REACTION OF HOTH Y D R W E ATom N
June, 1960
WITH
CARBOXYLIC ACID
785
THE REACTIOS OF HOT HYDROGES ATOMS KITH CARBOXYLIC ACIDS' BY ALI M, EL AT RASH,^ RUSSELL H. JOHNSEN AND RICHARD WOLFGASG Chemistry Department, Florida State Vnirersity, Tallahassee, Florida Received January 11, I960
The reaction of hot hydrogen atoms with a nuniber of liquid carboxylic acids has been studied. Recoil tritium from the nuclear reaction Li6(n,a)H3 as used as the source of the hot hydrogen. Labeled hydrogen ( H T ) , tritium labeled parent acid, CHaT antl other labeled degradation products which may be formed by the replacement of an atom or group by a tritium atom account for nearly the entire yield. These products arc, with the exception of HT, insensitive to the presence of IZ as a radica,l scavenger. This suggests that the primary reaction of hot atoms in the gas phase-a high-energy fast dieplacement machanism-is also operative in the liquid phase. The distribution of products reveals that a hot displacement reaction may take place a t any type of bond and that its probability is proportional to the number of bonds of that type. Several trends among these competitive modes of displacement attack emerge clearly: e.g., attack on C-H is more likely than on C-C; attack on C-H yields more HT than labeled acid, especially with secondary C-H bonds; etc. The stability of a lniieled species toward dissociation due to excitation introduced by the displacement reaction, has a detectable effert on the yield nattern. Comparison with data on hot hydrogen reactions with gaseous hydrocarbons suggests that the same simple mech:tnistic model of the hot displacement process is applicable to both phases.
Introduction Recoil tritium from the nuclear reactions Li6(n, a ) H 3and Ile3(11,p)H3has been used as a sourcc of high kinetic energy hydrogen atoms to study the properties of that species. The kinetics of its reaction in the gas phase have been determined.3-5 The chief mechanism of the reaction of such hot hydrogen :zt,oms with gaseous hydrocarbons appears to be a direct displacement reaction. I n sysCH, it was found that a large tems such as T fraction (about 70%) of the tritium atoms pnrticipate in a displacement reaction by virtue of their high kinetic energy t o form HT, CH3T and CH2T. The remainder lose their excess energy by collision and then undergo thermal reactions. The resctions of "hot" tritium atoms have also been investigated in condensed phases such as solid glucose,6-iliquid inethyl alcohol, ethyl alcohol and acctone.s The results of these studies also suggest a mechanism in which a hot hydrogen atom displaces an atom or a group in a one-st'ep reaction to produce t'he labeled parent compound. In the present work we have extended the investigation of hot hytlrogcn at'om react'ioiis to a homologous series of carboxylic acids, including the following: acetic, propionic, n-butyric, isobutyric, n-valeric, isovaleric, t-yaleric (pivalic), n-hexoic, and isohexoic. The nuclear react'ion Li6(n,a)H3 mas used as thc source of thc hot t'ritium atoms. The immediat'e purpose of the work was: (I) t'o survey reactions in the liquid phase and compare them with those in the gas phase; ( 2 ) t o determine the validity of a fast, localized direct displacement mechanism us. other possible mechanisms in which an intermediat'e well-defined transition complex, .rvith an equilibrium distribution of energy in its various de(1) This work \vag supporter! in part by the U. s. dtonlic Energy
+
Commission un-ier Contract dT-(40-1)-2001 and in part by the FSU Research Counc!il. (2) Abstractcd from a thiws aubmittrc! h y A . 31. Elatrash in partial fulfillment of t t e rcquircmrnts for t h e degrce of Doctor of Philosophy. ( 3 ) P. J. E s r u p a n d R. Wolfgang. J . A m . Chem. Soc., 82, 2G6l (1960).
( 4 ) AI. Arnr ElSayed and R. Wolfgang, i b i d . , 79, 3286 (19.57). 62, 1356 ( 5 ) M. Amr EISayed a n d R. Wolfgang, THIS JOURNAL, (1958).
