connected to the 24/40 joint and fixed in position by means of a spring. The outer flask containing the jacketing substance was attached to the apparatus. The complete assembly was clamped in position and the outer flask heated by means of a Glascol mantle (size for a 250-ml. boiling flask). When the jacketing substance was heated to boiling, steam generated from a boiling flask was introduced into the distilling flask through the side arm by attachment to the 12/9 ball joint. The distillate was collected in a 125-ml. Erlenmeyer flask until the acetic acid was quantitatively transferred (the volume required for quantitative transfer is shown in a subsequent section). The contents of each
Erlenmeyer flask were boiled to expel carbon dioxide, stoppered, and cooled in an ice bath. The acetic acid wv3s titrated with standard 0.01M sodium hydroxide. RESCLTS
Titration of Acetic Acid. The pH a t the stoichiometric end point in the titration of 0.002M acetic acid with 0.01-1.1 sodium hydroxide is approximately 8. As the titration error a t the phenolphthalein end point ( p H = 8.5 to 8.7) is about O.5a/o0,phenol red is a more suitable indicator. Bromothymol blue produces a smaller titration error than does phenolphthalein, if the change in color from green to blue is selected as the end point. In this study bromothymol blue vias used. Distillation of Acetic Acid. The volume of distillate required to transfer the acetic acid quantitatively from the reaction mixture to the receiver was determined by jacketing the distilling flask at the boiling point of three compounds: water, toluene (boiling point 110.5' C.), and n-butyl alcohol (boiling point 117.7" C.). The rate of distillation of acetic acid at the three jacket temperatures is shown in Figure 4. When the distilling flask was jacketed with n-butyl alcohol, 50 ml. of distillate were required to transfer 0.05 mmole of acetic acid quantitatively, while a toluene-jacketed apparatus required 60 ml. of distillate. 4 water-jacketed apparatus was not satisfactory, because the jacket temperature was of the order of 5" C. less than the boiling point of the reaction mixture. As a consequence, the volume increase of the reaction mixture during distillation seriously retarded the rate of transfer of the acetic acid, as shown by the figure. The apparatus was not jacketed Kith a substance boiling higher than n-butyl alcohol because t,he temperature differential between the jacket and the reaction mixture (boiling point approximately 105' C.) is sufficiently large to make the initial rate of distillation very rapid, and bumping may result. However, even when nbutyl alcohol was used to jacket the apparatus, no samples were lost by carry-over of sulfuric acid in the course of the study. Stability of Acetic Acid. The AF2pp K. per mole of acetic acid for the reaction: 4H+
+ 4HS04- + CHICOOH = 4SO*(g) + 2C02(g)+ 6Hz0
is -1 kcal. Consequently, it is possible that decomposition of acetic acid by sulfuric acid would be appreciable under conditions employed in the determination of terminal methyl groups. However, n-hen 0.05 mmole of acetic acid is heated in 14M sulfuric acid a t 180' and 120" C. in sealed tubes, the acetic acid is recovered quantitatively a t each temperature after a &hour digestion period,
'd 1 9 / 2 2 JOINT
4
1 2 / 9 BALLJOINT
-%I
B 5
.1
€.Of50 J O I N T 2 4 / 4 0 JOINT
Figure 3.
E
7 7
N
Jacketed apparatus for distillation of acetie acid
,
1306
ANALYTICAL CHEMISTRY
For the reaction: 32H+
+ 8HCr04-
+
3CH3COOH = 8Cr+++ 6COz(g)
+ 26H20
AF2g80 K. per mole of acetic acid is -2i9 kcal. Even with this very large value for the free energy favoring the oxidation, the rate of loss of acetic acid during digestion was slow. After a 4 hour digestion period a t 120' C. in O.1M dichromate, 96% of the acetic acid was recovered from a solution \T hich was 7.2M in sulfuric acid, 23% from a solution which was 10.8M in sulfuric acid, and 3% from a solution which was 14.431 in sulfuric acid. Throughout all of the distillations the dichromate in the reaction mixture was not reduced with hydrazine, because it was found that during the distillation the oxidation of acetic acid was negligible. Because the sulfuric acid concentration of the reaction mixture in the first of the above three experiments approaches that employed by Ginger ( 8 ) , the stability of acetic acid was studied using the quantity of acid employed by Kuhn and Roth and by Ginger. The results are shown a t three temperatures in Figure 5 . Figure 5 (left) shows that a t 150" C. the rate of oxidation of acetic
0 0
acid is rapid when the sulfuric acid concentration in the reaction mixture is 3.6M (1 ml. of concentrated sulfuric acid) and 6.OM (2 ml. of concentrated sulfuric acid). When 6 M sodium hydrogen sulfate is used as the source of hydrogen ion, however, the rate of oxidation of acetic acid is reduced sufficiently so that it may be used in place of sulfuric acid, if the oxidation of the organic compound to acetic acid in the bisulfate solution is sufficiently rapid.
