The fragmentation of cyclopentanol has been studied by employing
the deuterium labeling technique. Starting from the molecular
ion the following specific processes could be established:
loss of H from the position 1, loss of CH3 formed mainly from 2
(or 5) position after a-scission, loss of H 2 0 as a 1,3-elimination,
and loss of C2H 5 generated from 2 and 3 or 4 and 5 positions plus
one hydrogen from position 5 or 2, respectively. The results are
discussed

D. Kovačević

K. Humski

L. Klasinc

English

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CCA-773 CR 0 AT I CA CHEM I CA ACT A 45 (1973) 363 543.51 :547 .514.71 Original Scientific Paper Mass Spectra of Labeled Cyclopentanols* K. Humski**, L. KLasinc, and D. Kovacevic Institute »Ruder Boskovic«, 41000 Zagreb and **Faculty of Technology, University of Zagreb, 44000 Sisak, Croatia, Yugoslav ia and V. Kramer University of Ljubljana, »Jozef Stefan« Institute, 61000 Ljubljana, Slovenia, Yugoslavia Received November 27, 1972 The fragmentation of cyclopentanol has been studied by em-ploying the deuterium labeling technique. Starting from the mo-lecular ion the following specific processes could be established: loss of H from the position 1, loss of CH3 formed mainly from 2 (or 5) position after a-scission, loss of H 20 as a 1,3-elimination, and loss of C2H 5 generated from 2 and 3 or 4 and 5 positions plus one hydrogen from position 5 or 2, respectively. The results are discussed. INTRODUCTION Recently, it has been shown1•2 that dehydration of the molecular ion of bicyclo[2.2.l]-2-heptanol in the mass spectrometer is a 1,3- and/or 1,4-elimination process. Regarding the Joss of the second hydrogen, this reaction seems to be stereospecific. The first hydrogen exclusively comes from the hydroxyl group and the orientation of the hydroxyl group does not influence the mass spectrum. In the present study the mass spectral behaviour of cyclopentanol was inve-stigated, employing deuterium labeling, field free metastable peak observation, and exact mass determination. Investigations of this kind have shown to give valuable information about fragmentation mechanisms3•4 and the energetics thereof. The mass spectra of cyclopentanol and cyclopentanol-2,2,5,5-d, , and those of cyclic alcohols from c. to C8, have been already studied by Natalis5•6 . A detailed study of the mass spectrum of cyclohexanol by means of deuterium labeling was performed by Budzikiewicz, et al.7 RESULTS AND DISCUSSION The mass spectra of cyclopentanol (I) , cyclopentanol-d (I-d), cyclopentanol-1-d (I-1-d), cyclopentanol-2-d, cis- (I-cis-2-d) and trans- (I-trans-2-d), cyclopen-tanol-3,4-d, (mixture of cis- and trans-) {I-3,4-d2) and cyclopentanol-2,2,5,5-d. (I-2,2,5,5-d, ) have been measured. The position of the deuterium label was proven by the NMR spectra of the mixture of cyclopentanol or deuterated cyclopentanols * Reported in part at the 3"' Yugoslavian Congress for Pure and Applied Che-mistry, Ljubljana, June 1972. ** The correspondence should be addressed to K. Humski, Institute »Ruder Boskovic«. 364 K. HUMSKI ET AL. with the Pr(DPM) 3* complex. The ratio of deuterium in I-3,4-d2 was 1.17/0.77 for the trans/cis positions towards the hydroxyl group. The spectra were assigned by inspection of the corresponding Dreiding models and the mathematic expression for the pseudocontact shifts8. The study of NMR spectra of the deuterated cyclo-pentanols gave the identical assignations. The uncorrected and normalized spectra of these compounds at ionizing potential of 70 eV are reproduced in Fig. 1. The most characteristic masses are numbered. The fragmentation pathway(s) are indicated by arrows and are confirmed by first field free region metastable peaks; second field