Study of the reaction between boron trifluoride methanol complex and sodium methoxide

Article abstract of DOI:10.1080/00958972.2014.919388

The reaction between boron trifluoride methanol complex and sodium methoxide in methanol solution was investigated using conductivity as the reaction indicator. The reaction conditions were examined and a mechanism of this reaction was proposed. Moreover, proper reaction conditions were proposed for boric acid preparation using this reaction. 2014

Full text of DOI:10.1080/00958972.2014.919388

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Study of the reaction between boron
trifluoride methanol complex and
sodium methoxide
ab
a
a
a
Lang Wuke , Zhang Weijiang , Xu Jiao & Zhang Lei
a
School of Chemical Engineering and Technology, Tianjin
University, Tianjin, China
b
College of Chemistry and Life Science, Weinan Normal
University, Weinan, China
Accepted author version posted online: 09 May 2014.Published
online: 21 May 2014.
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To cite this article: Lang Wuke, Zhang Weijiang, Xu Jiao & Zhang Lei (2014) Study of the reaction
between boron trifluoride methanol complex and sodium methoxide, Journal of Coordination
Chemistry, 67:9, 1530-1540, DOI: 10.1080/00958972.2014.919388
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 9, 1530–1540, http://dx.doi.org/10.1080/00958972.2014.919388
Study of the reaction between boron trifluoride methanol
complex and sodium methoxide
LANG WUKE†‡, ZHANG WEIJIANG†, XU JIAO*† and ZHANG LEI†
†
‡
School of Chemical Engineering and Technology, Tianjin University, Tianjin, China
College of Chemistry and Life Science, Weinan Normal University, Weinan, China
(Received 21 January 2014; accepted 4 April 2014)
Boron trifluoride methanol complex was added to sodium methoxide at 65 and 25 °C, and the
reaction time was 18 and 6 h, respectively. This figure indicates that the feeding stage is the
controlling step of the whole reaction.
The reaction between boron trifluoride methanol complex and sodium methoxide in methanol solu-
tion was investigated using conductivity as the reaction indicator. The reaction conditions were
examined and a mechanism of this reaction was proposed. Moreover, proper reaction conditions
were proposed for boric acid preparation using this reaction.
Keywords: Boron trifluoride methanol complex; Sodium methoxide; Mechanism; Boric acid
1. Introduction
Our laboratory has engaged in the separation of boron isotopes and preparation of enriched
boron compounds for years. Our basic idea is to produce enriched B-10 boron trifluoride-
anisole or enriched B-10 boron trifluoride gas first, and then produce other boron-10
*
Corresponding author. Emails: xujiaohh@163.com, wjzh@tju.edu.cn
2014 Taylor & Francis
©

Boron trifluoride methanol
1531
enriched boron compounds such as enriched B-10 boric acid. For enriched B-10 boric acid
production, some methods are plausible. The first method is that BF complex, such as
3
boron trifluoride ether complex, reacts with calcium carbonate or lithium carbonate in water
to produce boric acid [1, 2]. The second method is that diethyl ether complex reacts with
calcium chloride in methanol; after the produced hydrogen chloride is neutralized, the pro-
duced methyl borate is hydrolyzed to boric acid [3]. The last one is that diethyl ether com-
plex reacts with sodium methoxide to produce methyl borate first, and then methyl borate is
hydrolyzed to boric acid [4, 5].
The last method interests us. The reaction between sodium methoxide and BF had been
3
investigated as early as 1894 by Lappert [6]. He claimed to have prepared methyl fluoro-
esters BF OCH , and BF(OCH ) by the interaction of boron trifluoride and sodium meth-
2
3
3 2
oxide as follows:
BF þ NaOCH ꢀ! BF OCH þ NaF
3
3
2
3
BF3 þ 2NaOCH3 ꢀ! BFðOCH3Þ þ 2NaF
2
However, Meerwein and Pannwitz [7] claimed to obtain the coordination compound,
BF NaOCH , and other researchers [4, 5, 10] obtained methyl borate from this reaction.
3
3
Not only the discordance of different authors about this reaction, but also the capability of
producing methyl borate and further producing boric acid interests us. This article is aimed
to clarify the knowledge about this reaction and to find proper conditions to produce boric
acid using this reaction. The following details our research about the reaction between BF3–
methanol complex with sodium methoxide in methanol solution.
2
2
. Experimental
.1. Reagents and apparatus
Anhydrous methanol, sodium methoxide methanol solution (30%), anhydrous calcium chlo-
ride, and dihydrate calcium chloride are of analytical purity, purchased from Guangfu Fine
Chemicals Co. Boron trifluoride gas is purchased from Liufang Gas Co. and the purity is
9
9.5%. Boron trifluoride complexes and the enriched boron-10 BF complex were obtained
3
in our laboratory. A DDS-11A conductivity meter was used to detect the reaction system’s
conductivity.
2
.2. Preparation of BF –CH OH complex
3 3
The complex solution was produced in a jacket packing tower as shown in figure 1. The
boron trifluoride entered from the middle of the tower and met the methanol pumping from
the methanol tank. Natural abundance BF complex or enriched boron-10 BF complex can
3
3
be obtained depending on the BF source. The BF mass concentration of the produced
3
3
solution is within the range of 0.300–0.680.