Table I.
Acetic Acid Recovered from Oxidation of Simple Aliphatic Compounds
(Reaction mixture contains 4 ml. of 5 N chromic acid and 1 ml. of concentrated sulfuric acid. Digestion temp., 120' C.) HOAc, Digestion Moles/Mole of Compound Time, Sample Hours Found Present 0.25 0.98 1 Acetone 0.5 4
0.99 0.99
0.5 1
2
1.95 1.98 2.00
Isovaleric acid
1 2 4
0.96 0.99 0.98
tert-dmyl alcohol
1 2 4
1.92 1.96 1.96
Trimethylacetic acid
2 4
0.92 0.96
2
0.98 0.99
Mesityl oxide
WATER TOLUENE
tert-Butyl alcohol
4
0
2
Figure 5 (center) shows the rate of oxidation of acetic acid a t the temperature usually prescribed for sealed tube digestions, 120" C., when the sulfuric acid concentration is 3.6M and 6M and when 6M sodium hydrogen sulfate is used as the hydrogen ion source. The oxidation is sufficiently slow a t this temperature to avoid serious loss of acetic acid with digestion periods as long as 8 hours. Figure 5 (right) illustrates the rate of oxidation a t 100" C. for the two reaction mixtures. Figure 5 also shows the rate of oxidation of benzoic acid in 4 X chromic acid and 3.6M sulfuric acid, a t the three temperatures studied. The comparatively rapid oxidation of benzoic acid permits the determination of terminal methyl groups in the presence of benzoyl or benzoate groups, if the digestion time is extended to a period considerably longer than usually recommended for the terminal methyl determination.
20 40 60 80 100 VOLUME DISTILLED (ML.)
Figure 4. Effect of jacket temperature on rate of distillation of 0.05 mmole of acetic acid From solution containing 4 ml. of 5 N chromic acid, 1 ml. of concentrated sulfuric acid, and 5 ml. of water
0 7.5 G. OF N O H S 0 4 . H 2 0 0 1.0 M L . CONCENTRATED H
0 1.0 M L . CONCENTRATED H2S0, 0 2.0 M L . C O N C E N T R A T E D H 2 S 0 4
L i l & . A 1 8
0 0 T I M E OF D I G E S T I O N
(HOURS)
Figure 5.
4
8
12
16
2I 0
24
4 8 12 16 20 24 TIME OF DIGESTION (HOURS)
T I M E OF D I G E S T I O N
(HOURS)
Rate of decomposition of 0.06 mmole of acetic acid at 150' C.
0 , Rate of decomposition of 0.06 mmole of benzoic acid in reaction mixture containing 1.0 ml. of concentrated sulfuric acid and 4 ml. of 5h chromic acid
1307
V O L U M E 2 8 , NO. 8, A U G U S T 1 9 5 6
1
0 1.0 M L . C O N C E N T R A T E D H 2 S 0 4 0 2.0 M L . C O N C E N T R A T E D H 2 S 0 4
I 1
0 1.0 M L . C O N C E N T R A T E D H 2 S 0 4 2.0 M L . C O N C E N T R A T E D H2S04
~~
0
I
3 8 12 16 20
0 TIME
OF DIGESTION. ( H O U R S ) Figure 6.
DIGESTION
12
16
OF D I G E S T I O N
20 (HOURS)
Rate of formation of acetic acid at 120" C.
Left. From 0.06 mmole of s t e a r i c a c i d Center.
TIME
(HOURS)
8
4
24
4
T I M E OF
Right. From 0.015 m m o l e of 2,3,4-trimethylhexadecanoicacid R e a c t i o n m i x t u r e c o n t a i n s 4 ml. of 5N chromic acid
From 0.03 mmole of 18-n-propylheneicosanoicacid
Determination of Terminal Methyl Groups in Some Simple Aliphatic Compounds. Campbell and Morton ( 1) concluded, on
0 1.0 M L . C O N C E N T R A T E D HZSOq
0 2.0
M L . CONCENTRATED H z S O ~
/
the basis of their work, that (CH3)~-C and (CH3)3-C-
groups
\ form acetic acid upon digestion with the chromic acid reaction mixture and that acetone is not a stable product in the oxidation step. However, this leaves unexplained the loss of 15 to 25% of the expected amount of acetic acid in the relatively simple samples they analyzed. For comparison, this study includes determinations of a number of simple compounds (tabulated in Table I). With the exception of trimethylacetic acid, the Kuhn-Roth reaction mixture yields the theoretically expected amount of acetic acid for the simple compounds studied. Although the data are not listed, the reaction mixture containing 2 ml. of concentrated sulfuric acid used by Ginger yields comparable results. T I M E OF DIGESTION ( H O U R S )
Table 11.