free metastables are denoted by asterisk. The results indicate that four parallel processes are responsible for the fragment-ation of the cyclopentanol molecular ion: a) the loss of a hydrogen atom, b) the loss of a methyl radical, c) the dehydration process and d) the loss of an ethyl radical. Process d) as first field free metastables show, is to a small extent also a two step loss of C2H 4 + H. As will be shown later, one of the most important characteristics of these processes is that they are well defined in respect to the origin of the expelled neutral fragment. For this reason also a number of exact mass determinations were performed and the mass spectra of I, (I-3,4-d2) and (I-2,2,5,5-d ) have also been investigated at ionizing potentials of about 10, 12, 15, and 20 eV. a) The Loss of Hydrogen As previously found on cyclohexanoF, the loss of hydrogen in cyclopentanol also occurs from position 1. Thus, in the case of I-1-d the [M-1]-ion becomes an [M-2]-ion with about the same intensity, whereas in all other compounds the loss of hydrogen resembles that in the undeuterated compound. b) The Loss of Methyl A methyl radical loss from the molecular ion is indicated in the mass spectrum of I. Its relative importance within the range of ionizing energies between 10 eV and 70 eV remains the same as shown by the nearly constant relative intensity to the molecular ion of 1/3. No further loss of a hydrogen is observed. Labeled compounds can give an answer about the origin of the expulsed methyl, but care must be taken about possible interference of this process with the dehydration mechanism (CD3 has the same mass as H20 or OD, and CHD2 the same as OH). Therefore also exact mass determination of the corresponding ions in the molecules containing two and four deuterium atoms have been performed. Thus, m /e 71 in I-3,4-d2 is a singlet peak corresponding to C5H 7D2 i. e. [M-OH], whereas in I-2,2,5,5-d4 both m/e 72 and m/e 73 are doublets: m /e 72 corresponds to C4H6DO [M..:_CD3] and C5H 4D4 [M- H 20] in a ratio of 1/5, and m/e 73 corresponds to C,tH5D20 [M-CHD2 ] and C5H 5D4 [M-OH] in a ratio of about 1/5. The observed relative loss of labeled methyl radicals CH3 : CH2D in I-1-d (5 : 1), I-2-d (1 : 1) and I-3,4-d2 (2 : 1) is, within experimental error, consistent with the assumption that the CH2 group from position 2 (or 5) is the precursor of the expulsed methyl, taking statistically one of the remaining hydrogens. Theoretically CH3 : CH2D ratios of 6 : 1 (I-1-d), 1 : 1 (I-2-d}, and 5 : 2 (3,4-d2) are expected. It might be expected that a-cleavage precedes this process. Unfortunately the results on I I !! 'L 111 * Tris(dipivalomethanato)praseodymium(III). ·u; Ji -H 0 5' 1 58 ~ c "' * :i> .~50 -H 50 C/J -CH3 C/J "' I > * C/J -H 44 87(W) "d 0 44 r 86{M;J 45 M "' 45 59 87{M;J 3 ~ Ji 0 fr: 41 I 58 69 ,.., 39 .. I 69 I 68 ;u 72 ,( 7Z ;J> 0 so 60 70 80 SJ "Ye 40 so 60 70 80 40 50 60 70 80 90 rrve "'.l 40 90 '% I:"' :i> td M I:"' 58 I D 58 SB ~ 100 -j 100 D 100 0 p-0H Q-oH 0-oo 0 ~ 0 D I I:"' D D 0 [-d "d >. I-3,4-d2 l-22,5,5-d4 M VJ ;;:: c ,.., "' ~50 so 50 :i> 57 z "' 46 I 0 .2: 59 I:"' d 44 C/J a; 45 87(Mt) fr: 59 SB ft-1;) 60 90(M;) 40 50 60 70 80 90 '% 40 50 60 70 80 90 '% lO 50 60 70 80 90 Ye Cl.:l m Fig. 1. The mass spectra of cyc!opentanol (I), I-1-d, I-2-d, I-3,4-d2, I-2,2,5,5-d 4 and I-d at 70 ev. c.n 366 K. H"CMSKI ET AL. I-2,2,5,5-d4 do not completely support this assumption since about 1 : 2.5 : 0 intensity ratio of the ions m / e 72, m/e 73 and m/e 74 ([M-CD3 ], [M-CD2H] and [M-CH2D]), respectively, would be expected. Thus, it seems that more than one mechanism plays a role in this process, but what certainly can be ruled out is a complete randomization of hydrogen prior to methyl