second complex spcct,rophotonict~ric~ally :ml cvon t'o determine it,s formation constant, such measurements would have to be carried out in solutions having an excess of iodine monochloride which has a considerable absorption in the near ultraviolet region and the results would not be too reliable. Also, concentrated IC1 solutions have a t.endcnc,y to halogenate organic compounds and such a side reaction with dioxane would render the measurement,s valueless. Thc procedure used to analyze the infrared data was that described earlicr.6 However, in using the Benesi-Hildebrand--Scott equation, we made two modifications. Instead o f using the initial concentration of dioxane as the independent variable, we used the actual concentration (obtained by assuming a value for K f ) . Also, we used the integrated absorbance, Bnl, instead of the absorbance, A , . The rrsulting value of k'fwas 35 I./mole. However, the cxperirnental scatter of the data, was sizable, and lines rould he drawn t~hrorigh the data giving values of Kf as high as 55 :and as low its 25. It should be noted that the infrarcil mt~muremenis were made on solutions with rclativdy high coric*ent,r:ttionsof ICL. Thus, it is possible that, thr higher valiic of Kf is significant, and duo t,o some contribiit.ion to oiir data from t.he 2: I complex. 1 3 ~riw 1 of elit! corrc'la tions fount1 c::trlierQ~ll in sttidiw of t,he iiifr:trccl iipcitr:~of charge-transfer comp1esc.s i t is of some iritercst. to determine the values of the frequency, halfintc.nsity width, and intenbit,y for thc I-C1 absorption band in thr rompl(,s. These were, respectively, Y = 352 cm.-l, A U ~ =: / ~ 1?1 c ~ i i . - ~and , B = 4000 cm.? millipole-l From this, A k / k = 0.12 and sa = 2 . 6 D / A . (see wnces cited for definitions). This value of sa is quitc: high for t,he corresponding value of A k / k , and t,hc resulting point f:ills quite high on the correlation plot (Fig. 7 of ref. 11) and outside the average deviation of the values shown i n that plot. This may also be due to the presence of a tw~~-to-onc complex.
Acknowledgment.-The authors are grateful to Professor L. Lilich for the discussion of this problem. (11) '7.B. Person, R . E. Erioksonand R. E. Buckles, J . A m . Chena. SOC.83, 29 (1960).
THE EFFECT OF HIGHER FATTY ACIDS O S THE D EC~~RBOSYL;ITIOS OF JLYI,OSIC ,lCID BI 1,cirlis \VATIS ( ' L ~ R R Coriliibu'ioii
110n (ha D e p n r f n e n t
o/ Chemisfry. Sarnl H n r y of the I'lnzias College, Dodoe Czty, Kansas
ReceiLed J a n u a r y 8 , 1900
Kintitic studies have been reported on the decomposition of malonic acid in forty-seven nonaqueous solvents comprising representatives of fourtern homologous groups.' The rate-determining step of the rraction in every case appears to he the form,ition of a transition complex, the nucleophdic c:trbo113d carbon atom of un-ionized malonic acid coordinating n ith an unshared pair of electrons +xian electrophilic atom of the solvent molecule, facilitating cleavage. The delicate affinity of un-ionized malonic acid for urishared electrons, mide manifest by the different rates of evolution of carbon dioxide, makes possible its use as n tool or technique for studying the electron and stclric strncturc.; of all kind5 of polar molcculcs. IntereLtiiig rr~11Jti ha^^ t)wn olitnincd prcviou4y from : study of the reaction i i i sm-era1 nf the lomrr fatty wids and their For the sake of cvmpleteness it was helieved n.orthwhile t o extend thr investigation into some of the higher membc rs of the series. ( I ) L W. Clark, Tirre J O [ J R \ % L , 64, 508 (1900) ( 2 ) L W. C l u h ibid , 64, 4 1 (19bO).
The presciit paper describes the results of kinetic studies carried out in this Laboratory on the decarboxylation of malonic acid in four additional monocarboxylic acids, namely, 2-methylbutanoic acid, n-valeric acid, hexanoic acid and decanoic acid, making nom a total of 51 solventb in which the reaction has been investigated. Experimental Reagents.-(1) Reagent grade mziloiiic nc id, 100 0% :may, was used in this investigation. ( 2 ) Solvmts: The fatty arids used in this research were either rmgent grade or highcst purity chemicals. Each sumple of eac 11 liquid M as fractionally distilled at atmospheric presbure into the reaction flask immediately before the beginning of earh experiment. Apparatus and Technique -Thc dct,iil. of t :q)p:wAis .tnd technique h a w 11cen desciibcd prc\ ioii4y I n thew csxpwimt.nts a sample of mitlonic atid nc~iglilng0.1857 g. ((*orrespondingto 40 ml. of COzat STP on c*omph%crcwtion) h ( b
calibrated Iiy the I;S. Bureau of St:tndaitlB.