1
532
L. Wuke et al.
3 3
Figure 1. Apparatus for the preparation of BF –CH OH complex.
2
.3. Reaction between BF –CH OH and sodium methoxide
3 3
The complex (80.10 g, 0.314MBF ) was added dropwise to a stirring solution of sodium
3
methoxide methanol (169.79 g, 30%, 0.932MNaOCH ) in a water bath. After addition, the
3
temperature of water bath was set and a conductivity electrode was placed in the flask. The
conductivity was recorded every 30 min and about 5 g of mixture was extracted using
1
0 mL syringe every 40 min. The mixture was weighed and centrifuged; the solid was dried
and weighted. Then the solid content of the reaction mixture was calculated.
When the reaction was stopped, the mixture was centrifuged, the wet solid was dried in
an oven and the liquid was distilled. Due to the formation of an azeotrope between methyl
borate and methanol at 54.6 °C, the distillate of 50–64 °C was collected.
2
.4. Determination of the concentration of sodium methoxide solution
The concentration of sodium methoxide methanol solution was determined according to
HG/T 2561-1994.

Boron trifluoride methanol
1533
2
.5. Determination of BF concentration of the complex
3
About 1–2 g (accurate to 0.0001 g) of BF complex was extracted using 5 mL syringe and
3
injected to about 20 g CaCl solution (20 wt%) which was neutralized to methyl orange
2
endpoint by HCl solution (0.1 M). Then the mixture was titrated to the methyl orange end-
point using standard sodium hydroxide (0.5 M) and boiled for 30 min till the methyl orange
no longer turned red. At last, the mixture was cooled to room temperature; 5–6 g of mann-
ital added and titrated to the phenolphthalein endpoint using standard sodium hydroxide
(
0.5 M), and this volume of sodium hydroxide was used to calculate BF concentration.
3
2
.6. Determination of methyl borate and boric acid
The distillate of 50–64 °C was detected using GC, Agilent, db-1701p column. The methyl
borate content was determined by external standard method. The distillate was always
directly hydrolyzed and the produced boric acid was used to calculate the methyl borate
content of the mixture. The boric acid was determined by IR (TENSOR 27). The purity of
the boric acid was determined by acid–base titration using standard sodium hydroxide
(0.2 N) after the addition of mannital.
2
.7. Determination of the side product
The side products were determined by energy dispersive X-ray spectroscopy (EDS),
1
5 keV. The purity of sodium fluoride was determined according to GB/T 1264-1997.
3
3
. Results and discussion
.1. Synthesis
Most authors used BF –ether complex as a reactant, but we used BF –CH OH complex
3
3
3
as a reactant. The advantage is that there is no need to isolate methanol when it reacts
with sodium methoxide in methanol solution. The BF –CH OH or enriched boron-10
3
3
BF –methanol complex was prepared first as shown in figure 1. It was used without
3
further purification.
If boric acid is the target product, reaction (2a) is needed. The first reaction (1a) is to pro-
duce methyl borate. The second (2a) is the hydrolyzation of methyl borate to boric acid.
CH OH
3
BF3 þ CH3ONa ꢀ! BðOCH3Þ þ NaF #
(1a)
3
BðOCH3Þ þ 3H2O ꢀ! H3BO3 þ 3CH3OH
(2a)
3
In order to produce boric acid, reaction (1a) should be studied and the following sections
focus on reaction (1a).

