July, 1957
MASSSPECTRUM OF NEOPENTANE
of neighboring micelles. The shearing stress exerted by the micelles near the region of contact leads to the deformation of the micelles and an increase in the area of contact over which adhesion occurs. As rise of temperature reduces the rigidity of solids, adhesion will increase with rise of temperature and so also sintering. This process seems to have very little effect on the surface properties of magnesium oxide prepared by the dehydration of brucite under vacuum a t temperatures below 660" since there is almost no change in the specific surface area of the oxide with increase of time of heating, though the process is probably effective during calcination in the presence of air. On the other hand, surface diffusion resulting from the mobility of the ions of the solid a t its surface becomes sensible a t temperatures about one-third of the m.p. of the solid, while lattice diffusion where the ions can move through the bulk of the micelles occurs only at temperatures above half the m.p. The last effect probably does not play an important part in the present work as the highest temperature used is 1100" which corresponds to 46% of the m.p. Thus in the dehydration of brucite under vacuum
89 1
a t lower temperatures, the development of the surface area of the product is a direct consequence of the loss of.water from the hydroxide, and the time of dehydration will only influence the surface area of the product in as much as the amount of hydroxide decomposed is a function of time. The oxides prepared above 650" will, however, undergo increased sintering with rise of temperature and prolonged heating. The effect of increase of time will be quantitative but that of temperature rise will be qualitative as well, since this will produce different effects on the three factors which determine the rate of sintering. It is important to note that the effect of time and of temperature are so interrelated that increasing the time will not cause a continued decrease of the surface area indefinitely, but that each temperature of dehydration is characterized by a limiting surface area, and further experiments are required before understanding the relation between the rate of sintering and temp eratur e. Acknowledgment.-The authors wish to thank Professor M. Prettre and Mr. M. Perrin for their measurements of the low temperature nitrogen adsorption on calcined brucite.
REARRANGEMENT PEAKS IN THE MASS SPECTRUM OF NEOPENTANE TERMINALLY LABELED WITH ONE C13 BY ALOISLANGERAND C. PETERJOHNSON Contribution from the Chemistry Department, Research Laboratories, Westinghouse Electric Gorp., Pittsburgh, Pa.' Received August 0, 196%
Rearrangement peaks were observed in the mass spectrum of neopentane labeled in one methyl group with 0 8 . For the three- and four-carbon fragments, the results indicate they could be formed in the expected manner of carbon-to-carbon bond scission. I n the two-carbon fragments, however, the experiments seem to lead to the conclusion that the fragments result from an almost statistical combination of any two carbon atoms of the molecule.
Introduction When polyatomic molecules dissociate under electron impact in a mass spectrometer, a number of charged fragments generally are observed. The formation of the majority of these ions can be ascribed to the scission of bonds in the molecule, but in certain cases ions have masses, such t,hat their origin must in addition involve some sort of intramolecular rearrangement. Far from being infrequent, such fragments often contribute to the most prominent peaks in the mass spectrum. Suggestions have been presented2 as to the mechanism by which they might be formed. To clarify the means of formation of its so-called rearrangement peak, Honig3 labeled isobutane with carbon-13 and came to the conclusion that the peak could be explained through isomerization of the molecule. However, in isobutane a unique distinction could not be made between this and other possible mechanisms. The present investigation follows the same method. By using neopentane labeled with C13 (1) Paper presented at American Chemical Society Meeting, Chicago, Illinois, 1950. (2) A. Langer, THIS JOURNAL, 54, 618 (1950). (3) R. E. Honig. Bull. A m . Phy8. SOC.,3 4 , R12 (1949).