(6) F. S. Rowland, C. P. Turton and R. Wolfgang, J . A m . Chem. Soc.. 78, 2354 (1956). ( 7 ) H. Keller and F. S. R o w l a n d , THISJOCRNAL,69, 1373 (1958). ( 8 ) W. J. H o f f ,,Jr., antl F. 6 . Rowlami, J . A m . Chern. Soc., 79, 4867
(1957).
grces of freedom, decays in a statistically determined manner to give the various products; (3) to study the mechanism of the various cbompetitive modes of the direct displacement process. Experimental Materials.-.411 the acids used in this work were pure reagent grade and were further purified by fract.iona1 dist'illation (except pivalic arid) in a 60 cm. column packed with 3/16'' Pyrex helices. Center cut,s having a boiling range of less t,lian 0.2" were used. Pivalic acid was prepared by a standard method,S and was purified by fractional crystdlization. Sample Preparation and Irradiation.-Solutione of anhydrous lithium chloride in the various arids wtw prepared in a dry boxlo having a t room temperature a relzttive humidity not greater than 5%. The concentration of these solut,ions ranged from 470 by weight for acetic and propionic acids, 27, for butyric acids to lyofor valeric and hexoic acids. To prepare iodine cont,aining solut,ions, ten milliliter fractions of the acid-lithium chloride solutions were t,ransferred to a volumetric flask and the amount of iodine required to make the necessary concentration was added. Solutions of different iodine concentrations were preparrd for acetic acid, ranging from 2.4 x 10-4 M to 1.4 x 10+ M . For the remaining acids solutions 5 x 10-3 $1 in iodine wcrc prepared. Quartz ampoules of about 10 mm. 0.d. and 10 em. long with a breakoff seal were used for irradiation. All samples were thoroughly degassed by repeated freezing, pumping and thawing. They were then irradiated in the Brookhaven Laboratory graphite reactor using the water-cooled facility. I n order to produce the same number of tritons, solutions of different concentrations in LiCl were irradiated for varying lengths of time. The 4% solutions were irradiated f o r one hour, the 2% for two hours and t,he 1% for four hours. Some of the samples were irradiated a t a flux of -1.7 X 1OI2 n/cm.2/sec., while others a t a flux of -3.9 X 10" n/cm.2/sec. In order to determine the flux to which rwh group of samples was subjected, a solution of LiCl in m t c r was irradiated with each group and the activity of the n-aLtcr sample was used as a monitor for the neutron flux. Tritium Assay: Gaseous Products.-After irradiation, samples were opened in a vacuum system antl cooled n i t 11 an ordinary ice trap. Thus only those contituents whic,h have a reasonably high vapor pressure at 0" wcre determined. The radioassay of thc gaseous activity was made by :t p : : ~ chromatographic method described by Kolfgang and Revland." An aliquot of the sample was injected into a strc:tni of helium passing through a c,hromatograph column. Aftc.r separation the gas stream passed through a thermal conductivity cell which detected and recortled an?- rnacrosco1)i:. amounts of material. The helium was then converted into a counter gas by continuous injection of methane and flowed through an internal flow proportional counter. The
i8ti T.4BLE EFFEt'T OF
Itun n u .
8
SIJSIE ~ X P E R I J I E N T I Ll'ARAMETERS
Iri.ai1. conditions
So air present
1
32 ID 1 16
A4irpres. 115; H2C) added 5 x 1 0 - 3 -11 I?
lilrix X 10-12 (n/cin .?/set.)
1.7
1.7 :%,9 3 9 ,
1.i
1.7
1 1 T.. l < ~ ~ i l ~: ~: iw lIV, f;, 13
C-T A c t . i n parent acid
"c
70Tot. T
(c.p.m./mg.i x 10-3
C b Tut. T a s C-T in parent acid
1 -1
1 1
3.3
'. 1
2 2
58 58
1
61 53
4 0 4 0 10 5 10 ti 3 3 4 0
18 18 19 l!) 1; 18
2 4 4
Results Table I shows the effect of several variables on the per cent. entry of recoil tritium in liquid acetic acid. It is observed that the flux, the presence of oxygen or iodine, both of which act as radical scavengers, and also the presence of small amounts of water have 110 effect on the yield of the parent. acid. Similar results were observed for the ot'her acids investigated. The distribution of activity anioiig the various t,ypes of labeled products foriiied in the absence of 1 2 is shown in Table 11. The perceiit'ages listed are t'hose of the total calculat'ed actiI-it,y using water as a monitor for the neutron flux. Column 2 lists the artivit'y of the gross liquid before t'he separation of thc parent acid as the p-toluidide. Column 4, l‘>
0 .1 6
18 17
.21 1I) 08
13.0 3 3 .0 1.3 19 72.0 31.6 0.96 18 e c 1i 72.0 27.8 1, I 18 l’cr cciit. listcd iii I, nnis :trv coinputctl on the Ixisis of t,hc t,ot:tl measured gtseoua x t i v i t y i i i ruris xhvre iodine Per cent. listed is that of the total calculated activity. Xot determined. :iltscnt ( i . ~ .r r, i n Yo, 81.
