THE REACTIVITY OF HYDROGEN ATOMS I N THE LIQUID PHASE. 11. THE REACTION WITII SOME ORGANIC SOLUTES BY T. J. NARDWICK Gulf Research & Development Company, Pittsburgh 30, Pennsylvania Recezaed Auguat 8 , 1961
Using a method developed previously, the rractivity of a number of organic compounds with hydrogen atoms was measured in n-hexane solution a t room temperature. Tht>following classes of cornpounds react by addition of hydrogen only: esters, methylbcnzenes, disulfides, coniugated or rondenved aromatic compounds, vinyl monoinrrs, and stable free radicals. lleacting by both addition and hydrogen abstraction me: aliphatic acids, aldehyde.;, ketones, alkynes and alkyl (other than methyl) benzenes Alcohols, e t h m and mercaptans are relatively inert. The effect of temperature on these reactions was studied, but the results are not easily interpreted. Absolute values for the rate constants a t room temperature were obtained from a comparison with values previously established.
Introduction A method of measuring the reactivity of hydrogen atoms in solution has been described recwitly.lJ The technique involves dissolving small amounts (up to 27,) of the material in question in a saturated hydrocarbon and, after exposure to relatively low doses of ionizing radiation, measuring the hydrogen gas yield as a function of solute concentration. Hydrogen atoms, produced in situ by the absorption of radiation by the solvent, react competitively with sol\.ent and solute. A generalized plot of the radiolytic hydrogen yield (GFIJ as B function of solute concentration is shown in Fig. 1. The number and type of reactions taking place in such a system during radiolysis are many and varied. In the present case, however, it is necessary to consider only those reactions which influence the formation of hydrogen gas. The pertinent reactions on radiolysis of solvent RIT containing solute S are RII
GI -&
Ilz
+ prodwts
Gz
RTI --+ H -E R ka TI RTT --f 132 R
+
TI Ir
+
+s
+s
kb
(1)
t 2) (3
k4
-+ rIs
-+ T
T ~-tR’
(4) ( 5)
Reaction 1 is the so-called “hot hydrogen atom” or “molecular hydrogen” reaction which produces hydrogen gas in yield GI, unaffected by small concentrations of solute. “Thermal” hydrogen atoms are produced in yield Cz, and react with solvent (reaction 3), or solute by addition (reaction 4) or abstraction of hydrogen (reaction 5 ) . Using a steady-state treatment for hydrogen atoms, the foIlowing kinetic expression is derived
where GH,(~)is the hvdrogcn gas vield in pure solvent G H , , ~is) Ihe hydrogen gas vic4d at solute concn. R
IS t h e decrease in hvdrogrn ga.7 vield resulting from the addition of solute to the pure solvent
AGlr2
(1) T J. Hardwick, .I. Phiis Chem , 64, 1623 (1980). ( 2 ) T.J Hardwick, rbzd., 6S, 101 (l9Gl).
A plot of l/AGI12 vs. [RII]/[S] gives a straight line of slope (k3/k4)(1/(3z) and intercept (1 /Gz) (k6/Ic4 1). Where hydrogen atoms react with the solute by addition only, i e . , 165 = 0, the intercept is l/Gz. Values of GI and Gz have h e n determined for a number of saturated hydrucarhom2 I n the generalized study of hydrogen atoms with various classes of solutcs, n-hexane has been used as the hydrocarbon solvent. In this case GI = 2.12, Gz = 3.16. The rate constant ha has been measured as 4.0 X lo9 cc. mole-’ sec.-I a t 23°.2 Relative rate constants obtained from the kinetic plots have been converted to absolute values using this value for Ita. This paper reports values of the reactivity of hydrogen atoms with organic compounds of several classes, e.g., aliphatic esters, acids, aliphatic alcohols, ethers, aldehydes, ketones, alkyl aromatics, condensed aromatics, etc. A comparison of mcthanol and n-hexane as solvents and sources of hydrogen atoms is made. Preliminary studies have been made on the effect of temperature. Comment is given on the accuracy and precision of the results. Experimental
+
Materials.-Pure Grade n-hexnne was obtainrd from Phillips Petroleum Company, and was purified further by prolongc-d stirring with sulfuric acid. In all eases the unsaturation content of the purified solvent was less thnn 0.2 mM/l., as me:tsured by bromination. Baker and Adams’ Reagent methanol was purified further by refluxing over alkaline silver o d e and distilling. The mcasurrd aldeIiyde content wss less than 0.1 miZ.I/l. There are too many chemicals to list the sourccs in detail. The purest available were used, and several were freshly distilled before use where deterioration on standing was suspected, e.g., aldehydes. Alcohols, however, were rigorously purified to remove carbonyl compounds. I n genrral, the solutes used were sufficiently reactive that small amounts of impurities would not significantly affect the mrasured valuc of the rate ronstants. Irradiation and Analysis.-The methods of irradiating degassed solution and measuring the resultant hydrogen yield have been described in dtltnil previously.1 BrirRy a 100-ml. sample of solution was drKassrd and irradiated to :tholit 20 kratl. by X-rays from a \’:in tic. GrimtY :trcvlwatc~r The cw’rgy ahsorl)c~diri ( w h samr)l(. W:M monitorcd L)v a sirnrilt:tncoue irradistiori of H solution of the Frickch dosiin(iter. ITydrogen gas WLS dctrrmintd I I V isolating it from other gasw and nwasiii-ing the amoiint in a RlcJ,eod gngc. I n a typical expcrirnent , 5-10 pmolrs of r:diolvt ic h v d r o ~ ~ r n gas was producrd. :ill rim8 wrr(’ made a t 23 =k 1’ unless otherwise stntcd. Irradiation a t Various Temperatures.-In e-qwriments drsignrd to study t h r eTwt of tempwaturr, t h r irradiation cells were submrrgctl in :L constant temperature h t h tu thc
118
rr.