in one of the four methyl groups, a number of high intensit,y rearrangement peaks can be investigated, and because of the high order of symmetry of the neopentane molecule, a study of the isotopic mass distribution in these peaks allowed a characterization of the prevailing mechanism. Preparation of the Labeled Neopentane.-The terminally C13-labeled neopentane (neopentane-l-CI3) was prepared by allowing C*3-labeled dimethylzinc to react with t-butyl chloride in toluene to form a reaction complex, which was in turn decomposed by water to form labeled neopentane. The dimethylzinc was made by allowing approximately 50% Cia-labeled methyl iodide (Eastman Kodak) to react with a 96% Zn-4% Cu alloy under reflux a t 47' for 48 hours in an all-glass micro apparatus filled with carbon dioxide, then slowly heating the methylzinc iodide to 180' over a period of several hours to decompose it, and finally distilling the resultant dimethylzinc into the reaction vessel. A solution of t-butyl chloride in toluene was then added at Oo, and reaction was allowed to proceed a t that temperature for 10 hours. The product was warmed to 50" for 1 hour, and then decomposed with water added dropwise. The neopentane formed was passed through concentrated sulfuric acid, water, 40% potassium hydroxide solution, magnesium perchlorate and silica gel and then condensed by an acetone-Dry Ice-bath. The gas was further purified by several transfers between evacuated flasks kept successively in Dry Ice. On analysis in the mass spectrometer, a small trace of methyl iodide (less than 0.1%) was found as the only detectable impurity.
892
ALOISLANGER AND C. PETER JOHNSON
Concentration of C'* in the Molecule.-The concentration of C'3 in the labeled position was determined by combustion of the neopentane to carbon dioxide, since the mass spectrum of neopentane does not have a Iarge'parent mass peak from which the enrichment could be determined directly. A sample of neopentane, under a pressure of several cm. of mercury, was heated for about one hour at 600" in a quartz tube containing several grams of copper oxide in wire form. Prior to the introduction of the sample, the tube had been heated and evacuated until carbon dioxide was no longer formed from the small amount of organic matter clinging to the wall of the tube and the capper oxide. The mass spectrum revealed that all the sample was converted to carbon dioxide. Several combustions were made independently and the scanning of the 44 and 28 mass regions repeated to minimize the effect of instrument fluctuations. The very small but persistent background peaks of COZ and NZ were subtracted in the final analysis. The isotopic abundance ratio R = M45/(M44 M45) of masses C1aOPand C120~16 was found to be R = 0.1070 i 0.0002 as an average of twelve readings. The ratio of peaks a t masses 29 and 28, R' = M29/(M29 M28) was found to be R' = 0.104 0.003, which is in agreement with the previous determination within the limits of experimental errors. Because only one out of the five carbon atoms in neopentane was labeled, the isotopic concentration in the labeled position is given by L = 5R - 4 N where N = 1.1% is the isotopic abundance of G I 3 in natural carbon. Thm, the G I 3 concentration for the labeled methyl group L = 0.491 or 49.1%. This is in agreement with expectations from the C13-enrichment of the methyl iodide, which was approximately 50%.
+
+
Mass Spectrum of Natural and Labeled Neopentane.-The mass spectra of natural neopentane supplied by the National Bureau of Standards and the labeled compound were recorded under identical conditions in a 90"-sectorial-type mass spectrometer using a 50 e.v. electron beam for ionization and employing magnetic scanning. I n order to compare the two spectra, the masses were separated into groups containing the same number of carbon atoms, and the sum of the intensities of the ions in each group was normalized to 100. The results obtained are given in TabIe I, columns S n and Hn. I n order to be able to draw any conclusions from these results, the mass spectrum of neopentane containing C12 only was computed by the equations indicated in Appendix A. The result is shown in column T,. From the mass spectrum of the labeled synthetic compound (23%)and the mass spectrum of monoisotopic neopentane (Tn),the concentration of ions of a given formula in the mass spectrum of neopentane can be divided by the equations derived in Appendix B, into contributions (Xn) of fragments Containing carbon from the labeled position and contributions (Yn) of fragments without the carbon from the labeled position. The fraction X , / T n of ions of a given formula containing the labeled carbon is tabulated in column F, of Table I. An assumption was made in all cases, that the probability of dissociation to a particular fragment ion is unaffected by the substitution of C13 for CI2, although there is evidence that an isotope effect exi s t ~ . ~Because '~ the natural abundance of deuterium is smalI, its contribution to the spectrum was neglected. This type of analysis reaches its maximum accuracy for the largest peaks and particularly when a large peak is preceded only by a small one. The (4) 0. Beeck, et al., J . Chem. Phus., 16, 255 (1948). (5) 0. A. Schaeffer. ibid. 18, 1501 (1950); 23, 1305 (1955); 23, 1309 (1955).