I :1 16
17:3,000 164,500
--
W ~ F
TABLE FROM
Tiitiatril product
REACTIOS 01,‘RECOIL T ~~
.icetic
I’voliionic
71
WITH
L i ~ c i uA i . i i w k * r i c , .icii)s I I:! I’RESENI, !5 y 10-J -11)
.icid irradiated % ‘1
__ But .vric---
~
i
71
v2]cric---f
t
-
78 68 7” 0 73 Hydrogen 78 0 71 . 9 88 1t i 4: 8.4 Jlcthanc :iI (j t3.8 2 8 i 1 1. < I 0.2 I .:3 0.7 0. I 9 Ethane I ) . !)ti 1S.5 0 . (i :3 0 .4 .3 1 Ethylene 0 13 1.8 0 ,2 2 0 1 I . -) Acetyleiie 0.22 1.5 Propane 0.08 0 .I 6.(i 6.0 I .o 8 .I l’ropylcric 0.6 1 4 0 2 -I 4 i-Biitmo , . . , 0.07 2 1 11 2 tr-131it.ttnc _. 0 . -1. 0 . 0-i ... :3.1 1. 5 0 1 13utylene . . ... ... . . . 0 . :< i- Ptat’anc: .. ... ... . . 0 15 n-Pen taiit: .. ... 0.08 . . listotl is computed froni t h e ohscrvt:tl activity iii identical control r u m wlicrt: I, is a b w i t . 0.1 .1[. . . Viidctectable amorints. ,
pecially TI, which would not be det,ec*tedby t8he aiialyt,icalmethod used, mould be expected. I t is observed that in both cases a large percentage of the gaseous activity is in the form of HT and is relatively higher for the normal acid than for t’he cwrrespondjng iso-acid. It is also to he noted that the activit,v in CHaT is about two to three times as grcat for t6e branched chain acid as for the normal acid with the same molecular weight. In addition to hydrogen, methane and the hydrocarbon forined hv decarboxylation of the parent acid (H.C.A.)are among the main gaseous products. In the presence of iodine as a radical scavenger the production of labeled hydrocarbons higher than that of thcl parent arid, i.c.,“synthesis” products, is rcltlric*c>ti ))tit i i o t cmnplctcly (~liniiiinic~tl.‘I’liis i q
--
-.
~~~
,,
Hemic---
80 2 5,5 1 :3 0 2 0. I 1.o
1
79 7.4 0 ,3 )
,I
.I .8 . :i
0 oti 1
0 ti
.o
0.08 2,1
1.7
‘i’hv iodine cwncentr:t-
rlearly demonstrated with propionic acid where I? concent’rationswere as high as 0.1 JI. The hydrogeii yield is slightly decreased. However, the yield of t,he labeled hydrocarbon of the parent arid ( e . g , , methane from metic, ethane from propionic, etc.) as well as the yield of other degradation products is uiiaffccted within the exl7erimerita.l error.