1
534
L. Wuke et al.
3.2. Study of the reaction conditions
Bowlus [8] claimed that the BF –CH OH system is featured by high conductivity.
3
3
Nieuwland et al. [9] showed that the conductivity of the system BF –CH OH is of the same
3
3
order as that of the system H SO –SO . O’Leary and Wenzke [10] had measured the mole
2
4
3
conductivity of the BF –CH OH system. While the other reactants, reagent methanol, and
3
3
sodium methoxide, or the product methyl borate have low conductivity compared with the
BF –CH OH solution, it is feasible to choose conductivity as the reaction indicator. The
3
3
reaction is deemed completed if the system conductivity does not change or changes as
slowly as possible. At the same time, the solid content was detected as described in
Section 2.3.
3
.2.1. Reactant ratio. Lappert [6] reviewed Gasselin’s results that, if the mole ratio of
BF and NaOCH is 1 : 1 or 1 : 2, the main product is the methyl fluoro-esters, BF OCH ,
3
3
2
3
and BF(OCH ) , as mentioned in the Introduction. But this result was not confirmed at that
3
2
time. Our experiment obtained methyl borate from the reaction between BF –methanol and
3
sodium methoxide methanol solution. Especially, when the ratio was 1 : 3, the yield of
methyl borate was the highest; when the ratio was greater than 1 : 3, the yield dropped shar-
ply; and when the ratio was about 1 : 4, methyl borate was hardly obtained.
When the ratio is less than 1 : 3, sodium methoxide is insufficient to remove the three
fluorine of BF , which leads to a low conversion of BF . When the ratio is greater than
3
3
1
: 3 such as 1 : 4, the produced methyl borate will be further consumed by excess sodium
methoxide, which leads to low yield of methyl borate. The latter observation was proved
indirectly by a patent [11] which reported that methyl borate reacts with sodium methox-
ide to form NaB(OCH ) . The following is the supposed mechanism of the 1 : 4 reaction.
3
4
Reaction (1b) is endothermic and slow; reaction (2b) is exothermic and rapid; and reaction
3b) is the overall equation. Reaction (2b) was repeated by the addition of methyl borate
(
to sodium methoxide methanol solution; the produced solid is almost equal to the theoreti-
cal yield of NaB(OCH ) and the EDS denotes the mole ratio of Na/O is close to 1 : 4. In
3
4
a patent [11], reaction (2b) was used to isolate methyl borate from its methanol azeotrope
and to produce high purity methyl borate by decomposing NaB(OCH ) .
3
4
CH
3
OH
3
NaOCH þ BF3 ꢀ! BðOCH Þ þ 3NaF #
(1b)
3
3 3
CH OH
3
NaOCH3 þ BðOCH3Þ3 ꢀ! Na½BðOCH3Þ ꢁ
(2b)
(3b)
4
4
CH ONa þ BF ꢀ! Na½BðOCH Þ ꢁ þ 3NaF #
3
3
3 4
3
.2.2. Feed mode. O’Leary and Wenzke [10] obtained sodium fluoroborate if sodium
methoxide methanol solution was added to boron trifluoride methanol complex. They
believed that reaction (3c) occurred. We had a similar result, but CH OBF cannot be
3
2
obtained by distillation, while a small amount of methyl borate was obtained. Hence, if

Boron trifluoride methanol
1535
Figure 2. Effect of the different feeding temperatures.
methyl borate is needed, BF –methanol should be added to the sodium methoxide methanol
3
solution to avoid the formation of NaBF4.
ꢀ
þ
2
ðCH OBF Þ H þ CH ONa ꢀ! NaBF þ CH OBF þ 2CH OH
(3c)
3
3
3
4
3
2
3
When BF –methanol solution is dropped to sodium methoxide methanol solution, heat will
3
be liberated. If the adding rate is fast, the temperature will rise quickly. So the effect of the
feeding temperature to the reaction was investigated. The experiments were conducted
under the same reaction conditions (mole ratio 1 : 3, water bath 65 °C), except changing the
feeding temperature. Our results showed that the difference between 12 and 25 °C feeding
temperature is little. However, the difference between 25 and 65 °C is great as shown in
figure 2.
Figure 3. Effect of the different reaction temperatures.