J. HARDWICK
5
:4
. 0
2 0 A
N
W 1:
3
I-
--2
1
1 50
I
I
_.
I
IO0 I50 (SOLUTE) MlLLlMOLESlLlTER
Fig. 1.-Generalized plot of hydrogen yield (GII:) I L ~a function of solute concentration in 7t-hexane, X.4 = 1012m. molt,-' scc --l. The iiiost reliable results are obtained in rc,gioii U. point whrre thc' levrls of the Irrttdiated liquid and the bath liquid \wre thc swne. All samples received the same ruposure, for tht. t o t d elrrtron charge from thc acechator wits the same for c w h run and the cw.igy of the electrons n r ~ smonitored on : ~ i ioupand(>d scale, which prrmitted a voltage setting rrpi-oducihlc t o =!=lo kv. The results indic a t i d a precision of ~t0.6r/;~.Thc relative hydrogen \rields were pl:md on an absolute. basis by taking G I I ~= ~5 ~28 at 23". Concentration of Solutes.--Thc concentration of solutc varicd considwhly, dqmiding on thv I eactivity. I n g r n c d , four to ei(;ht difrcrcnt solute conceiitrationa w e i ~ rlwd, varying ovw a facator of four The concentration range was lrw than tliis foi solutes: ( 1 ) where the reactivity is low (a 25&/volurnc n s s the maximum solutr coricmtiation used) or (2) \vlieic the solutes aie sparing!y solriblr, as is the case for some of the solid polyaromatics.
of l i d and lii, were cnlculated from the measured rat 10s. Esters. - The rc:icti\.ity of hydrogrn atoms wit ti some aliphatic (IsterF IS +hoirii in l':iblc I. IYith the excrptioii of mc>thyl ate, the nile of hydrogen atom adclition to sur 1ers i~ the samc within experimental m o r , t'zx., 8.6 X 1Oio cc. mole-' sec.-l. Hydrogen atom abstraction is less than 5% of this value or < 4 X lo9 cc. mole-' sec.-l. The invariance of k4 in the series implies that the wine reaction is taking pln~c,nnd we ascribe this io the additioii of hydrogen atoms to the carboxyl group. In the kinetic development of the renctior, mechanism, one iiicludcs some unknown coristarit relating the effective collision dianieter of nhexane and the ester carboxyl group. A variation in rate with increased chain length of either alkyl group of the ester therefore would not be expected. Reduced reactivity might be expected if steric hindrance were a factor, e.g., from a compound 6uch as t-butyl pivalate. TARIX T I?JCiCTIYITT
OI" HYDROGPS ATohis IYITIJ ~ h I l ~ I J ~ \ T IT':STERI C. REWTIOX BY IDDITIOS OE.II,Y 7' = 23 Zt l o Arctatc
Propionate 13iit) rate X 10'0 cc. molr-1 3rc.-1
Valcrnte
LIrthyl G 9" 8 3 8 6 1Sthj1 83 8 3 8 5 8 2 ??-Propyl !I 2 9 2 IEopropyl 84 9.0 n-Butyl 9 0 8 1 n-..lmyl 84 M e m 8 G i 0 4 X 1010 cc mok' -I set.-' a S o t included in calculating mean valrit.
