Mass Spectra of Multi-Halogen Compounds A Student Practical Project David Holdsworth, Goh Siah Ching, and Mohd Jamil bin Hj Abd Hamid Universiti Brunei Darussalam, Gadong 3186, Brunei Chemistry education majors a t the Universiti Brunei Darussalam carry out chemistry projects during their final semester. One student's project involved the synthesis of bromochloro analogues of DDT and examination of the products by gas chromatography-mass spectroscopy. The data were analyzed using two specially written microcomputer Dromams to facilitate the identification of characieristic molecular ions and fragmentation abundance clusters for the halogen molecule atudv. Thc program also helps to assign ion formulas. Bromine (79Brand "Br) and chlorine (35C1and 37C1)both have laree naturallv abundant heavier isoto~es.which lead to &aracterist$ molecular-ion clusters a t M , (M + 2Y, (M + 4)+etc. We were interested to see if we could distinguish characteristic halogen isotope patterns X, X + 2, X + 4 in molecular and fragmentation clusters (1) and to identify the compounds formed, using gas chromatography with low resolution mass spectroscopy only. Apmgram that predicts the number of bromine and chlorine atoms in a molecular-ion or fragment-ion cluster with a programmable calculator (2)was modified for use with a microcomputer to display the relative heights of the first three ion abundances a s a bar diagram, to predict the number of bromine and chlorine atoms in the cluster, and to display the calculated heights of the cluster a s a bar diagram for direct comparison. Another calculator program (3) that displayed all relevant combinations of C, H, N, 0 atoms for a given mass was modified for a microcomputer to display all combinations of C, H, Br, C1 atoms possible a t a particular mass. Thus, we could use the first program to identify the number of Br and C1 atoms in a molecular ion or fragment ion cluster commencing a t Xi d e , and we could use the second promam to predict the ion formula. . DDT (1)is prepared readily in the laboratory from chloral and chlorobenzene ( 4 ) in the presence of sulfuric acid. Table 1. Mass Spectra Analysis of the Major ChloralChlorobenzene Reaction Products, Indicating Some of the Main Halogen Isotopic Abundance Clusters %X = 100
mie 352
program 1
program 2
computer decision
ion formula
Cls
C14 H9Cl5
317
CI4
C14 H9 C14
282
C$
C14 H9 C13
246
C12
C14 He C12
235
CIz
C13 Hg Clz
212
CI
Cin Hg CI
117
Cl3
C Cl3
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M+
I t was decided to attempt the synthesis of bromo-analogues using combinations of chloral or bromal, and chlorobenzene or bromabenzene The reaction products were examined by GC-MS and the molecular and fragment-ion data were analyzed by the microcomputer programs described. Compounds prepared were assigned molecular formulas and general structures, though identification of particular stereoisomers on the phenyl ring was not feasible with low resolution mass spectroscopy alone. The Chloral-Chlorobenzene Reaction The crystalline product separated on the GC mlumn and several peaks were noted a1 retention rimes of 16-20 min. The mass spectrum of the main GC peak (about 80%) was examined in some detail (Table 1). Asecond peak gave a very similar spectrum, while a much smaller peak showed a quite different series of isotopic patterns. I n the first and second GC peaks examined, the M+ cluster of ions, each two-mass units apart, began a t d e 352. From the relative abundances of the ions in the cluster microcomputer program l predicted Br 0, C1 5. The second program indicated the molecular ion formula to be C14 H9 "-
GI5'.
The compounds had molecular formulas CI4H9 C15, consistent with isomers of DDT. The base peak clusters showed fragment ions beginning a t mle 235, indicating Br 0, C12 (program 1)and Cl3H9C12(program2). Some of the main clusters are shown in Table 1. The minor GC peak analysis we indicated Cla H8 C14 (Mi = 3161, perhaps DDE (21, the main environmental decomposition product of DDT (5).See Figures 1and 2. The Chloral-Bromobenzene Reaction The GC-MS of the purified product indicated two peaks, both of which had isotopic abundance peaks expected of B h , CI3commencing a t mle 440. The second computer program indicated a molecular formula of ClpHg Br2 C13, consistent with dihromo-DDT isomers (1:X=C1, Y=Br). The base peaks were similarly shown to be CISH9 Br2 (Table 2).
X
I
X-C-C-H
base peak
A@ Y
Figure 1. DDT, X = CI. Y = CI,
Table 2. ChloraCBrornobenzene Reaction-Major Product %X = I00
m/e
%X
+2
%X
program 1
program 2
+ 4 computer
ion formula
decision
Figure 2. DDE,X = CI, Y = CI.
GC peak (M+=404) was shown to be C14.H8Brz A Clz, probably a dibmmo-DDE(2: X = C1, Y= Br), m t h bmmine atoms on the phenyl ring.
