Products of tetramethyllead dealkylation with bromine

Article abstract of DOI:10.1134/S1070427214050152

Products of tetramethyllead dealkylation with bromine in the temperature interval from -20 to 20°C were analyzed by X-ray diffraction, IR spectroscopy, and spectrophotometry. Specific features of their thermal behavior were studied by differential thermal

Full text of DOI:10.1134/S1070427214050152

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 5, pp. 624−628. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © N.B. Egorov, 2014, published in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 5, pp. 636−641.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Products of Tetramethyllead Dealkylation with Bromine
N. B. Egorov
Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia
e-mail: egorov@tpu.ru
Received March 3, 2014
Abstract—Products of tetramethyllead dealkylation with bromine in the temperature interval from –20 to 20°С
were analyzed by X-ray diffraction, IR spectroscopy, and spectrophotometry. Specific features of their thermal
behavior were studied by differential thermal analysis and mass spectrometry.
DOI: 10.1134/S1070427214050152
Stable lead isotopes find growing use in modern sci-
ence and engineering [1–4]. Tetramethyllead Pb(CH3)4
is used as the working substance in separation of lead
isotopes by centrifugation. The physicochemical proper-
ties of this compound have been comprehensively studied,
and it has been shown to be compatible with materials of
the centrifugal equipment [5].
unstable and tend to disproportionate to form various
lead-containing products.
Akimov et al. [9] evaluated by quantum-chemical cal-
culations the thermodynamic probability of the reactions
of Pb(CH3)4 dealkylation with halogens and substantiated
the choice of bromine for Pb(CH3)4 dealkylation.
Here I report an experimental study of the products of
Pb(CH3)4 dealkylation with bromine and of their chemical
transformations on heating.
After the production on separation cascades, isotope-
enriched Pb(CH3)4 should be converted to lead metal,
as this form is more convenient for storage and sale and
is the starting form for preparing lead compounds with
altered isotope composition. In conversion of Pb(CH3)4
to lead metal, the loss should be reduced to a minimum,
the isotope dilution should be excluded, and the required
chemical purity of the product should be ensured.
EXPERIMENTAL
The following chemicals were used: Pb(CH3)4
(99.5%), bromine (analytically pure grade), carbon tet-
rachloride (ultrapure grade), potassium tetrahydroborate
(98%), ammonium hydroxide (analytically pure grade),
hydrogen (99.995%), and Pb(CH3)3Br (Aldrich).
Thermal and photochemical processes can be used for
converting Pb(CH3)4 to lead metal [6, 7]. However, both
in photolysis and in thermolysis of Pb(CH3)4 lead metal
is formed as dusty deposits, various films, and flakes
with nonuniform distribution over the reactor surface.
Therefore, the lead metal can be removed from the reactor
only by mechanically scraping it off or by dissolving it in
appropriate reagents. This process requires manual labor
and is difficult to automate. Therefore, both procedures
are characterized by low productive capacity and high
processing cost.
The reaction of Pb(CH3)4 with Br2 was performed in
a CCl4 solution in the temperature interval from –20 to
20°С. The dealkylation was performed in a KRIO-VT-06
cryostat. Tosol (aqueous ethylene glycol) was used as
thermostating liquid. The temperature was maintained
with an accuracy of ±0.1°C. The unchanged starting re-
actants were removed in a vacuum at room temperature,
and the solids formed were analyzed.
The content of Pb(CH3)4 dealkylation products was
determined spectrophotometrically following the proce-
dures described in [10]. The electronic absorption spectra
Lead metal can be prepared from methyllead halides
of types PbR3Hal and PbR2Hal2, formed by dealkylation
of Pb(CH3)4 with halogens [8]. These compounds are
624

