1565
J. COLLINAND F. P. LOSSING
and 4, where the amino function partakes in salt formation, essentially linear behavior is obtained over the whole p H range. The lines have slopes varying from -0.04 t o -0.07. Though Hammett’s equation is applicable for the polarographic reduction of quinoxalines t o the dihydro stage, the influence of substituents is relatively weak. For example, Brockman and Pearson6 obtained slopes of -0.25 in their study of the polarographic reduction of benzophenones. The variations in heights of the second major wave with pH are influenced by the electronic nature of the substituents. The electron-acceptor substituents, bromo and chloro, cause the heights t o decrease with increasing PH whereas electrondonor substituents, ethoq- and inethoxy, cause the heights t o increase with increasing fiH up to and
[COSTRIl3UTIDN
ISSUEDAS s I < C NO, 4G43,
VOl. 80
including pH 6. These trends can be explained qualitatively on the basis t h a t catalytic activity increases with increasing dissociation of the 1,4-dihydroquinoxalinium ion. Electron-acceptor substituents cause the pKB’s t o be lower than for the electron-donor containing systems. In each instance the proton removed from the monobasic 1,4-dihydroquinoxalinium ion comes from N1, the “para” nitrogen. Acknowledgment.-M.P.S. wishes to express his appreciation to N. J. Klein for preparing the 6amino-, 6-chloro- and 6-methoxyquinoxaline1 to J. C. W. Chien for the 6-bromoquinoxaline and to C. H. Hibbitts for the 6-ethoxyquinoxaline. hlADISON,
KEW JERSEY
PROM TtIE D I V I S I O X OR P U R E
CHEMISTKY, ?;ATXONAL RESEARCH COUNCIL]
Mass Spectra of Propyne and Propyne-d,, and the Appearance Potentials of C3H4+, C3H3+and Equivalent Deuterated Ions Bs J. COLLIN’AND F. P. LOSSIXG RECEIVED OCTOBER 21, 1957 %: comparison cif the mass spectra of CH8CCH and CDaCCH sholrs that extensive migration of hydrogen atoms occurs in the propyne ion, the loss of E1 arid 1) atoms occurring on a statistical basis. The appearance potentials of the C3D3+and C3D&1’ ions from CLlaCCH do not show the difference in the energy of formation which would be expected if these ions were CI)&C+ and +CD2CCH,respectively. I t is coricluded that the CsDz+ and C&hIi+ ions are both propargyl ions, and that their formation involves an extensive rearrangement of hydrogen atoms. From a consideration of the value of D(HCaHa) derived from these appearance potentials it is concluded that the excess energy available for the rearrangement is appreciably less thari 9 kcal./mole, and is possibly almost zero. The ionization potentials of CsD$Hand CsHl are found to be the same within tile csperimental error.
Introduction From a study of the appearance potentials of the CaXI3+ ion produced from allene, propyne, butync and 1,2- and 1,3-butadienes by electron impact2 it was concluded t h a t this iori had the propargyl
+
(CHs-CrCH) structure. Xt might therefore be expected that the formation of the C3H3+ ion from propyne would be a simple process in which a H atom is spIit off from the methyl group, without any rearrangement of hydrogen atonis being involved. On the other hand, some preliniinary experiments on the mass spectrum of CD1--CsCII showed that, a t least with 50 volt electrons, the loss of H and D was in the ratio 1: 3, suggesting that in the C3H4+ion all the hydrogens were equivalent.$ Furthermore, the ionic heats of formation indicated that the formation of the C3Ha ion from 1,:?-butadiene and 2-butyne, for which a migration nf a hydrogcn atom is necessary, proceeds with little or no activation energy. In view of this, i t is of interest t o find whether the apparently straight-forward process for formation of the CzH3+ ion from propyne itself also involves a migration and, if so, whether the required activation e n e r p is sufficient to invalidate the determination of D(C:jH3-ICI) in propyne.’ f
(1) National Research Council of Canada r o s t d o c t ~ r : r t r F d l o w Institut d e Chimie Generale, TJniversit6 de Liige, l B 1956-1957. Quai Roosevelt, Lit.ge. Belgium. ( 2 ) J. Collin and F. 1’. Lossina, Trm J O U R N A L . 79, 5848 (1957) (3) J. B. F a r m e r and F. P. Lowing, C a n . J. Ciiem., 33, 861 (1955).