loss. c) The Dehydration Process The mass spectrum of cyclopentanol-d shows that the label is removed by the dehydration process, i.e. the hydrogen of the hydroxyl group is included in the eliminated water. On the other hand a great problem in the study of the dehydration process in cyclopentanol arises from the unusually high further loss of a hydrogen (and even one more) from the [M-water]-ion. The relative intensity of [M-water] to [M-water-hydrogen]-ions depends highly on the ionizing energy ranging from 3 : 1 to 1 : 1 for energies between 10 to 70 e V respectively (vide infra). Here we give the results concerning the dehydration process at 70 eV. For the reasons mentioned some calculation was necessary to get an insight into this process. The relative intensities of ions relevant for the study of the dehydration process in cyclopentanol are given in Table I. Here the measured ion intensities in the labeled compound are normalized to the sum of the corresponding inten-sities of the undeuterated compound and quoted as »experimental« . F or each compound also the actual deuterium content is given. Taking into account this TABLE I Dehydration of Labeled CycLopentanoLs in the Mass Spectrometer at 70 eV [M-18] Cyclopentanol (I) 100 exp." 104 I-1-d calc.b 99 exp.• 103 I-2-d calc." 97 exp." 65 I-3,4-d2 calc." 97 exp." 85 I-2,2,5,5,-d4 calc." 86 Relative Intensities 'i I [M-19] I [M-20] I [M-21] --- . 76 9 63 18 76 10 65 17 77 11 72 36 12 77 11 55 35 10 87 13 1 ' Deuterium Content D , 0/o 100 Do 99 Di 1 Do 97 Di 3 Do 94 D2 6 Di 86 D4 13 Da 1 D2 Loss of HOD ? yes ? • Normalized to the sum of intensities of the undeuterated compound; " Assuming that _the compound loses only undeuterated fragments as in I . and taking into account actual deuterium content. MASS SPECTRA OF LABELED CYCLOPENTANOLS 367 content, the ion intensities for the relevant ions of the labeled compounds have been calculated assuming that only undeuterated fragments are lost in the same ratio as from the unlabeled compound. These values are quoted as »calculated« in Table I. An inspection of »experimental« vs. »calculated« values gives information about the loss of the label in the eliminated water. In this case [M- 18] should be reduced on account of the [M-19] and lower mass ions. Now, Table I shows that this holds only for I-3,4-d2 • Dehydration is again shown to take place as a 1,3-elimination process. But it is interesting to notice that I- 1-d, and I-2-d showed loss of H 20 which was higher than calculated. This could be explained by competition of the fragmentation processes (isotope effect) . The enhancement of the lower masses [M-20] , [M- 21] results from the subsequent loss of deuterium instead of hydrogen as calculated. Never-theless, some possibility for a small fraction of 1,2-elimination could not be ruled out completely as it is indicated in Table I. d) Loss of Ethyl Radical The spectra of deuterated cyclopentanols in Fig. 1 and the intensities of the relevant ions in Table II show that the ethyl is formed from methylene groups 2 and 3 (or 4 and 5) and one hydrogen from position 5 (or 2, respectively), as + suggested previously5- 7 and in accordance with the structare CH2 =CHCHOH for m /e 57. Similarly, for m /e 44 exact mass determinations yielded the com-position C2H4 Q+ and from the mass spectra of the labeled compounds it can be found that positions 1 and 2 (5) generate this ion. TABLE II Loss of C2H5 from LabeLed CycLopentan oLs in the Mass Spectrometer at 70 eV I [M-28] Relative Intensities [M-29] [M-30] [M-31] [M-32] Cyclopentanol (I) 14 100 I-1-d 13 100 I - 2-d 10 57 100 I-3,4-d2 2 15 100 I-2,2,5,5-d4 4 16 27 100 Low Energy Mass Spectra Besides the expected reduction in the total ion current