PRODUCTS OF TETRAMETHYLLEAD DEALKYLATION WITH BROMINE
625
I, %
I, %
(a)
(b)
2θ, deg
2θ, deg
Fig. 1. Diffraction patterns of products of Pb(CH3)4 dealkylation with bromine at 20°С. (I) Intensity and (2θ) Bragg angle. Br2 : Pb(CH3)4:
(a) 0.5 and (b) 2.5.
were recorded with an Evolution 600 spectrophotometer
using a 1-cm quartz cell.
The IR spectra showed that the substances formed at
the Br2 : Pb(CH3)4 molar ratios from 0.1 to 1 corresponded
to trimethyllead bromide Pb(CH3)3Br. In the IR spectra of
the substances obtained at the molar ratios from 1 to 2, the
bands of Pb(CH3)3Br disappeared, and the bands of dimeth-
yllead bromide Pb(CH3) appeared. The IR spectra of the
precipitates obtained on adding excess bromine contained
only the absorption bands characteristic of Pb(CH3)2Br2.
The diffraction patterns were recorded with a D8
Discover diffractometer (CuKα radiation, λ = 1.54056 Å).
The IR spectra of samples prepared as KBr pellets
were recorded with a Nicolet 6700 Fourier IR spectrom-
eter in the range 400–4000 cm–1 at room temperature.
Thermal gravimetric studies were performed with an
SDT Q600 analyzer in the temperature interval 20–400°С
in a nitrogen stream at a heating rate of 10 deg min–1.
The released gases were analyzed with a ProLab mass
spectrometer in the m/е interval from 2 to 200 with unity
resolution.
The frequencies of the vibration band maxima and
the band assignment in the IR spectra of the dealkylation
products formed in the reaction of Pb(CH3)4 with Br2 at
10°С and different molar ratios of the reactants are given
as example in the table.
The mass spectra of thermolysis products were also
studied with a TRACE DSQ gas chromatograph–mass
spectrometer equipped with a direct sample inlet system.
Ions were recorded in the m/e interval 40–400 with unity
resolution at a heating rate of 50 deg s–1.
It should be noted that, according to the IR spectra,
Pb(CH3)2Br2 decomposes at room temperature within 1 day
to form Pb(CH3)3Br, but below 0°C Pb(CH3)2Br2 is fairly
stable. The Pb(CH3)2Br2 decomposition rate increases
with temperature. Pb(CH3)3Br is stable and suitable for
prolonged storage.Apparently, the Pb(CH3)2Br2 decompo-
sition is associated with lower strength of the Pb–CH3 bond
in the compound containing two bromine atoms, compared
to the compound containing only one bromine atom.
RESULTS AND DISCUSSION
To find the optimum conditions for dealkylation of
Pb(CH3)4 with bromine and reduce to a minimum the
loss of isotope-enriched lead, we studied the products of
the reaction of Pb(CH3)4 with Br2 at –20, –10, –5, 0, 5,
10, and 20°С. At each temperature, a solution of Br2 in
CCl4 was added to a solution of Pb(CH3)4 in CCl4, with
the reactant molar ratio varied from 0.1 to 2.5. Colorless
crystalline precipitates were obtained.
The purity of the compounds obtained at the Br2 :
Pb(CH3)4 molar ratios of 0.5 and 2.5 was proved by X-ray
diffraction analysis. As seen from the diffraction patterns
(Figs. 1a, 1b), each compound has a set of specific inter-
planar spacings differing from each other. Comparison
of the diffraction pattern of the compound formed at the
molar ratio of 0.5 with that of the reference compound,
Pb(CH3)3Br (Aldrich), showed that they were identical.
The IR spectra of the dealkylation products obtained
at the examined molar ratios and temperatures are iden-
tical. They were compared to the spectra of methyllead
bromides described previously [11].
Figure 2 shows how the yields of Pb(CH3)3Br and
Pb(CH3)2Br2 at 20 and –20°C (determined spectropho-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 5 2014