The rate constant for methyl acetate is low, and \vas not inclitdd in determining the meail valuc~. Results and Discussion So satisfactory rxplanation is apparent at this Treatment of the Data.-A correction wa~h ap- time, hiit the discrepalicy is not likely to be due t o plied to : ~ lrcsults l to account for the absorption of impurities, for poesiblc impiirit ies (acids, aldehydes, energy directly by the solute. For simplicity it ctc.) are much morc reactive toward hydrogen was assiiined that no hydrogen atoms were pro- atonis than iire eb1er.s I t mav be that the unique duced from the solute (a doubtful assumption), ( x s c of tn-o methyl grotips attached to the carboxyl but in most cases low yiclds of such atoms will group results in a slightly loncred reactivity. Acids.--Wit h acids as solutes the intercept of the have little effect on the final result. The correction, as applied, involved normnlizing the energy kinetic plot was always > 0.316, indicating that, k 5 absorbed by n-ht.xanc to 100 ml. of solvent, taking has a sigiiificaiit value. The rates of addition into account the ratio of electron densitics of solute (k,) and abstraction ( k 6 ) arc reported in Table 11. I'rom these data sevcral factors may be noted: and n-hexane. All compoiinds sludied gave a straight line on (1) IIydrogcn atoms both add to, and abstract plotting l / A < ~ ~ ZJS. ~ x [n-hexailel/ [solute]. The from, organic acids. The ahqi raction of hydrogen value of the intercept in all cases was e q ~ a to l or from triflrioroacetic acid is particularly significant,. grwter than 0.316 (I/(&) within an cqcriment:rl ( 2 ) For all aliphatic acids the rates of ntlditioii and ahstractiorl are ahout the same. (3) The ratio error of f50/& As a preliminary stcp, a modified lcast squares of kS/kd is constant for all acids despite large fit was made on tho kinetic plot of the corrected charges in individual rates. It seems clear from the resiilts that hydrogen data. For those solutes where the value of the intcrrtpt was within *5yo of 0.316, (l/G), the atoms arc attacking the carboxyl part of the organic lcast squarcs fit was recalculated :issuming the acid, either adding to this group or abstracting the rarbosyl hydrogen. Somewhat surprising is the intercept fixed at 0.816. The relativt. rate constants (ka/lc4) were calcu- constaiicy of the ratio k5/lcd despite large changes lated from the slopes of the straight lines in the in reactivity iind structure. This may be due kinetic plots The value of k 5 l k r was determined either to the requirement of a suitable collision from the intercept; if k 5 / k 4 was less than 0.05, geometry for each type of reaction, and the ratio kg was taken as zero or negligihle. .Ibsoluie \dues of 0.27 is aboiit the valne expe d, or to a transi-
,Jan., 1962
REACTIVITY
OF
HYDROGEN kLTOMS WITH
O R G a N I C SOLrJTES
119
propionic acid, with an activation energy between 7.7 and 9.2 kcal., assuming a steric factor of unity. In view of the preliminary nature of earlier results, it perhaps is not fair to make a quantitative comparison. The values of the rate constants obtained, however, indicate that in the liquid phase the activation energy for hydrogen abstraction is less than TABLE I1 5 kcal. REACTIVITY OF HYDROGEN ATOMSWITH ORGAXICACIDS Alcohols.-The radiolytic hydrogen gas yields 1' = 23 =I=1' for the system 2%/volume aliphatic alcohols in n$4 Ics (abhexane are shown in Table 111. Values range from (addition) straction) X 1011 c c . mole-18ec.-l kdkr slightly above to slightly below 5.28 mole/100 e.v. Either the alcohols are relatively inert to hydrogen Acetic acid 6 1 1 5 0.24 Propioiiic acid 6 9 1 6 .23 atoms, or reaction 4 is insignificant compared to reaction 5. Values of G H ~> 5.28 probably arise Butyric acid 5 8 1 6 .27 from hydrogen gas derived directly frorn the alValeric acid 5 4 1 6 .29 Isobutyric acid 5 5 1 6 28 cohol, for in the correction for energy absorbed by Trifluoroacetic acid 16 4 4 28 the solute, no hydrogen gas formation from this Benzoic acrd 29 8 3 .28 source is postulated. Values below 5.28 probably 0.27 & 0 02 are due to traces of aldehyde or other reactive impurity which is not easily removed from the Trifluoroacetic acid and benzoic acid were chosen alcohol. to illustrate that hydrogen indeed is abstracted TABLE I11 from the carboxyl position. This of course must HYDROGEN GASYIELDSFROM THE SYSTEMSnbe so in the case of trifluoroacetic acid. Studies RADIOLYTIC HEXASE-ALIPHATIC ALCOHOLS with other aromatic compounds vide injra indicate T = 23 i 1' that abstraction of hydrogen from the benzene ring is an unlikely process when compared to hydrogen atom addition. 