and enhancement in the relative intensity of the molecular ion (at about 8 eV it becomes the base peak), lowering of the ionizing voltage does not bring great changes in the fragmentations observed at 70 eV. A direct function of the ionizing energy are only the relative intensities of m /e 39, 41 , 43 and 67 in I , i.e. peaks which can be attributed to secondary fragmentations. Inspection of the dehydration process at lower energies which is now less disturbed by the loss of further hydrogen(s) shows clearly the loss of HOD only from the I-3,4-d2 compound amounting to about 400/o at 10 eV. Unfortunately, no information about possible higher 368 K. HUMSKI ET AL. importance of the cis (or trans) position of deuterium relative to the h ydroxyl group for dehydration could be obtained since our sample was a mixture oi both. EXPERIMENTAL Technique The mass spectra were recorded under identical operating conditions w ith a CEC 21- 110 C m ass spectrometer at about 90 °C with 70 eV and a heated gas inlet system. The metastable peaks in the first field-free region were measured on the same instrument by decoupling the acceler ating voltage. Exact mass measurements were performed at a resolving power of approximately 20.000. The NMR spectra were recorded on a Jeol 100 MHz spectrometer. The concentrat-ions of alcohol and Pr(DPM)J* in CC14 were 0.2 M and 0.05 M respectively. Tetra-methylsilane was used as an internal standard. CydopentanoL (I). - Compound I , b. p. 139-40 °c, was prepared in 970/o yield by lithium aluminum hydride reduction of cyclopentanone9. The NMR spectrum of the mixture of I and Pr(DPM)J complex was recorded and the following chemical shifts were obtained : i: 11.1, 2H, trans-3,4-H2 ; i: 12.3, 4H, cis-3,4-H2 and trans-2,5 - H2; and i: 15.1, 3H, 1-H and cis-2,5-H2. CycLopentanoL-1 - d (I - 1-d). - Alcohol I-1-d was obtai ned by lithium alum inum deuteride reduction of cyclopentanone in 93'0/o yield9 • The deuterium content was 0.99 atoms per molecule according to mass spectral analysis (Table I). The NMR spectrum of I-1-d and Pr(DPM)3 showed the following chemical shifts : i: 11.1; 2H; :r 12.3, lH; i: 15.1, 2H. trans-CydopentanoL-2-d (I-2-d) . - Alcohol I-2- d was prepared by the lithium aluminium deuteride reduction of cyclopentene oxide9, which was prepared in the usual m anner by the oxidation of cyclopentene with perbenzoic acid .10 The overall yield was 299/o. The deuterium content was 0.97 atoms per molecule (Table I). 3-CycLopentenoL' 1• - This alcohol was prepared in 629/o yield based on sodium borohydride (3.8 g, 0.1 moles) by the hydroboration of cyclopentadiene (45 g, 0.68 moles) and subsequent hydrolysis of the formed alkyl borane by a hydrogen peroxide --sodium hydroxide mixture. The product, b. p. 58-60 °C/20 mm, 5.44 g) w as 76°/o pure according to GLC analysis (100/o FFAP on 60/80 Chromosorb W). In order to obtain high purity, 2.3 g of the product was purified by preparative GLC (20°/o OS 138 on Chromosorb W 60/80 at 125 °c) giving 1.1 g of 3-cyclopentenol which was 99.0"lo pure (according to GLC analysis) . Cydopentano1-3,4- d2 (I - 3,4-d2). - 3-Cyclopentenol (0.4 g, 0.0048 moles) was refluxed for 2 hrs. with 7 ml of methanol-d (98.2°/o D) in order to exchange hydroxylic hydrogen with deuterium. Otherwise hydroxylic hydrogen could exchange with deuterium during the homogeneous catalytic deuteration and only partial deuteration could be achieved in position 3 and 4. Methanol was removed by distillation through a 30 cm Vigreaux column. The residual 3-cyclopentenol-d was placed in a pear-shaped flask with 20 ml of methanol-d and 12 m l of dry benzene. Into this solution 0.092 g (10"6 moles) of t ris(triphenylphosphine)rhodium(I) chloride' 2-13 was added and