I CI
'H
H'
CI OH
I
CI-C-C-H
I
' 0 CI
PhCl
290
138
92
D. D. T.
CI-C-C
I
I
-
CI OH
H'
phc' c l - ~ - c ~ c l-H20 c-b'-L*c' I 7
I
I
I
CI H
CI H
6
Figure 3. Proposed mechanism for the synthesis of DDT.
C14 Hg Brw C13 C14 Hg Br2 C12 C14 Hg BrzCI C13 H9 Brz
Br Cl
C14 HE Br CI
M+
base peak
Conclusions GC-MS proved sensitive enough to detect a series of related isomeric compounds a s major and minor GC peaks. The microcomputer programs allowed rapid analysis of the detailed mass spectral output. Molecular and fragmentation clusters were identified successfully by microcomputer analysis and molecular formulas were assigned to several compounds in the mixture. The proposed reaction mechanism, involving electron withdrawal by the three halogen atoms to activate the carbocations required of electrophilic attack, accounted for the ready synthesis of target five halogen compounds using chloral, compared to similar bmmal reactions. Four halogen products predominated in bromal syntheses. The chloral syntheses only and GGM%microcomputer analysis of the products took about 10 hours of student time. Several features of these chloral syntheses will be modified and used in future undergraduate laboratory exercises when students will combine laboratory skills with organic spectroscopic analyses.
4
CI @
Brz CIS Brz Cl2 Brz Cl Brz
These effects are reflected in the relative difficulties encountered in the attempted syntheses of the related bmmo compounds with five halogen atoms, from bromal.
Proposed Mechanisms The synthesis of DDT (see Fig. 3) involves the initial electrophilic aromatic substitution by protonated chloral (3) on the aromatic ring of chlorobenzene. Attack in the p-position is favored. The combined electmn-withdrawing effects of the three chlorine atoms result in a stronger positive charge on carbocation (31, facilitating its electrophilic attack on chlorobenzene. The intermediate carbocation (4) loses a proton to regenerate the aromatic ring (51, which is reprotonated in acid to form another carbocation (6). The reaction is completed by further electrophilic aromatic substitution involving a second chlorobenzene molecule
O
300 198 148 92
Electron withdrawal by bromine atoms is less than that of chlorine, thus the positive charges on carbocations (3)and (6) are not as strong and electmphilic attack is less ready. Electophilic attack of the trihalogen carbocation on the aromatic ring is shown by scale models to be substantially hindered by the three bulkier bromine atoms, compared to those of chlorine.
The Bmmal-Bromobenzene Reaction No five-bromine compound was detected. Two compounds were identified a s Cg H4 Br4 (Mi= 416) and Clp Hg Br4 (M+= 4921, with one and two aromatic halogen rings, respectively.
I CI-C-C#
228 241 222 193
and carbocation (61, strongly positive due to its three electron-withdrawing chlorine atoms, Protonated bromal carbocations involved in the electmphilic attack would differ from their chloro equivalents,
The Brornal-ChlorobenzeneReaction The crystallized pmduct showed two major peaks (Mt= 404) with a Brz Cl2 cluster which analyzed for C14 Kg B h Clz. However, examination of the washed, dried, uncrystallized reaction product showed a small GC peak (1.5%), a t a higher retention time. This analyzed for the target fivehalogen --~ compound, C14 Hg Bra C12, with a base peak of C14
CI
440 405 370 323
5
Experimental 1.0 g of bromal or chloralhydra t e was magnetically stirred with 1.5 mL bromo or chlorobenzene and heated gently until the mixture dissolved. A 10-mL portion of sulfuric acid was added slowly, and the mixture was agitated for about 40 min. A hexane solution of the washed, dried precipitate was purified on a n alumina column a n d crystallized from hexane. Volume 69 Number 10 October 1992
857
To avoid exposure to the halogen compounds, all t h e reactions w e r e carried o u t in a n efficient f u m e cupboard. As a precaution t h e s t u d e n t w o r e a protective nose m a s k a n d laboratory gloves. About 0.5 pL of a 10 mg1100 mL hexane solution was injected into a HP5890 series I1 GC-MS instrument, with a flow rate of 40 mL of helium per minute. The GC temperature gradient was 50-200 ' over 30 min. Experimental yields of the target five halogen compounds under the same laboratory conditions were chloral-chlarobenzene 82%(14%), chloral-bromobenzene 86% (9%),
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bromal-chlorobenzene 1.5%(69%),and bromal-bromobenzenenil (69%). Several four-halogen compounds were detected especially in the bromal reactions, and their yields are shown in brackets.
Literature Clted 1. Holdsworth.D. K J. Chem.Educ. 1982.59, 78CL781. 2. Holdaworth. D. K J. Chem. Edue. 1983,60.103-104. 3.Holdsworth. D. K J. Cham.Edue. 1980.57. W l W . 4. Waddington, D. J.: Findlay, H. S. Organic Chamisfry fhmvgk Erperimnf; Milk and Boon: London. 1965. 6.Solomons,T W . G. OrgonicChemiatry; John Wiley: New Ymk, 1988.