626
B, %
EGOROV
B, %
(a)
(b)
Br2 : Pb(CH3)4
Br2 : Pb(CH3)4
Fig. 2. Yield B of (1) Pb(CH3)3Br and (2) Pb(CH3)2Br2 in reaction of Pb(CH3)4 with Br2 at (a) 20 and (b) –20°С as a function of the
reactant molar ratio.
tometrically) depend on the Br2 : Pb(CH3)4 molar ratio.
Pb(CH3)3Br is formed when Br2 is in deficiency, with the
highest yield attained at the Br2 : Pb(CH3)4 molar ratio of
1. In the range of molar ratios from 1 to 2.1, the yield of
Pb(CH3)3Br decreases and that of Pb(CH3)2Br2 increases,
i.e., a mixture of the reaction products is formed.At molar
ratios higher than 2.1, Pb(CH3)2Br2 is the only reaction
product.
IR characteristics of products formed in reaction of Br2 solutions with Pb(CH3)4 at 10°С
Absorption maximum, cm–1, for indicated compound
Band
assignment
product formed at indicated Br2 : Pb(CH3)4 molar ratio
Pb(CH3)3Br [11]
Pb(CH3)2Br2 [11]
0.5
1.5
2.5
3023
2928
3020
2920
3021
2925
3022
2928
3020
2920
νa(CH3)
νs(CH3)
2774
2288
1641
–
2323
–
2773
2286
1641
–
2286
1639
–
2324
–
2xδa(CH3)
2xδs(CH3)
1609
1396
–
1610
1610
–
2xρ(CH3)
δa(CH3)
1414
1397
–
1391
–
1391
–
1390
1167
1154
1149
1160
1154
1167
1150
1165
1154
1164
–
δs(CH3)
784
655
–
821
602
523
777
–
–
780,797
825
–
–
ρ(CH3)
–
–
νa(PbC2)
492
463
–
–
493
460
493
–
–
–
νa(PbC3)
νs(PbC3)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 5 2014

PRODUCTS OF TETRAMETHYLLEAD DEALKYLATION WITH BROMINE
DTG
627
DTG
TG
DTA
TG
DTA
T, °C
T, °C
Fig. 3. Thermogram of Pb(CH3)3Br. (Т) Temperature; the same
for Fig. 4.
Fig. 4. Thermogram of Pb(CH3)2Br2.
Thus, the data obtained by spectrophotometry, IR
spectrometry, and X-ray diffraction suggest that the
dealkylation of Pb(CH3)4 with bromine occurs with
replacement of one or two methyl groups, yielding in
the examined temperature interval two compounds,
Pb(CH3)3Br and Pb(CH3)2Br2, depending on the molar
ratio of the reactants.
The IR spectra of Pb(CH3)3Br and of the substance
deposited on the capsule walls in the course of isothermal
heating of Pb(CH3)3Br at 130°С are identical. This fact
confirms partial sublimation of Pb(CH3)3Br on heating.
According to [8], decomposition of alkyllead halides
should be accompanied by the formation of tetraalkyllead
compounds, Pb(CH3)4 in our case. It can be formed via
the following pathways:
Thermolysis of Pb(CH3)4 occurs with the formation of
lead metal [7]. We found no data in the literature on trans-
formations of Pb(CH3)3Br and Pb(CH3)2Br2 on heating;
therefore, we studied the products of their thermolysis.
·
2Pb(CH3)3Br → PbBr2 + Pb(CH3)4 + 2CH3,
(1)
(2)
3Pb(CH3)3Br → PbBr2 + 2Pb(CH3)4 + CH3Br.
Figure 3 shows the thermogram of Pb(CH3)3Br, in
which there are two endothermic effects at 138 and
372°С. The first endothermic effect is associated with the
decomposition of Pb(CH3)3Br. The results of X-ray dif-
fraction analysis show that the final thermolysis product
is PbBr2, with the endothermic effect in the DTA curve
at 372°С corresponding to its melting.
To confirm these pathways, Pb(CH3)3Br was studied
by mass spectrometry with direct sample inlet. In the mass
spectrum obtained, the strongest peaks belong to ions with
the following mass numbers m/e (the relative intensity,
%, is given in parentheses): 317 (100) Pb(CH3)2Br+, 208
(82.5) Pb+, 287 (72.6) PbBr+, 223 (61.2) PbCH3+, 94 (55.1)
CH3Br+, and 253 (54.3) Pb(CH3)3+.
In the mass spectrum of the gas phase, peaks of ions
with the mass numbers m/e of 14, 15, 16, 28, 29, 30,
40, 44, and 94 (hereinafter, for 1H, 12C, and 79Br) were
detected. These peaks belong to the fragment ions CH2+,
CH3+, C2H5+, and C3H4+ and to the molecular ions CH4+,
C2H+, C2H+, C3H8+, and CH3Br+. The CH3+ and CH2+
ions4are det6ected from the very beginning of heating of
Pb(CH3)3Br. This means that the decomposition starts
with cleavage of the Pb–CH3 bond, and other gaseous
products are formed in secondary reactions.
Also, the following less intense peaks were detected,
m/e (relative intensity, %): 383 (13.9) PbCH3Br2+,
302 (10.1) РbСН3Вr+, 238 (8.2) Pb(CH3)2+, 268 (7.1)
Pb(CH3)4+, 79 (6.3) Br+, 57 (1.3) C4H9+, and 43 (3.5) C3H7+.
The presence of the strong Pb+ fragment peak and of
the Pb(CH3)4+ molecular peak suggests the formation of
Pb(CH3)4 in the course of thermolysis. Under electron im-
pact, this molecule undergoes radical decomposition [12]:
The relative amount of the residue in thermolysis of
Pb(CH3)3Br is 31.9%. The calculated amount assuming
the formation of PbBr2 is 56.6%. This fact indicates that
a part of lead passes into the gas phase in the course of
decomposition. The lead loss may be due to volatilization
of the starting compound Pb(CH3)3Br or to formation
of a gaseous lead-containing product in the course of
decomposition.
Pb(CH3)4 → Pb+ + 4CH3+.
(3)
Thus, the thermolysis of Pb(CH3)3Br is accompanied
by its partial sublimation and decomposition.
The thermolysis of Pb(CH3)2Br2 follows more com-
plex pattern and is characterized by three endothermic
effects at 37, 117, and 375°С (Fig. 4). The thermolysis
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 5 2014