Results Decarbosylatioii experiments were carried out in each solvent a t three or four different temperatures over approximately a 20" temperature range. Two or three experiments were performed a t each temperature in each solvent. First-order kinetics were observed for the first 50-75uo of the reaction, after which the reaction rate conqtant decreased slightly with time, due, undoubtedly, to slower side reactions. About 50 ml. of solvent was the amount generally used in the experiments HOWever, a wide variation in the ratlo of solvt~ntto wlute did not appear t o have any effect upon the rate of reaction. The average values of the appnrcnt first-order rate constants for the reaction in the four acids a t the various temperatures studied. obtnincd from the slopes of the experiment:d logarithmic plots, are listed in Table I. The parametrrs of the Eyriiig equation are shown in Table 11. Corresponding data for the reaction in propionic arid and butyric acid, as well as for the dec~nrboxylntlo~i of molten malonic acid, are included for co1np:trison. Discussion of Results In the c a v of the dccarhoxylation of nialoiiic acid iu the l(~\vcrnioiiocarboxylic acids it has been shown that the transition complex is probably formed by the coordination of the elcetrophilic carbonyl carbon atom of the malonir acid with one of the unshared pairs of electrons on the hydroxyl oxygen atom of the solrent inolccule.* Since the AIZ* of the reaction decreases n. thp effective ncgati\-e chargc on thc nurleophilic atom of thc v ) l \ ~ ~ir~crrnscs~ it thc tlecrcnsr in AII* on going from propionic ncitl to n-l)utyrit acid (line.: 5 and 6 of Tablc 11) is coiisisttwt ~ i t thc h fnct that the ethyl group exerts a greater positiJr. inductive effect than does the methyl group6 The slight (7) I, W Clark zbzd 6 0 , l l i 0 (1056) K J Laidler "Chemical Kinetics lnc., New York, N. Y., 1950, p. 138 (2)
"
J J c ( ~ r ~ s - I l dBook 1 Co
.
JIay, 1960
s o . of
Solvrnt
2
3 2 2
2 2 >
3
.7 3
2 3 3 3
I>ecnnoic ncitl
3 3
(1) 2-31ethylhutanoic acid ( 2 ) n-Valeric ncitl (3) Hexanoic i t ~ i d (4) Decanoic arid ( 5 ) Propionic acid? (6) n-Butyric acid2 (71 1Iolten mnlonica acid6
k X 104, sec. - 1
runs
2-E+,liylhiit~anoic acid
Solvent
693
SoTES
AH*, kcal.
32.1 32.2 :32.5 2ti.O
?J.G 32.3 :1:1.0
1 . 2 4 f 0.01" 2 . 4 6 i: .01 5.12 .02 0 . 2 3 i .01 1.58 =I=.01 2 . 2 3 f .02 4 . i S i .04 10.10 ==! .02 2.10 i .01 3.8!J : t .02 (;.?A* .01 10,;173Z . O 1 1 . S 9 f .02 3.84 f .01 ,5.48 f .01 8 . . i i i .01
AS*. ?.ti.
+1.1
+2.4 +3.2 -11.0 +F.1
+2.5 +4.5
*
klro
AFiro*,
X IO', see.-1
31.5 31.1 31.2 31.1 31.1 31.3 31.1
1.8 2.5 2.4 2.5 3.5 2.3
kcal.