Acetic Acid Recovered from Oxidation of Simple Aliphatic Amines
(Reaction mixture contains 4 ml. of 5 N chromic acid and 1 ml. of concentrated sulfuric acid. Digestion temp., 120' C.)
Sample n-Butylamine
n-Amylamine n-Hexylamine
Digestion Time Hour; 0.25 0.6 1 2
Tri-n-butylamine
HOAc
Moles/Mole of Compound Found Present 0.64 1 0.66 0.73
o
I
:1.00 f a
1
0.72 0.89 0.89 0.69 0.88 0.12
3
0
4
2.22
3
1
2
1 2
Triethylamine
Figure 7. Rate of formation of acetic acid from 0.06 mmole of 5,5-dimethyl-1,3-cyclohexanedione at 120' C. in reaction mixture containing 4 ml. of 5 N chromic acid
2
0.06
1
7.5
G.
NOH'S04
~
0 I O ML.CONCENTRATED
H2504
'
I
c, 0.80 0
0.60
I
$ 0.40 Amines of low molecular weight do not yield acetic acid quantitatively when oxidized with the chromic acid reaction mixture. Table I1 lists the results of the digestion of several such compounds using the Kuhn-Roth reaction mixture a t 120" C. Determination of Terminal Methyl Groups in Several More Complex Aliphatic Compounds. The effect of varied digestion conditions on four compounds was studied: Three of these were the fatty acids-stearic acid, 18-n-propylheneicosanoic acid, and 2,3,4trimethylhexadecanoicacid. The fourth, 5,5-dimethyl-1,3cyclohexanedione, was selected for study because it was thought
0
z 0.20 LL
I 0
I
J
I2 16 20 24 T I M E O F DIGESTION ( H O U R S ) 4
8
Figure 8. Rate of formation of acetic acid from 0.06 mmole of 5,5-dimethyl-1,3-cyclohexanedione at 150" C. in reaction mixture containing 4 ml. of 5 N chromic acid
24
1308
ANALYTICAL CHEMISTRY
that it might be a bridge to a better understanding of the oxidation of compounds containing a benzene ring. Figures 6 and 7 shon- the fractions of acetic acid obtained in the dichromate oxidation by using the Kuhn-Roth digestion mixture and the increased amount of sulfuric acid recommended by Ginger a t 120" C. The dashed lines represent the rate of decomposition of acetic acid in the reaction mixture a t the same temperature as shown in Figure 5 (center). All four compounds yield a greater qua,ntity of acetic acid a t the end of a 90-minute digestion period xyhen Ginger's modification is used. Figure 6 (left) shows that the time required to produce the maximum amount of acetic acid from stearic acid is increased fourfold by using the Kuhn-Roth procedure rather than the Ginger modification, but within experimental error the same total amount of acetic acid is obtained by either procedure. The oxidation of 18-npropylheneicosanoic acid by the Kuhn-Roth procedure yields 2 moles of acetic acid per mole of sample very nearly quantitatively, as shown in Figure 6 (center). For this compound the greater amount of sulfuric acid employed in the Ginger modification apparently enhances the destruction of the terminal methyl groups a t a rate sufficiently rapid to result in the formation of about 13y0less acetic acid than was produced by the Kuhn-Roth reaction mixture. Figure 6 (right) shows the results of the oxidation of 2,3,4-trimethylhexadecanoic acid by the two mixtures. Again the oxidation is more rapid when 2 ml. of concentrated sulfuric acid is used, but a greater yield of acetic acid is obtained from the four terminal methyl groups when the smaller amount of acid is employed. The results of the analyses of these two samples suggest that still further reduction of the sulfuric acid concentration may yield acetic acid more nearly quantitatively, if the required digestion period is not increased unreasonably. Figure 7 shows the very slow oxidation of 5,5-dimethyl-1,3cj-clohexanedione by either reaction mixture. After a digestion period of 24 hours the oxidation is nearly complete, when the Ginger modification is used. With the Kuhn-Roth osidation mixture, digestion of the sample is not complete a t the end of 24 hours and the slow rate of formation of acetic acid illustrated in Figure 7 indicates that considerably more time is required for complete digest ion. Because of the rapid destruction of acetic acid a t 150" C., few samples were digested a t this temperature. The effect of increased temperature upon the digestion of 5,5-dimethyl-1,3cyclohesanedione is interesting, howver, because it indicates that a temperature greater than 120" C. may be desirable for the digestion of samples which are relatively stable t o dichromate Oxidation. Figure 8 shows the rate of formation of acetic acid when the Kuhn-Roth procedure is used, and also the milder oxidation mixture containing 7.5 grams of sodium hydrogen sulfate. Both curves indicat,e that the gem-dimethyl group yields one acetic acid molecule but that the relatively long time required for the oxidation destroys sufficient acetic acid, so that the maximum amount produced approximates 75 to 80% of the total possible amount. CONCLUSIONS