5.28 3 47 If the postulate of a constant value of k 6 / h is None 5.48 3.46 correct, it follows that hydrogen atoms react with Methyl alcohol 2%/vol. 5 24 3 58 benzoic acid only in the carboxyl position. If n-Propyl alcohol 2%/vcl 5.28 3.33 reaction with the ring occurred to any significant Ispropgl alcohol 2%/vcl. 5 21 3.38 extent k5/lc4 would decrease. As will be shown n-Butyl alcohol 2%/vol. 5 33 3.56 later the rate of addition of hydrogen atoms to Isobutyl alcohol 2%/vol. 5 20 3.50 benzene rings is about 2-3.5 X cc. mole-l n-Amyl alcohol 2%/vol. 5 30 3.56 sec.-l, and such a contribution to the rate might n-Hexyl alcohol 2%/vol. __ not be detected easily in the case of such a reactive Mean 5 29 3 48 rt 0.08 compound as benzoic acid. To determine whether reaction 5 is significant, The rate of addition of hydrogen atoms to alithe hydrogen yield from solutions of 1%-hexane phatic acids is seven times greater than to aliphatic esters. Apparently the addition takes place on containing O.Ei%/volurne of benzonitrile, with and the carboxyl group in both cases. It is suggested without 2%/volume aliphatic alcohol, was measthat the energy of activation for attack on an ester ured. The results are shown in the last column carboxyl group is larger than for the corresponding of Table 111. If reaction 5 were significant, the attack 011 an acid carboxyl. It also is possible value of GH* should increase on adding alcohol. that the replacement of hydrogen by an alkyl group For example, consider the system 0.5% benzonitrile, 2% n-propyl alcohol. If the abstraction of hydroexhibits a small steric effect. The absence of reaction 5 in the hydrogen attack gen from the alcohol occurred a t a rate = 2 X 1010 on aliphatnc esters (k5 < 4 X lo9 cc. mole-I sec.-l) cc. mole-I sec.--l, GI€, would be 3.91. As GH%is tends to confirm that hydrogen abstraction in not measurably changed on adding alcohols, the reactivity of hydrogen atoms on alcohols must be acids occurs at the carboxyl group. The higher reactivity of hydrogen atoms toward low. This raises the possibility of using such alcohols fluorinated acids is unexpected, and little comment as a source of hydrogen atoms on radiolysis. can be macle a t this time. The results presented here agree qualitatively Adams and Baxendale6 in effect did this in their with gas phase work. Burton3t4photolyzed acetic studies on the methanol radiolysis. Strong and acid vapor and suggested that a hydrogen atom Burr7 studied the addition of hydrogen atoms to abstraction from the carboxyl group might occur acetone in isopropyl alcohol solution, where hydrogen atoms came from the radiolysis of the solvent. CH36OOXI XI -----f CHaCOO + Hz (6) A comparison of methanol and n-hexane as solwith an energy of activation in the range 7-11 kcal. vents and in situ sources of hydrogen atoms appears In a later paper Henkin and Burtons postulated an later in this paper. Ethers.-Experiments to determine the reactivanalogous reaction between hydrogen atoms and ity of hydrogen atoms on diethyl and diisopropyl (3) M. Burton, J . Am Chem Sac., 88, 692 (1936).
tion complex in which addition to the carboxyl group or abstraction of hydrogen occur with fixed relative probability. As in the case of aliphatic esters the length and structure of the alkyl chain seems to have little or no effect on the rate of reaction.
*
+
(4) M. Burton, z b z d , 58, 1645 (1936). ( 5 ) H. Henkin and VI. Burton, rbzd , 60, 831
(1938).
(6) G. E. Adams and J. H. Baxendale, tbzd., 80, 4215 (19581. (7) J. D. Strong and J. G. Burr, %bid., 81, 775 (1959).