3.5
decrease in S I * on going from n-lnityric acid to 2-methy1but:inoic acid (lines G and 1 of T:tble 11) reflects the small increase in the I effect due to the branched alkyl group in the latter. I t is apparent, however, on comparing lines 1, 2, 3 and G of Table 11, that there is very little difference in the A H * of the reaction in n-butyric acid, 2-methylbutanoic result acid, n-valeric acid or hexanoic acid-a which is not surprising in view of the fact that a11 normal alkyl groups beyond the 3-4 carbon stage possess about the same inductive effect. The decrease in the AS* of the reaction on going from propionic acid to n-butyric acid (lines 5 and G of Table 11) is indicative of the increased complevity of the butyric acid dimer over that of propionic On going trom n-hutyric acid to 2-methy1but:moic acid (lines G and 1 of Table 11) A S * decreases hy only 1.1 e.u. This decrease does not appear t o he commensurate with the greater steric hindrance one n-ould expect from the presence of the branched methyl group. The addition of a methyl group t o the Q carhon atom of butyric acid apparently interferes to some extent with the tendency toivard dimerization (screening effect) . 7 ( 5 ) C. N. Hinshelaood, J Chem. Soc , 117, 156 (1900) ( G ) C. IC. Ingold, 'Strncture and I l r r h a n i s m in Organic Chemist r y , ' Corneli Unirersity Press Ithaca, N. Y . , 1933. p. 71 (71 R . IIiichel, "Tiieorrtlcnl Princi1)lrs ot Organic Clieniistrj ," Vol 11, Elsm if'r l ' i i l ) l i ~ I ~ ~(10 n g , N t n T o r h , N. T , 1958, p 363.
Both the AH* and AS* of the reaction in n-butyric acid and in n-valeric acid are very nearly equal (lines 6 and 2 of Table 11). This suggests that the &carbon acid is not as extensively dimerized as is the &-carbon acid. The AS* of the reaction in hexanoic acid is slightly higher than it is in n-butyric acid or nvaleric acid (lines 6, 3 and 4 of Table 11). The addition of another carbon atom t o the chain has undoubtedly resulted in considcrahle intcrfercnc~ with the tendency toward dimerization, so that, in spite of the larger size of the monomer of hesanoic acid, the reduction in the degree of dinierizntion actually results in a simpler transition complex with malonic acid than in tiit) case of n - h t y r i c acid or n-valeric acid. At first glance the data for the decarl~o\.ylationof malonic acid in dtwmoic acid (line 4 of Tahle 11) appear rather surprising. If all alkyl group< lieyond the 5-4 carbon stage have about the samc inductive effect one would have expected the AH* of the reaction in decanoic acid to be approximately the same as in n-valeric acid, n-butyric acid or n-caproic acid. We find, instead, that the A H * of the reaction in decanoic acid is almost qix ltilocalories lower than it is in liesanoic acid Evidently, the nine-carbon alkyl group iii decanoic acid has profoundly changed the electron propertiei of the carboxyl group to which it is attached. Steric hindrance due to the complexity of the hydrocarbon chain probably entirely prevents the formation of the dimer, and strongly hinders the formation of the transition complex, as evidenced hy the very large decrease in AS*. It has been pointed out that in the case of the lower molecular weight alkaiioic acids the H-bond in the dimer (being formed 1)etweeii the hydroxyl group of one molecule and the carhonvl oxygen atom of another molecule^ prohahly itrongcr than that which is formed between t\vo hydroxyl groups.R This strong hydrogen bonding in the dimer will oh.\-iously drcrcnsc the electron dciisity on the hydroxyl oxygen atom. Consequently, in the case of fatty acids which do not dimerize, the electroil density on the hydroxyl oxygen atom would be expected to be considerably greater than it would lie for those acids which exist largely in the dimeric form. The large decrease in AII* on going from hexanoic acid to decanoic acid is in line with these considerations. Of considerable interest is a comparison of Hinshclwood's data for the decomposition of molten malonic acid with that for the deconiposition of malonic acid in propionic acid (lines 3 and T of Table 11). (It is worth noting that a period of nearly 40 years intervened hetween these two set.: of data.) The transition complex for the rractioii in propionic acid is formed by coordination of thP carhonyl carhon atom of the malonic arid with the hydroxyl oxygen atom of the propionic acid In molten malonic acid the tranqition c*omplt.x is formed evidently hy coordination hetwern the clectrophilic carbonyl carbon atom of oiie molecule of malonic acid and one of the urisharrd p:iiri of t~lrctrnnson the hydroxyl ouggen atom cd' a rccoiid ( 5 ) I\'. IIucAel. ref. 7,
13.
341.