Vol. 61 TABLE I
EACHCARBON GROUPNORMALIZED TO 100 Ma88
20 27 28 29 30
37 38 39 40 41 42 43 44 50 51 52 53 54 55 56 57 58
Sn 1.30 22.68 3.48 71.00 1.50 0.33 1.28 18.24 2.00 70.75 3.83 3.42 2.10 0.41 .G3 .23 1.02
0.17 2.46 5.09 8.5.93 3.92
Tn
Hn
H"n
1 . 3 3 1.05 0.84 23.1G 18.71 1 4 . 8 2 3 . 0 0 7 . 3 0 11.18 72.52 58.94 47.07 0.0 14.00 26.10 0.34 0.27 0.21 1.09 0 . 9 1 1.31 18.80 14.41 10.64 1 . 5 0 6 . 3 4 10.79 73.08 52.54 84.00 1.52 21.39 39.77 3.47 2.84 1.34 0.0 1 . 1 2 2.3G 0.43 0.29 0.17 .64 .60 .58 .21 .3G ,51 1.06 .70 .37 0.13 .45 .75 2.56 1.55 .61 5.21 3 . 6 0 2.07 89.65 50.83 26.92 0 . 1 1 34.52 67.84
Xn Ya F n Fragment 0.49 0.84 0.37 CzHe 8 . 8 3 14.33 .38 CzHs 0.65 2.35 .22 CzHi 26.10 46.42 .36 Cn€Is 0.13 0.21 0.38 0.53 0.78 .40 8 . 0 9 10.11 .4G -0.GO 2.10 , 38.48 34.00 .52 1.29 .1G 0.23 2.36 1.11 .G8
CaH CiHe CaIIB CaHa CaHa CsHs CrHi
0.2G 0.17 0.60 .32 .32 .50 ,I9 .02 .71 .35 .G7 .09 .04 .69 2.04 .52 .SO 5.18 .03 1.00 67.84 21.74 0 . 7 6
C4H2 CiHs CiH4 CiHi CaHo C4H7
.
..
CdHe CaHi
method becomes inaccurate in the opposite case, because the C13 contribution from the previous peak niaaks the small C12 peak. However, the rearrangement peaks with which we are concerned were relatively large, and in this case the accuracy is principally limited by the reproducibility of the mass spectra, being about 4% in the two-carbon group and somewhat better for the three- and fourcarbon fragments. Discussion In the four-carbon group, 76% of the dominating C4H9+fragment contained carbon atom from the labeled position in agreement with the 75% to be expected from simple bond splitting. Smaller peaks in this mass region showed approximately the same labeling, though the variations were larger, as one would expect, from experimental errors. In the three-carbon group, 52% of the CaH5+ fragments contained carbon from the labeled position as compared to 50% that one would expect from an equal probability of C12-C13and C'2-C12 carbon-to-carbon bond scission. The smaller peaks likewise showed approximately 50y0 labeled fragments with the exception of a small C,Hs+ peak which lies between two much larger ones and is subject to greater error. I n the two-carbon group about 37% of the C2H3+ and C2Hs+ fragments contained carbon from the labeled position. A lower value was obtained for the C2He+ fragment, but this value was derived from a small peak. The experimental data for the major peaks in the three- and four-carbon regions show no evidence of carbon-skeletal regrouping. I n the two-carbon fragments, however, the figure of 3G-38% for the ratio of the number of fragments containing carbon from the labeled posit,ion to the total number of fragments of a given formula is unexpected and experimental errors could conceivably alter it only a few per cent. Ordinarily, one would suppose that neopentane will split, four ways leading directly to two-carbon fragments, any one of which would contain the central carbon atom. This would lead to
MASSSPECTRUM OF NEOPENTANE
July, 1957
893