120
T. J. HARDWICK
ether were made in the usual manner. KO measurable addition of hydrogen atoms was found with either compound. It is probable that aliphatic ethers are in the same category as aliphatic alcohols and paraffinssolvents producing hydrogen atoms on radiolysis, but relatively inert to attack by them. Evidence for this is found in Newton's work on the radiolysis of a series of pure aliphatic The initial hydrogen yield is of the same order as is found for the corresponding alcohols. lo Highly branched ethers have lower initial hydrogen yields than those with n-alkyl groups (cf., paraffins and alcohols). With increasing dose the hydrogen yield decreases, a result which occurs only when the reactivity of hydrogen atoms with the solvent is low. The hydrogen yield from pure isopropyl alcohol was 2.42 moles/100 e.v. ; with added iodine this yield dropped to about 1.6 moles/100 e . ~ . ~ Although no pertinent experiments have been carried out in the present study it would appear that aliphatic ethers, like paraffins and aliphatic alcohols, may be used as a solvent and source of hydrogen atoms. Aldehydes.-The reactivity of hydrogen atoms with several aliphatic aldehydes was measured. Individual yields were less reproducible than in other systems, with a consequent greater deviation in the slope and intercept of the kinetic plot. Among four aldehydes studied, viz., propionaldehyde, n-butyraldehyde, isobutyraldehyde and nvaleraldehyde, no significant difference in slope or intercept was observable. To obtain the best values, the modified least squares fit was made for all data on all aldehydes. The rate of addition (k4) is 6.6 X 10" cc. mole-' set.-' (.tl50/,); the rate of abstraction ( k ~is) 2.8 X loll cc. mole-' set.-' ("25%); kg/k4 = 0.42. Deviations greater than usual perhaps are due to the chemical nature of the aldehydes. Some dimers or polymers may be present, perhaps in equilibrium with the monomer. As the normal method of sample preparation includes a rapid cooling (to -lQOo) and a slower thawing, equilibrium between monomer and polymer may not always be fully established before irradiation. The modes of reaction have been interpreted as addition to the carbonyl group and abstraction of the carbonyl hydrogen, in a manner analogous to the reactions with aliphatic acids. The structure of the alkyl chain apparently does not affect the reactivity of the hydrogen atoms. Abstraction of hydrogen from the carbonyl group has been suggested previous1y1l-l6 as one step in the gas phase photolysis of aldehydes. The S. Newton, J . Phys. Chem., 61, 1488 (1957). (9) A S. Newton, ibid., 61, 1490 (1957). (10) W. R. McDonell and S. Gordon. J. Chrm. Ptiys., 23, 208 (1955). (11) J. A. Leermakers, J . Am. Chem. Soc., 6 6 . 1537 (1934). (12) F. Patat. 2, physzk. Chsnt.. B32, 274 (193R). (13) W.R.Trost, B. de B. Darwent and E. W. R. Straoie, J . Chem. Phga , 6. 203 (1947). (14: D. C Grahame and G. K. Rollefson, dbzd., 8. 98 (1940). (15) F. E. Blaaet and J. N. Pitts, Jr., J. A m . Chem. Sop., 74, 3382 (1952) (16) P. A. Leighton, L. D. Levanas. F. E. Blacet and R. D Rowe. tbid., 69, 1843 (1937). (8) A.
Vol. 66
value of 6 kcal. reported for the gas phase at room temperature ( P = l ) I 3 is higher than can be allowed from our results. Curiously, we find no previous suggestion that hydrogen atoms add to the carbonyl group. Ketones.-Hydrogen atoms both add to, and abstract hydrogen from, aliphatic ketones (Table IV). For dialkyl ketones the rate of addition is in the same general range, but individual values arc significantly different. The rates of abstraction vary widely, :md attempts to relate the reactivity to chemical structure have not been successful. I n view of the general inertness of alkyl chains to hydrogen atoms, as exemplified by alkanes, alcohols, acids, esters and aldehydes, one is loath to attribute the hydrogen abstraction reaction to simple abstraction from the alkyl group, although such a step'has long been considered part of the mechanism for ketone photolysis. l7 Although we know of no data for or against the postulate, it is proposed that hydrogen abstraction takes place from the enol form, e.g. R-C=CH2 I
+ H --+RC=CH2 I
$. 132
(7)
I
OH
0
-+RC=CH 4- Hz
(8)
A
H
In the enol form, hydrogens on both the olefinic carbon and on the oxygen are expected to be reactive. Addition of hydrogen to the olefin group of the enol would in one case give the same product as addition to the keto form. RE.4CTIVITY OF
TABLE IV HYDROGEN ATOMSWITH T = 23 =k '1
Ketone
KETONES
ka k6 (addition) (abstraction) 101' eo. mole-' aec.-l
x
Acetone Methyl ethyl ketone Diethyl ketone Methyl n-propyl ketone Mcthyl isopropyl ketone Methyl isobutyl ketone Diisopropyl ketone Cyclohexanone Acetophenone Benzophenone
3.2 3.2 3.8 2.8 3.9 3.0 3.5 6.8 12.0 27.7
Dichlorotetrafluoroacetone
10
1.6 1.5 1.7 0.75 0.6 1.32 0.7 2.4 l o l l cc. mole-l sec.-l. Sulfur Compounds. (a) Carbon Disulfide.-Hydrogen atoms react with carbon disulfide by addition; k, = 26.4 X loll cc. mole-l sec.-l. (b) Dimethyl Disulfide.-Hydrogen atoms react by addition only; IC4 = 44 X 10" cc. mole-' sec.-1. This confirms the known high efficiency of this compound as a radical scavenger.