Consequently the abundance of CSions and Cz ions in the spectrum of propyne and of propyne-d3 have been compared at lower electron energies, and the appearance potentials of C3D3+ and CsDzH+ ions have been measured and compared with the corresponding ion in propyne itself. Experimental Thc propyne was obtained from Farchan Research Laboratories. The sample of propyne-d3, prepared by Merck and Company (Canada) was found t o be 97.7-98.0% CDa-C=CH, the remainder being mainly C3D21l2. Corrections were made to tlie mass spectra t o remove the small contributioas from this impurity. The mass spectrometer was a 90” Sier-type, and the method of measuring the appearance potentials was the same as that used previously.’
Results and Discussion The relative intensities of C3 and C? peaks in the mass spectrum of propyne a t different electron accelerating potentials are given in Tallle I. I n Table I I a are shown the corresponding peak intensities for propyne-d: I n both cases the peaks have been corrected for contributions from carbon-13, and the propyne-dz spectrum has been corrected for contributions from the propyne-dz impurity. I n Table I I b the spectrum of propyne-d3 has been simplified by combining peaks of similar composition to facilitate comparison with the propyne spectrum. It can be seen that the intensities are quite similar for CBpeaks but less so for Cz peaks. The most interesting feature of the spectrum in Table IIa is the ratio of the C3D3+and CaD2H+
MASSSPECTRA OF PROPYNE AND P R O P Y N E - ~ ~
April 5, 1958
1560
TABLE I MASSSPECTRUM OF PROPYNE AS A FUNCTION OF ELECTRON ACCELERATING POTENTIAL* m/e
Ion
40
50
40 CJHl 37.40 39.20 C3HS 32.20 33.00 39 C3Hn 12.58 13.03 38 37 CsH 9.20 9.13 36 C; 2.42 1.83 27 CoHt 0.25 0.28 26 CzHr .98 1.03 25 CzH .92 0.73 24 CI .37 0.15 0 The abundances are expressed
MASSSPECTRUM
14
12
13
...
Electron accelerating potential (volts) 17 16 18 19 20
25
30
TABLE IIa FUNCTION O F ELECTRON ACCELERATING POTENTIALa
38.81 40.23 45.45 49.71 63.21 65.94 67.88 73.40 7.97 8.72 9.49 9.73 9.28 8.56 8.51 9.32 17.34 21.49 21.30 25.36 25.25 24.87 27.11 23.56 6.00 4.93 1.39 1.13 0.80 0.44 5.62 6.06 5.73 2.20 1.78 1.23 0.78 6 . 6 1 6.67 6.94 4.10 1.43 0.13 0.01 0.01 ... 6.61 6.29 ... .04 ... ... 1.81 1.75 1.28 0.52 0.23 .02 .01 . . . ... ... 2.09 1.61 .21 .ll .03 .03 .02 .01 0.22 0.23 .06 .02 .02 .01 .01 .14 .13 .12 .17 .03 .02 .005 ... .91 .53 1.16 ... .13 .005 .06 ... .41 .27 .41 ... ... ... 1.15 .91 .24 .09 ... ... ... .70 .70 .32 .21 .07 ... .57 .31 .07 ... . . . ... ... ...
43 CSDIH 42 CtDi 41 C;DiH 40 CsDo 39 CaDH 38 CaD 37 CaH 36 CI 30 CtDt 29 CiDoH 28 CzDt 27 CzDH 26 CaD 25 CaH 24 c,
15
42.77 40.85 61.54 64.42 68.01 71.26 75.43 80.97 87.92 94.89 98.05 1.95 34.80 36.55 33.19 31.91 29.31 26.79 23.18 18.24 11.57 4.90 1.41 1.08 0.73 0.50 0.17 ... 13.21 10.99 4.01 2.88 2.06 ... ... 5.85 1.98 0.01 ... ... ... 0.21 0.01 ... ... ... ... ... ... ... ... ... ... .26 .14 .04 0.04 0.03 0.02 ... . . . ... ... ... ... .62 .31 .19 0.19 0.18 0.17 0.15 ... ... . . . ... ... ... ... ... ... .23 .09 ... ... ... ... ... ... ... .02 ... in percentages of the total ionization.
O F PROPYNE-d3 AS A
40
50
Ion
m/e
Electron accelerating potential (volts) 20 19 18 17 16
25
30
15
13
14
12
78.14 82.62 88.15 95.60 98.47 7.03 5.68 3.96 1.37 0.43 14.14 11.25 7.78 3.02 1.os 0.21 0.12 0.11 ... ... 0.43 0.29 ... ... ...