694
moleciile of imalonic acid. (A certain analogy exists between these two acids inasmuch as they are both carboxylic acids containing three carbon atoms.) The hpecifio reaction velocity constant for the decarboxylation of molten malonic acid a t 140" is the Same as that for the decomposition of malonic acid in propionic acid a t that temperature. Furthermore, the AH* of the reaction is nearly equal in both cases, being slightly less in molten malonic acid than in propionic acid. Also, the AS* of the reaction decreases by 1.6 e.u. on going from propionic acid 10 molten malonic acid. All these results are easily explicable on the basis of the known properties of the mono- and diIt is known that dimerization carboxylic a c i d ~ . ~ in the case of the dicarboxylic acids proceeds beyond the dimer stage, however the associated complex which forms still possesses a carboxyl group a t both ends. Consequently, in molten malonic acid, the transition complex represents a larger aggregate than in propionic acid, resulting in a lowering of AX*. The fact that there are free carboxyl groups a t the termini of the associated complex results in a slightly higher effective negative charge on the hydroxyl oxygen atom of molten malonic acid than in the case of propionic acid, and hence in a somewhat lower value of AH *. Further n ork on this problem is contemplated. Acknowledgments.-The support of this research by the National Science Foundation, Washington, D. C., is gratefully acknowledged. (0)
I'ol. til
n'OTES
W. I I u c h d ref 7 , p. 329, et
seg.
1
06 00
04
08
I2
16
1 20
24
28
LOG (AQ. E S T E R P M O L A R I T Y ) t 4 .
Fig. 1.-Ester phosphorus molaritiefi, organic V S . aqueous: 0 ,no H,POI; 0 , 1.004 W H+POI; A, %.008MH3POr; 0, 3.70 JI HJO ' ,.
The dibutyl phosphate was prepared by further purifying the previously used material mixed with =iN EXTRACTABLE COMPLEX OF DIBUTYL P32-labelled dibutyl phosphate from the Yolk Radiochemical Company. Any pyro- and triPHOSPH-iTE AXD PHOSPHORIC ACID esters were removed from the mixture by the proceBY B. J. THAATER dure of Peppard, et aL13followed by acidification, extraction into the kerosene, extensive wa5hing Contribut on from Los Alamos Sczentific Laboratoru of the University of Caalzfornza, Los Alamos, New Alezzco with water to remove monobutyl phosphate, and Received January 2, 1960 drying. The kerosene solutions of the dibutyl The extract,ion of uranium by a several-fold phosphate were equilibrated in the previous mangreater concentration of dialkyl phosphate has been ner4 with the desired aqueous phase, after which the dibutyl phosphate concentration was determined in formul:it,ed as each phase by P-y counting. The method thus U09+*(aqj+ :!H?Us(org) = was similar to that of Dyrssen.6 UOdHD&(org) 2H+(aq) ( I ) Figure 1 contains a log-log plot of the ester conby Baes, et aL12in the extract,ion into n-hexane with centrations a t equilibrium. The lines have been di-(2-et hylhexyl) phosphate and by Dyrssen3 in the drawn with a slope of 2. This slope is verified in extraction into chloroform wit,h di-n-butyl phos- the absence of HdP04 for grobs organic concentraphate. It, was t,he purpose of the present experi- tions of ester of 0.00008 to 0.002 X ,but it increases ments 'to inv'estigate further the state of the extrac- rapidly when the concentration is raised above 0.03 tant without- uranium under the conditions of the iM where higher polymers apparently hecome iniauthor's st'udy of uranyl phosphate c ~ m p l e x e s . ~portant. The presence of H3P04 augments the The aqueous phases thus were HC104-XaC104 extraction of the ester into the organic phase, thus solutioiis having 0-3.7G M total H3P04, 0.510 M indicating one or more extractahle, probably unH+, arid 1.07 for the ionic strength, and the previous charged, complexes between the two. The series of points of each of the three concentrations of kerosene T K L Sused. H3POa each shows a slope of 2 within lop4to 0.002 (1) Woi k pcrfolmed under t h e auspices of t h e -4tomic Energy ComM ester in the organic phase. Higher concentramission. tions of ester give higher slopes. However, the Xingaro and C. F. Coleman, THIS JOURN A L , 62, 129 (IY58). average extracted complex probable has the com(3) D. Ilyrssen. "Recent Investigstions on Extractable Rletal Combination 2HD.zH3P04within the region of a dope
+
plexes." i n Italian National Research Council. et al., "Chemistry of t h e Co-ordinatc Comoounds, a Symposium. Sept. 15-21, 1957," Pergamorl Press, Inc , S e w l - o r k . 1!158. I). 291-295. 11) R .I 'r!l:irii(>r .r. .!,,I. ('hem. Soe., 79, 4298 (1957).
( 5 ) D. F. Peppard. J. R. Ferraro and G. W. Mason J Inorg. Nuclear Chem., 7 , 231 (19.58). (6) D Dyrssen, Acta Chem. S e a n d , 11 ( Z ) , 1771 (1957).