Vol. 66
(c) &Butyl Mercaptan.-This compound proved relatively inert, k4 < l o l l cc. mole-' sec.-l. Part of this reactivity may be due to disulfide impurities, which would markedly increase the apparent reactivity. Spot checks with other simple mercaptans confirmed a low reactivity with hydrogen atoms. Free Radicals.-Diphenylpicrylhydrazil (DPPH), a stable free radical, is sparingly soluble in nhexane (-0.5 mM/l.). Various concentrations, made up by weight and checked by e.s.r., were irradiated. Although the decrease in G Hwas ~ small, the data mere sufficiently reproducible to demonstrate that the kinetic plot gave a straight line of intercept 0.32 =!= 0.06. Assuming no hydrogen abstraction, 1r4 = 1.05 X cc. mole-l see.-' (*10%). A second free radical t-Bu
\
/
/
\
t-Bu
O a = C H - o - __ - O t-Bu'
t-Bu
(Coppinger's Radical-CR) l9 was found to be more soluble in n-hexane. The hydrogen yields from the system n-hexane-CR, plotted kinetically, gave a straight line of intercept 0.320. The rate of hydrogen atom addition was found to be 9.0 X 10l2cc. mole-l sec.-l. The kinetics indicate that these radicals react as true hydrogen atom scarengers. As might be expected the rate of addition is the highest which has been measured in our experimental work. Miscellaneous Aromatic Compounds.-The reactivity of hydrogen atoms with a number of aromatic compounds is shown in Table VII. All react by addition only. Roughly speaking they can be divided into two classes, the first group having reactivities below 6 X cc. mole-I sec.-l, the other group having reactivities above 10 X 10l1 cc. mole-l sec.-l. To the first group can be added the alkyl benzenes (Tables V and TI). To the second group can be added acetophenone and benzophenone. In the first group the compounds are similar to alkyl benzenes in that a relatively inert group is attached to the benzene ring. For comparative purposes one-half the value of the rate constant for bibenzyl and diphenylmethane should be used, for this gives the rate of attack for one benzene ring. Assuming that steric hindrance is the same for monosubstituted groups, the reactivity due to substituents increases in the order H < C,&, < C6H&Hz < CH, < alkyl < KHz < OH < CH20H. The distinguishing feature of the second group is that all compounds have a reactive group conjugated to the benzene ring. As might be expected this results in greatly increased reactivity. The mode of attack of hydrogen atoms is not known, except that reaction occurs by addition. This uncertainty, coupled with the varying sizes of the compounds, nullified any attempts to assign reactivity to structure in this group. Normally benzyl acetate would not belong to (19) G . A I . Coppinger. J. Am. Chem. Soc., 79, 501 (1967).
,Jan., 1962
REACTIVITY O F HYDROGEN !LTOMZS
123
WITH ORGANIC SOLUTES
TABLE VI1 REACTIVITY OF HYDROGEN ATOMSWITH SOMEAROMATIC
COIIPOUNDS T=23&1' k4 (addition) X 1011 cc. mole-'
.50
sec. -1
Aniline 4.0 Phenol 4.2 Bibeiizyl 4.4 Diphenylme thane 4 6 Benzyl alcohol 6.0 8.8 Benzoic anhydride Beneonitrile 10.0 Kitrobenzene 12 5 Biphenyl 13.5 Toluonitrile 14 Monoisopropylbiphenyl 14 Naphthalene 14.6 Triphenylene" 15 Acenaphthene" 15 or-Methyhaphthalene 15.5 Phenanthrenen 18 o-Terphenyl 20 m-Terphenyl 20 trans-Stilbene 21.9 5-Butylanthraquinone 23.5 Benzyl acetate 27 Anthracene" 31 p-Terphenyl" 37 Tetraphenylbutadiene" 39 Compounds were insufficiently soluble to give data over a suitably wide concentration range. An intercept of 0.316 was assumed, and a mean value of the slope from several oointR was used to determine relative rates.
the group, for the structure is not obviously conjugated, nor would the sum of the reactivities of the benzene ring and the ester add up to the experimentally determined rate. It may be that for this compound a hydrogen bond exists between the ortho hydrogen and the doubly bound oxygen group> in effect giving two conof the densed six-membered rings. The results in the system t-butylanthraquinone (TBA) in %-hexane are in contrast to those obtained by Dewhurst.20 his however, solutions of 1 and 2 m&f TBA were irradiated to -5 Mrad,, probably producing 10-15 mM/l. of unsaturated products. It is suggested that for much of the radiolysis hydrogen atoms &sappeared by reaction with these unsaturates, rather than by reaction with TBA. Effect of Temperature.-Radiolytic hydrogen yields from pure n-hexane a t various temperatures are shown in Table VIII. A slight but steady decrease is observed with decreasing temperatures. The radiolytic hydrogen yields from so~utionsof methacrylate (MMA) in n-hexane were measured i n the same temperature range. The results for three temperatures are plotted kineticj the -intercept ~ ~G~ is~obtained, ally (pig. 2). ~ and from the slope and G ~ the , relative values kg/krl are obtained. The results are in Table VIII.