The present investigation is concerned only with the determination of terminal methyl groups attached to aliphatic compounds. A relatively limited number of substances have been studied, but the information permits a number of conclusions, and points the wag to further study of the procedure. The rate of distillation of acetic acid is speeded up appreciably by jacketing the distillation flask with toluene or n-butj-l alcohol. The oxidation of acetic acid at the temperature of distillation is so slow that there is no need to reduce the chromic acid i1-ith hydrazine or hydrogen peroxide. -4t 150" C. the Kuhn-Roth reaction mixture and the one containing an increased amount of acid proposed by Ginger oxidize acetic acid so rapidly that satisfactory results are not obtained. At 120' C., the rate of Oxidation of acetic acid is decreased-a 20-hour digestion period is required to destroy 570 of the total
acetic acid present if the Kuhn-Roth reaction mixture is used; approximately 7 hours are required to destroy a like quantity if the Ginger reaction mixture is employed. No marked decrease in oxidation occurs when the temperature is further lowered to 100" C. a t either acid concentration. Acetic acid is considerably more stable to oxidation by chromic acid than is benzoic acid. Consequently, by extending the digestion period, it is possible to remove the benzoic acid completely, while losing only a small amount of the acetic acid formed. By minimizing the digestion period and digestion temperature it is possible to estimate the sum of the two acids. The chromic acid oxidation of tert-butyl and tert-amyl alcohols shoxs that one molecule of acetic acid is obtained quantitatively
/ for the (CH,),-C
and( CHP)3-C-gr~upspresent.
These results,
\
plus the rapid and quantitative formation of acetic acid from acetone, confirm the conclusion of Campbell and Morton that acetone is not a stable product in the oxidation of these tn-o groups. The determination of terminal methyl groups attached to aliphatic amines requires further study. The results of the oxidation of the more complex aliphatic compounds reported in this investigation illustrate that a general procedure for the terminal methyl determination cannot be so rigidly established as that generally employed. The results in Figure 6 (center and right) shon- two instances where less acetic acid is obtained by using the increased amount of sulfuric acid recommended by Ginger. However, Figure 7 indicates that for 5,5-dimethyl-1, 3-cyclohexanedione the greater sulfuric acid concentration is necessary a t 120" C. As a result of this investigation the authors have adopted a polivy of determining terminal methyl groups under a single set of conditions only when it has been previously determined that the sample is easily digested or when the quantity of material available for analysis is the limiting factor. In general, the KuhnRoth reaction mixture is employed and the digestion periods for two samples of the substance to be analyzed are varied by a factor of 2. These two results then serve as a basis for determining whether digestion is complete and, if necessary, what the conditions should be for further study. I n each terminal methyl determination the rate of destruction of acetic acid aft,er digestion of the sample was slightly greater than that for the digestion of acetic acid itself. Westheimer (4) states that +4 and +5 chromium intermediates are produced during the reduction of dichromate and that a t least one of these species is capable of oxidizing manganous ion to manganese dioxide. I n the digestion of acetic acid no appreciable reduction of dichromate occurs, whereas for a sample this is not t'rue. It seems, therefore, that the presence of very small amounts of such a powerful oxidizing agent could be responsible for the slight difference in rate of oxidation of acetic acid in the two cases. Of course, one of these intermediates also could oxidize the terminal methyl group directly and thus be responsible for the low results shown in Figure 6. The design of the equipment used in this study is more elaborate than that usually employed for the terminal methyl determination. However, the authors believe that the simplicity of operation, the reduced quantity of distillate required for quantitative recovery of acetic acid, and the complete elimination of sulfuric acid carry-over in the distillation step represent improvements to the method of analysis. I n addition, the appreciably better results which have been obtained using sealed tubes and the rocking furnace for the digestion of samples justify their use. LITERATURE CITED
(1) Campbell, A. D., hlorton, J . E., J. Chem. SOC.1952, 1693. (2) Ginger, L. G., J . B i d . Chem. 156, 452 (1944). (3) Kuhn, R . , Roth, H., Ber. 66, 1274 (1933). (4) Westheimer, F. H., Chem. Reti. 45, 419 (1949). RECEIVED for review February 13, 195G. Accepted May 5 , 1956.