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25% of the fragments containing the labeled carbon j = 1 S, = T,(1 - N ) T,IN T,IN* j = 2 S, = T,(1 - N ) * T,I 2(1 - N ) N atom. j Therefore, some of the two-carbon fragments do Twr Cri (1 - N)j-rNI 8, = not include the central carbon and must have arisen r=O by the only possible alternative process: the bondj! ing of two previously unconnected methyl groups, c,i e: -(j r)!r! to form a two-carbon fragment. Such a combha,tion, if it were the sole process involved, would lead Because Tu = T-I = T-a = . . = 0 and since 8, to a 50% contribution of the labeled position in two- and N are known, there are n equations for the n carbon fragments, and the range of 25-50% label- unknown T,, and it is easy to see that T , can be caling represents the limits the labeled position can culated. It is assumed that a given mass is not contxibute to a two-carbon fragment, assuming any found in different j groups. unimolecular reaction whatever in neopentane. B. Calculation of the Fraction of a Fragment The actual experimental value obtained, 36-38%, Containing the Labeled Compound.-Let is close to the value that would result from random H , = abundance of the n 1 fragment in a mass region combination of two-carbon atoms within the moleof j carbons containing N C1* random1 and L Cl3 in a labeled position (synthetic labeLd comcule, Since out of the ten possible ways of obtainpound) ing two-carbon atoms from a set of five, four would X, = part of a fragment derived from the labeled position contain the isotopic carbon, 40% of the Cz peaks in T, would be enriched. Y . = part of a fragment of the same formula not deThe fact that in the mass spectrum of the synrived from the labeled position thesized neopentane an isotope shift of only one L = concn. of CIS in labeled position mass unit is observed, after correction for natural F , = X / T , the fraction of a fragment of a certain formula (313, is proof that just one carbon position in the containing the labeled carbon molecule was labeled and that no exchange occurred We have between the zinc dimethyl and the methyl groups X, + Y , = T , and also X.-I Y , = H", = monolabeled of the t-butyl chloride. compound Acknowledgment.-We are greatly indebted to Dr. G. Comentz for his help in the derivation of the j 1 H , Xn (1 - L) Xn-I L Y n (1N-) Y - I N Appendix. j = 2 H , = X, (1 - L)(1 - N ) X,i[L(l - N ) Appendix (I - L ) N ] Xn-&N Y n (1 - N)' Y,-I ZN(1 - N ) Yn--2 N' A. Calculation of the Monoisotopic Compound. j-1 -Let H , = X, (1 - N)i-' (1 - L ) X,, j abundance of the n l t h peak in tt mass region of j
+
.
+
+
+
E
+
s,
=
+
randomly carbons and with N abundance of distributed. Spectrum of natural neopentane N = natural abundance of C13 .I = no. of carbons in mass group l t h peak with N = 0. T. = abundance of the n Monoisotopic compound containing only C1z
+
n = number of hydrogens attached in T ,
Assume that there is no difference in C12-C12,C12(213, C1,3-C13 bond scission and that independent probability applies, then
+
+
+
+ + +
+
+ r-I [CJ-l(l - N)j-1-r Nr (1 - L) + C :; (1 i N)i-rW-'L] + X,-jNi-'L + C r=O
Yn-?C,i (1 - N)i-'N'
the summation is omitted a t j = 1. Since L, N and Hn are known and the T nhave been calculated, the X nand Yn can be found from a set of linear equations for each carbon group and F, calculated.