Discussion 1. Hot 11s. Thermal Reactions.--There arc three experimental criteria for establishinq that a reactioiiis caused hy hot atoms. (1) Diminution of the yield of hot product by the addition of a large excess of moderator, i e . , chemically inert substances such as helium which remox-r the excwa kinetic. ellcvgy from the hot atom. ( 2 ) T i i w i i h i t i I ity of yirltl
788
A. ill. ELATRASH, R. H. JOHNSEN AND RICHARD WOLFGANG
to temperature (since the hot atom provides essentially all the required activation energy). ( 3 ) Insensitivity of yield to small amounts of scavengers which react avidly with thermalized species. Most thermal reactions will be inhibited by such scavengers since the species causing them usually undergo many collisions before reacting and thus are liable to encounter and be trapped by the scavenger. Hot species are not sensitive to small amounts of scavenger since they must react in a few collisions, before losing their excess energy. The hot displacement reaction mechanism of recoil tritium with gaseous substances has been established by all of these ~ r i t e r i a . 4 ~I n the liquid phase, however, the use of only the scavenger technique has been practical to this time. Thus the finding of Hoff and Rowland8 that the products of reaction of recoil tritium with acetone and alcohol were insensitive to the presence of diphenylpicrylhydrazyl may be taken as evidence that the reactions proceeding were due to a hot mechanism. Similarly in this work, we find (Tables 111, IT’, and T’) that the presence of 5 X ill 1 2 eliminates only of the order of 10% of the gaseous products. (This 10% presumably forms T-labeled iodides which are not detected). Formation of the labeled form of the acid irradiated is apparently completely independent of the presence of either IZor 0 2 as scavengers (Tables I and 111). Conversely, those (rather minor) products which are affected most by the presence of IZare all “synthesis” products which, since they contain more atoms than the original acid, cannot have been formed by direct hot reaction. These products, for instance CzH,T and C3H7T formed in the reaction with acwtic acid (Table 111) presumably were formed hy interaction of thermal tritium with radiation produced species. Again, this is in complete analogy with gas phase systems.g ~ 5 However, in contrast to the gas phase situation these synthesis products are not entirely eliminated by scavenger-even Iza t a concentration of 0.1 14 (Table Sr). This residual yield-which corresponds to less than 1% of the recoil tritium-presumably is due to the combination of tritium just thermalized with a radical held in the same solvent cage. Such tritium will react before it can be reached by the scavenger. The yield of IIT is also somewhat affected by the presence of Iz. It appears that most of the tritium reaching thermal energies abstracts a hydrogen atom from a molecule of acid. This is the expected reaction for thermal hydrogen.13 2. Nature of the Hot Reaction.-For the hot reaction of hydrogen with saturated compounds in the gas phase a simple displacement mechanism in which the entering hydrogen replaces an atom or group h:is been postulated. This general mechanism also appears dominant for liquid acids sincc nearly all the observed products (99Oj,) can be made by breaking a single bond and replacing the atom or group removed with tritium. The low yield of rearrangement or isomerization products (e.g., see pentanes and butanes from hexoic acids ed
(13) E. XV R Steacie, “Atomic and Free Rartiral Reactions,” Prid , Reinhold Pub1 Corp , New York, K Y 1954 p 551.
Yol. 64
and butanes from valeric acid) already has been remarked on and seems difficult to explain by any other mechanism. In particular, any mechanism involving an activated complex or transition state in which an equilibrium distribution of energy in the degrees of freedom is approached must be excluded. 3. Competitive Hot Displacement Reactions.In a relatively complex molecule the hot displacement reaction can occur competitively a t several different types of bonds, and the hot atom may combine with either of the entities on the two sides of the bond. We will examine here the relative importance of the various modes of hot displacement reaction. To a first approximation the amount of a given product depends on the fraction of the total bonds, which on being broken in a hot displacement reaction can give that product. This is illustrated by Fig. 1 which shows that the ratio of I-IT to CH3T is approximately proportional to the relative number of C-H and C-CH, bonds. The approximate constancy of the yields of the labeled form of the parent acid and of HT, particularly whrn corrected for the fraction of C-€1 bonds in the molecule also supports this generalization. The observations just cited suggest that the concept of an approximately equal probability of attacking every bond in the molecule may have some limited validity. Obviously factors involving the structure and stability of the entities involved will cause deviations from this overly simple model. To determine the nature of such deviations it is instructive to compare actual experimental results with results calculated with the “equal probability” hypothesis of the hot displacement mechanism. In Table V I this is illustrated for n-hexoic acid. The first column has been Calculated assuming equal probability of attack on each bond and that there is an equal chance that the tritium will join with the entity on each side of the bond The good qualitative agreement in Table T-I strengthens the previous conclusion that nearly all products can be formed by simple replacement without rearrangement. In similar calculations for the other acids the qualitative experimental distributions follow equally well. However. the quantitative disagreement clearly shows deviations from the “equal probability” hypothesis. These deviations are summarized below with comments on their probable origin. (a) In every case except one, the yield of €IT is higher than that calculated by random attack, even when the thermal contribution is allowed for. (The exception, isohexoic acid, may be due to experimental error since the absolute ralues of gaseous activity are not accurate, being obtained by dlfference between liquid activity and total calculated activity.) Further, in most cases activity found in labile form and in liquid products other than the labeled parent acid is low. This indicates that attack on a C-H bond is relatively more probable than attack on any other. X similar conclusion may be drawn from results on reaction with gaseous ethane4 and other hydrocarbons. Since the C-H bond is a relatively strong one its higher susceptibility must be ascribed to its greater ex-
June, 1960
RE.iCTION O F
HOTHYDROGIX .*ITOMS WITH
posure to attack and to a greater ability to absorb the energy of and stop the hot hydrogen atom. RIXULTSOF
4 TT \(-K
CIF
TABLE VI Hwr TRITIUM ON
SORA141,
CARBOXYLIC
ACID
789
35
30
HEXOIC
‘lc.11,
“0 total nctlvlty Calcd. o n ’ ‘ eq u a 1 probability” hypothesis Exptl.