628
EGOROV
CONCLUSIONS
of Pb(CH3)2Br2 starts at a lower temperature than that of
Pb(CH3)3Br, suggesting thermodynamic instability of the
former.As in thermolysis of Pb(CH3)3Br, the final decom-
position product of Pb(CH3)2Br2 is PbBr2; therefore, the
endothermic peak in the DTAcurve at 375°С corresponds
to its melting. The relative amount of the residue for
Pb(CH3)2Br2 is 60.33%, and theoretically it should be
92.4%, which also confirms partial volatilization of lead.
(1) In the temperature interval from –20 to 20°С,
the dealkylation of Pb(CH3)4 with bromine occurs with
replacement of one or two methyl groups and formation
of trimethyllead bromide Pb(CH3)3Br and dimethyllead
dibromide Pb(CH3)2Br2.
(2) The thermolysis of Pb(CH3)3Br and Pb(CH3)2Br2
is accompanied by the formation of gaseous products
and lead bromide PbBr2. Pb(CH3)2Br2 decomposes via
formation of Pb(CH3)3Br.
In the mass spectra of gaseous thermolysis products,
we detected peaks of ions with m/e 14, 15, 79, and 80,
belonging to fragment ions CH2+, CH3+, and Br+ and to
the molecular ion HBr+. The generation of CH3+ ions is
characterized by two maxima at 37 and 117°С, whereas
(3) Thermal decomposition of methyllead bromides
is not an efficient route to isotope-enriched lead metal
because of probable increase in the loss of the isotope-
enriched material due to high volatility of Pb(CH3)3Br
on heating.
CH2 ions are recorded only at 117°С. The Br+ and HBr+
+
ions appear in the mass spectrum only at 362°С, with
melting of PbBr2.
The IR spectra of the substance obtained by isothermal
heating of Pb(CH3)2Br2 at 40°С and of the sublimate de-
posited on the capsule walls in the course of Pb(CH3)2Br2
heating are identical to that of Pb(CH3)3Br. Pb(CH3)3Br
may be formed via the following pathway:
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(70.2) Pb(CH3)3+, 223 (43.7) PbCH3+, 287 (46.1) PbBr+,
208 (40.4) Pb+, 302 (8.4) РbСН3Вr+, 238 (7.5) Pb(CH3)2+,
and 268 (1.1) Pb(CH3)4+. A distinctive feature is the ab-
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