...
...
...
...
...
.01
...
... ...
...
...
...
...
...
... ... ... ... ... ...
... ...
... ...
... ... ... ...
... ...
...
...
... ... ...
...
...
..,
...
...
...
...
... ... ... ...
...
... ...
...
TABLE IIb MASSSPECTRUM 50
:on
C3DaH CIDl C,D2H . . CaDl CzDH CzD CaH CI CzDtH CeDs CzDz f CIDH CzH CoD C2
+
+
+ + +
30
40
25
OF PROPYNE-d3 (SIMPLIFIED) Electron accelerating potential (volts) 20 19 18 17 I6
.
I
peaks. These ions are formed by the loss of a Hatom and a D-atom, respectively, and the processes which might be expected would be + CD3-C=CH
f e-
+CD,-C=C
CD3-C=CH
+ e-
----f
+
+ H + 2e-
CDo-C=CH
fD
+ 2e-
(1)
(2)
Since the energies and frequency factors involved in these two processes should be quite different, it is surprising to find t h a t the probabilities of loss of H and of D correspond almost exactly t o the statistical probabilities. The removal of two and three atoms also seems t o proceed on a statistical basis. This is clearly shown by the ionic ratios of C3D3/ ( C ~ D Z H C3D3), C ~ D Z / ( C ~ D HC ~ D Zand ), G D / (C3H C3D) given in Table 111, the ratios being nearly l/4, and 3/4, respectively. This is especially true a t the higher electron energies. At lower electron energies the ratios increase appre-
++
15
14
13
12
38.81 40.23 45.45 49.71 63.21 65.94 67.88 73.40 78.14 82.62 88.15 95.60 98.47 1.51 34.92 33.i3 34.19 36.84 32.84 30.98 30.02 25.31 21.17 16.93 11.74 4.39 1.22 0.64 0.41 0.11 . . . ... 12.23 12.73 12.94 10.66 3 . 5 9 2.91 2.03 ... ... ... ... ... 8.42 8.04 5.38 1 . 9 5 0.17 0.01 0.01 ... ... ... ... ... 2.05 1.61 0.23 0.02 -01 . . . ..* . . ... ... .05 .03 0.02 0.01 ... ... ... 0.36 0.37 .34 .17 .05 ... ... ... ... 1.57 1.32 .SO .30 -03 .08 ... ... ... ... ... ... .. 1.85 1.61 .56 .30 .07 ... ... ... . . . . . . . . . . . . . . . 0.57 0.31 .07 ... ...
+
.
I
.
.
.+
ciably, particularly the ratio C ~ D Z / ( C ~ D HC3D2), although the change is perhaps not much greater than would be expected t o result from the increasing importance of zero point energy differences between C-H and C-D bonds as the available energy is reduced. Further evidence for the equivalence of the H and D atoms can be obtained from the appearance potentials of the C3D3+ and C3D2H+ ions as given in Table IV. Using these data and the quantities A&(propyne) = 44.32 kcal./mole4" and A.Hf(H) = 52.1 k ~ a l . / r n o l ethe ~ ~ following heats of formation are obtained from CsH,, AHf(C,H,+) = 270.2 kcal./mole from C3D3H,AHf(CsDzH+)= 274.0 kcal.inole from C3DaH,AHf(C8D3+)= 272.7 kcal./mole (4) (a) F. D. Rossini, el al., "Selected Values, etc.," Carnegie Press, Pittsburgh, Pa., 1953; (b) F. D. Rossini, e: ol., Circular 500, National Bureau of Standards, Washington, D. C . , 1952.
1570
J. COLLINAND F. P.LOSSING
IOXIC
R a t i o of ions CaDd(C3DzH CJDa) CIDZ/(CJDH CIDZ) CaD,'IGH CID)
50 0.245 ,460 ,786
+
+
+
40 0.252 ,524 ,782
30 0.273 ,536 ,763
TABLE I11 ABUNDAXCE R.4~10S
25 0.264 ,538 .733
Electron accelerating potential (volts) 20 19 18 17 16 0.282 0.306 0.291 0.315 0.332 ,819 ,612 ,606 , G39 ,672 ,765 ... ... ... , . .
TABLE IS' A P P E A R A S C E POTENTIA1,S
Potential
Propyne
Propyne-da
Ionization potential A(C3H3-1 *4(C;