.45 N
I W
a -
\
.40
.35
I
VALUESOF GI, Gz, GH?(~) AND ks/k4
AT
VARIOUSTEMPERA-
TURES
GHW G1
zi,k4
(*o
03Y
Temperature 23" 00 -40 -190 5 28 5 21 5 17 5 14 2 12 2.06 2 01 3.16 3.15 3 13 1.92 X 10-8 1.66 X 10-3 I 79 x 10-3
a Indicates the reproducibility in a t least four runs a t each temperature.
tween $50 and -200. Taubman, et a1.,22 reported no significant change in the radiolytic hydrogen gas yield from %-octane and ?%-decane between -60 and +go", although no limits of error were given. Dewhurst20 reported GH~W) = 3.5 at -78" for %-hexane; in Preliminary experiments we find GH?~O)= 4.8 at this ture. Previously we had reported that the change in GH*(~) for several alkanes was less than 3'% between -25 and +400.2 It Perhaps is surprising to find the yield of hytemperature, while drogen atoms unaffected the L'mOleCU~ar" or "hot atom'' hydrogen decreases with temperature. KO satisfactory explanation has been found* The ratio 1 d k 4 varies with but has a minimum value a t about 10". This result is an indication of the complicated nature of the diff usioii and competitive reaction processes taking place in solution. Attempts to interpret the data
*
O a 2
124
T. J. HARDWICK
Vol. 66
are hindered by a lack of knowledge of certain known accurately. In view of the many corparameters (e.g., what constitutes a collision be- relations it is questionable whether results retween a hydrogen atom and a solvent molecule) ported here are known to better than a factor of and by the general elementary state of the theory 2 or 3 or ai1 absolute scale. On the other hand of diffusion processes in liquids as applied to a evidence was presented in a previous paper2 which system such as the present one. indicates satisfactory agreement between the An understanding of the effects of temperature rates of H atom reactions in liquid and in gas on hydrogen atom reactivity in the liquid phase phase. As more experimental evidence accumumust await further theoretical development. In lates the uncertainty in absolute values will be the meantime it mould appear most unwise t o less. For the present, we shall continue to obtain assign energies of activation to hydrogen atom relative rate constants (Jc3/k4 and k6/k4) with the processes in solution when measured by this greatest possible precision. radiolytic method. TABLE IX Comparison of n-Hexane and Methanol as SolREACTIVITY O F H ATOMSWITH SOLVENTS vents and Sources of Hydrogen Atoms.-As indiCOMPARISON OF kH+n.HEXANE WITH kW+biPTHANOL cated in the section on alcohols, and as shown by Rate constant ratio the experiments of Strong' and Baxendale,6 ali(k4 Jr ka) Ratio phatic alcohols may be used as solvents and i n ka kHtn-Hexene Solute Methanol %-Hexane kH+ Methanal situ sources of hydrogen atoms in the same manner 713 127 5.6 as saturated hydrocarbons. In a series of experi- ciq-Penttme-2 822 127 6.8 ments the hydrogen yields from pure methanol and Cyclohexene 106 25.6 4.1 from methanol solutions of (1) methyl methacry- Benzene 520 5.8 late and ( 2 ) benzene were measured. For pure Methyl methacrylate 3020 methanol G H ~= 3.96, in good agreement with some 5.6 f1.0 workers16J0but lower than was found by others.*3?2* It perhaps is more important a t this point to The hydrogen yields for the solution, plotted kinetically, gave G2 = 2.52, G1 = 1.43. Data designate the extent of systematic aiid random of Adams6 using benzoquinone as a solute give errors associated with the present experimental method. Systematic errors are (1) those involved substantially the same result. in apparatus calibration and operation, and are Hydrogen yields were measured using two other solutes, cyclohexene and cis-pentene-2. Both ad- considered to be low (5 x 10" cc. mole-1 see.-% f 3y0 Thc most suitable region for making hydrogen yield measurements is at that in which the solute In some instances (marked with a superscript a in concentrations give GII* = 4.2-3.0 (region B). Table VII) the solubility of the solute in n-hexane Ideally this should cover at least a three-fold range is too low to obtain a suitably wide range of solute concentration. The error accordingly is larger. in solute concentration. Acknowledgment.-The author wishes to thank Usually a compromise must be made. I n the case of esters the reactivity is so low that GH, = the Nuclear Physics Section for the operation of the 4.0 a t 2% concentration. With methyl meth- \-an de Graaff accelerator, and Mr. €1. 0. Strange acrylate (k3/k4 = 2.55 X 10'2 cc. mole-' s a - ' ) the for the preparation of Coppinger's radical.