the flask was attached to the atmospheric pressure hydrogenation apparatus. The mixture was deuterated with 115 ml. (0.005 moles) of deuterium gas at room temp. by stirring for 18 hrs. The reaction mixture was distilled in high vacuo c10-5 mm) at room tem-perature in order to separate the volatile components from the catalyst. The distillate obta ined contained benzene, methanol-d and cyclopentan-3,4-drol-d. The binary azeotropic mixture of methanol -benzene was distilled at 58 oc through a 30 cm Vigreaux column. During the distillation 7 ml of methanol was added three times in order to exchange hydroxylic deuterium with hydrogen. When the temperature started to rise the mixture was free of benzene and the excess methanol was removed at 65 °C. The residue was purified by preparative GLC (200/o OS 138) yielding 0.190 g of I-3,4-d2, 99.5°/o pure according to GLC analysis (100/o FFAP). The deuterium content was 1.94 atoms per molecule (Table I). The NMR spectrum of I-3,4-d2 with Pr(DPM)a gave the following signals: i: 11.1, 0.83 H; i: 12.3, 3.24 H; i: 15.1, 3H. That indicates that - 600/o of deuteriums in position 3 and 4 is trans to the h ydroxyl group (at , 11.1) and - 400/o cis (at i: 12.3). * Tris(dipivalomethanato}praseodymium(III). MASS SPECTRA OF LABELED CYCLOPENTANOLS 369 CyclopentanoL-2,2,5,5-d4 (I-2,2,5,5-d) . - Compound I-2,2,5,5-d4 was prepared in 210/o yield by the base catalyzed hydrogen exchange of cyclopentanone with D20 and subsequent reduction of this ketone with lithium aluminium hydride9• The deute-rium content was 3.85 atoms per molecule (Table I) . The NMR spectrum of I-2,2,5,5-d4 and Pr(DPM)3 showed the following chemical shifts: i: 11.1, 2H; i; 12.3, 2H; i: 15.1, lH. CyclopentanoL-d. - The triple exchange of 0.778 g cyclopentanol with 7 ml D20 (980/o) gave cyclopentanol-d with 670/o deuterium in hydroxyl group according to NMR analysis. REFERENCES 1. K. Hums k i and L. K.l as inc, J . Org. Chem. 36 (1971) 3057. 2. K. Hums k i, J. M. Jerk uni c a, L. K 1 as inc, and J. M a rs e 1, Croat. Chem. Acta 44 (1972) 221. 3. J. L. Ho 1 m es and D. Mc G i 11 iv ray, Org. Mass Spectrometry 5 (1971) 1339 and 1349. 4. H. G tis ten, L . K 1 as inc, J. Marse l, a nd D. Mi 1 iv o j e vi c, Org. Mass Spectrometry 6 (1972) 175. 5. P. Nat a 1 i s, BuU. Soc. Chim. BeLges 69 (1960) 224. 6. P . Nat a 1 is, BuU. Soc. Roy. Sci. L iege 31 (1962) 790. 7. H. Budzik i e w i c z, Z. Pe 1 ah, and C. D j er" s s i, Monatsh. 95 (1964) 158. 8. J. K. M. S a nder s and D. H. W i 11 i ams, J . Am. Chem. Soc. 93 (1971) 641. 9. A. Streit w i es er, Jr., R. H. Jagow, R. C. Fah ey, and S. Suzuki, J. Am. Chem. Soc. 80 (1958) 2326. 10. W. J. Hi ck in b o t tom, Reactions of Organic Compounds, Longmans Green and Co., London, New York, Toronto, 1950, p. 262. 11. H. C. Brown and G. Z we if f el, J . Am. Chem. S oc. 81 (1959) 247. 12. J. A. 0 s born, J . Am. Chem. Soc. 93 (1971) 2397. 13. C. D j er ass i and J. Gut z w i 11 er, J. Am. Chem. Soc. 88 (1966) 4537. IZVOD Spektri masa obiljezenih ciklopentanola K. Humski, L. Ktasinc, D. Kovacevic i V . Kramer Istrazivani su spektri masa ciklopentanola obiljefavan jem pojedinih mjesta s deu-terijem. Na temelju dobivenih rezultata bilo je moguce dokazati nekoliko specifienih procesa fragmentacije. To su gubitak vodika iz polofaja 1, gubitak metila iz polofaja 2 (ili 5), odlazak vode preko 1,3-eliminacijskog procesa i odlazak etila iz polofa ja 2 i 3 (ili 4 i 5) zajedno s jednim vodikom iz polofaja 5 (ili 2). INSTITUT »RUE>ER BOSKOVIC« 41000 ZAGREB TEHNOLOSKI FAKULTET 44000 SISAK I UNIVERZA V LJUBLJANI INSTITUT »JOZEF STEFAN« 61000 LJ"UBLJANA Primljeno 27. studenoga 1972. 

Mass Spectra of Labeled Cyclopentanols

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