HT CHaT C2Hs’r C3H7T n-C4HgT
z-C~H~T n-C5H11T z-C~H~~T
t-CsHnT Labeled parent acid Labile T and Jtlwr ! ~ r p i ~IirodiictP d Other prodiicts
32 6 2 6 2 6 2 6 2 6 0 0 2G 0 0 0 0 29 0 26 0 0 0
46 0 1 3 0 6
.4 .3 0
.o .o .o
27 0 23 0 0.2
(b) -ittack on a C-€I bond is more likely to yield H T than the labeled acid. Further, the yield of H T as a “hot” product is consistently higher with normal than with branched acids. Thus with nvaleric arid HT accounts for 65y0of the total gas yield, while with isovaleric acid it represents 48% and with t-valeric acid 43%. Evidently HT is more likely to be formed with secondary than with primary C-H bonds. As a corollary to this, a higher vield of labeled parent compound would be expected whcn interaction is with primary C-H bonds. This was observed by Hoff and Rowland,* studying C2HsOH,who found that the specific activity per I€ atom was higher in the CHI moiety than in the CII,. With the acids we would therefore expect a higher yield for the branched compounds and this is actually observed for isohexoic and isovaleric acid. However, the yield of labeled isobutyric n4d is about the same as that of n-bittyric arid and that of t-valeric arid as that of nvaleric acid (see below). This vari.ition In the ratio of H T to labeled parent was also observed in qtiidies of hot hydrogen attack on gaseous hydrocarbon^.'^ I n thr hydrocarbon study this trend was quantitatiyely predictable, on the basis that HT production followed attack along the axis of the C-H bond, while displacement cf H to form the labeled parent followed attack by the T toward the carbon atom. Tarying, but predictable, steric obstruction in different systems ton-ard the latter type of attack, causes a variation in the yield ratio of H T to labeled parent which correlates approximately with the ratio of secondary to grimarv C-H bonds. The fact that the ratio of HT t o labeled carboxylic acid follows a similar trend indicates that this model of the displacement process, in which the point and direction of impact of the hot atom primarily determines the course cf the reaction, is applicable in liquid as well as gas phase. The lower than expected yields of &valeric and isobutyric acids may br explained in terms of ail excitation ricclhnnism. A hot displacement warI
11) I> l-rrll
111
I T1 \\ > l f A l r + ,
I
I I H C‘hc m
\OC
111
pzt.+
25
Prop.
a
20
%HT */e
CH
?
3
/
15
10
5
0 4
6 e - C - H Bonds # -C-CH3 Bonds
#
,
IO
12
Fig. 1.-Relation between labeled products and the rrlative number of bonds which on rupture give rise to these products.
tion by an incident hydrogen atom is likely to leave a certain residual excitation energy in the molecule. In t-valeric and isobutyric acids the carboxyl groups are attached to the carbon atom where branching occurs. Decarboxylation should therefore proceed more readily than with the other acids since the relatively stable tertiary and secondary radicals, respectively, are formed. Independent evidence for this view derives from the finding1that the G-value for the production of COz from isobutyric acid irradiated with C060 y-rays is much greater than that for isovaleric, isohexoic and nbutyric acids irradiated under the same conditions. (e) Rupture of a C-C bond to form a gaseous hydrocarbon becomes progressively more likely as the end of the molecule away from the COOH group is approached. This conclusion is substantiated by the facts summarized in Table T’II. (Products formed by the rupture of the C-COOH have not been included here.) This means that the probability that T attacks the lst, 2nd, 3rd and 4th C-C bond from the CHa-terminal of the chain to form labeled alkane is approximately 4 : 2 :1.3:1. Work on the reaction of hot tritium with a number of gaseous hydrocarbon^'^ has shown that the position of the C-C bond does not greatly affect the probability of attack there. However, the same simple steric model that justifies this fact also prcdicts that the hot hydrogen is more likely to attack toward the sterically less obstructed side of the C-C bond. Thus, of the two possible products the Rmaller and less complicated is formed i n greater ( 1 5 ) 11. II.
J ~ ~ I i 1 i ~T