COI1SPAItATIVE STUDIES ON THE DECAIEBOSYLilTION OF I'ICOLINIC ACID AND JIALOSIC ACID ISTHE MOLTEX STATE ASD IX somrmx BY LOUIS\YL4T'l'S CLARK Department
OJ
Cheiriistry, Western Carolina College, Cullowhee, No7 th Carolina Received August 10, 1061
ICincitic data are rcportcd for the decarboxylation of picolinic acid in the molten state and in p-cresol, aniline, phenctolr, i:-c~lilorophenetole,p-dimethouybenzene and nitrobcnzenc, as wcll as for the decarboxylation of malonic avid in p-dimethosyImizcnc. The p:tramcters of the Eyring equation are calculated and a possible mechanism of thc reaction is suggested.
Schenltel and Klein studied the decarboxylation of picolinic acid in the molten state as well as in
out in this Laboratory on the decarboxylation of picolinic acid in the molten btate, as well as in the solverits phenetole, p-cresol, nitrobenzene, pchlorophenetole, p-dimethoxybenzcne and aniline. For the sake of comparison the decarboxylation of malonic acid was studied in pdimethoxybenzene. The results of this investigation are reported herein.
scvcral polar solvents by means of the loss in weight, allowing the GOz to escape.' Cantwell and Brown studied the decarboxylation of this acid in acid, Imsic and neutral media by weighing quantitatively the amount of COZevolved in measured time intervals.* Results of these studies led the authors Experimental to favor the zwitterion as the initial reacting form, Reagents.-Picolinic acid, 99.5% assay, and malonic and to propose a urlimolcciilar mechanism for the acid, 100.07, assay, were used in this research. The rewtion. They found, nevertheless, that the rate solvents used werr rragent grade or highest purity chemicals of decomposition of picoliriic acid mas lowered and and were freshly distilled immediately before use. Apparatus and Technique.-The apparatus and techthe activation energy raised by both acids and bases. nique used in studying the decarboxylation of picolinic acid l'raenkel and co-workers have shown that, in the and malonic acid in polar solvents (involving mrasuring the dcc:xboxylation of malonic acid in polar liquids, volume in ml. of COZ evolved a t different time intervals) a bimolecular mechanism is involved, and that, in has been described previously.8 The thermometer used the rate-determining step, the electrophilic, po- to dcterrnine the temperature of the thermostat, as wcll the buret in which the Cot was collccted, were calilarized, carbonyl carbon atom of a carboxyl group as bratcd by the U. S.Bureau of Standards. In each eyprriunites with an unshnred pair of electrons on a mcnt a 0.2212-g. sample of picolinic acid, or a 0.1870-g. nucleophilic atom of ~ o l v e n t . ~Other unstable sample of malonic acid (the quantities ncetled, respectively, acids, including oxalic acid,4 oxamic acid16 and to furnish 40.0 ml. of COZ a t STP on complete rcaction) weighed quantitativrly into fr,zgile glass capsule ant1 oxanilic acid16as well as the trichloroacetate ion,' was was introduced in the usual manner into thc reaction flask have been shown to dccompose by this same containing approximately 60 ml. of solvent previously mechanism. It appeared logical to assume, there- saturated with dry COngas. I n the study of the dccarbouylation of molten picolinic fore, that other unstable acids, including picolinic acid the same apparatus and technique as was ustd for aciid, should undergo dccalbvxykttion by a mech- studying tlic rcaction in solution W ~ L Hadopted, exrcpt that , uiiism simiktr to that of malonic acid. In order to for the 3-neck flask, there was substituted an IJ-shaprtl test this possibility kinetic studies have been carried 10 rnm. 0x1. Pyrex brand glass tube 30 cm. in length, sealid ( 1 ) 13. Sohenkrl and A. K l ~ i n .Ilela. Cham. Ada. as, 1211 (1915). (2) N. €1. Cantwell and E. V. Brown, J . Ana. Chem. Soc.. 76, 41613
(1053). (3) C. Braenkel, R. 1,. Bvlford and P. E. Yankwich. ibad., 7 6 , 15 ( 1 9 j I).
W. Clatk, J . Z'hys. Chem., 61, 699 (l9;7). (5) I,. W.Clark, abad., 65, 180 (1961). ( 6 ) L. W. Clark, rhad., 65, 572 (1061). (7) L. W. Clark, a b t d , 63, 99 (1959). ( 4 ) I,
a t one end, and provided with a 19/38 standard titper outer joint at the other end.
Results The decarboxylation of molten picolinic acid was studied at 5 different temperatures between 1G7 and 1 8 8 O , the experiment being performed twice (8) L. W. Clark, rbad., 60, 11.50 (1956).