1
536
L. Wuke et al.
The curves of figure 2 are the conductivity versus reaction time at 25 and 65 °C feeding
temperatures, respectively. If the complex is fed at 25 °C, the whole reaction time is about
h, whereas, if the feeding temperature rises to 65 °C, the whole reaction time will be pro-
6
longed to 18 h. There is no rising stage in the 25 °C feeding curve, which could be attrib-
uted to too large detection intervals to record it. The reason of the rising stage will be
explained later.
3.2.3. Temperature and reaction time. Changmei [3] selected reaction temperature within
55–60 °C. Other researchers [4] chose reflux temperature. Considering the boiling point
54.6 °C of the azeotrope of methanol and methyl borate, the reaction of 1 : 3 ratio from 55
to 75 °C water bath was investigated.
The conductivity was recorded from 55 to 75 °C in a water bath with a 5 °C rise of temper-
ature as shown in figure 3. Table 1 is the analysis of the curves of figure 3. Figure 3 shows
that the conductivity of the system decreases and approaches a constant value eventually, but
the dropping rate of conductivity at different temperatures is different. Table 1 shows that
6
0 °C is the dividing point; under 60 °C the conductivity decreases slowly compared with that
of above 60 °C. However, the difference of 65–75 and 55–60 °C is not significant.
The solid content of this reaction mixture was detected and calculated in figure 4. The
solid content rises sharply to a constant value in about 40 min, and is constant the remain-
ing time. The highest point in the conductivity figure almost coincides with the solid con-
tent figure at 40 min.
Table 1. Analysis of figure 3.
Water bath (°C)
55
60
65
70
75
Reaction time (h)
Slope (ms cm min
10
8
6–7
−0.0021
6
5–6
−0.0024
−1
−1
)
−0.00063 ~ −0.00089
−0.00087 ~ −0.00092
−0.0022
Figure 4. Solid content with time.

Boron trifluoride methanol
1537
3.3. Analysis
The IR spectrum of boric acid is shown in figure 5. The strong bands can be assigned as
−
1
−1
follows: 3206 and 1194 cm are ν O–H and δ O–H, respectively; 1458 and 546 cm are ν
BO and δ B–O, respectively. The purity of boric acid is 99.5%.
The elements of the side product are shown in figure 6. The calculated mass content of F
in sodium fluoride is 45.25% and Na is 54.75%; the found result of EDS is F 47 ± 1, Na
5
3 ± 1%; so the side product is mainly NaF. The impurity may be methanol and boride such
as NaBF . The purity of the side product is about 96%.
4
3
3
.4. Mechanism hypotheses
.4.1. BF –CH OH system. Before studying the reaction between BF –CH OH with Na-
3
3
3
3
OCH , one must be aware of BF and its numerous complexes. Boron trifluoride is a strong
3
3
Lewis acid. It is a typical electron-deficient compound and is easy to form coordination
compounds with electron-rich atoms or molecules. Seven elements (N, O, F, H; P, S, and
Cl) have been shown with certainty to be capable of coordinating to boron trifluoride [12],
Figure 5. IR spectrum of boric acid.
Figure 6. EDS spectrum of side product.

1
538
L. Wuke et al.
while the stabilities of complexes with BF are different. For example, if oxygen is the
3
donor, the stabilities decrease in the order ethers > alcohols > carbonyl compounds [12].
Many reactions can be attributed to its electrophilicity and the exchange of ligands with dif-
ferent stability.
Boron trifluoride can form 1 : 1 or 1 : 2 molecular adducts with methanol. Some research-
ers [13, 14] studied the BF –CH OH system or the complexes BF ·CH OH and
3
3
3
3
BF ·2CH OH. They proved that the acidity and high conductivity of this system are due to
3
3
–
the formation of some ionic species. Jean Derouault [13] claimed that there exist BF and
4
+
CH OH in both BF ·CH OH and BF ·2CH OH. Servis [14] reported that at methanol to
3
2
3
3
3
3
boron trifluoride ratios of greater than 2 : 1, formation of the conjugate base of the 1 : 1
–
+
adduct, CH OBF , and the conjugate acid of methanol, CH OH , occurs. Our system is
3
3
3
2
similar to the latter.
3
.4.2. Feeding stage. With the addition of BF –CH OH, solid is gradually precipitated.
3 3
Sodium methoxide is a strong base, so the acid–base reaction was proposed to be the first
step as (1d), and it corresponds to the feeding stage. This step is exothermic, so high tem-
perature will suppress it. Figure 2 indicates that this step is the rate controlling step. In other
words, if this step is delayed, the whole reaction time will be prolonged. Analysis of the
solid content indicates that BF distributes mostly in liquid phase after addition.
3
CH
3
OH
ꢀ
þ
ꢀ
þ
CH OBF þ H HOCH þ CH O Na ꢀ! CH OBF Na(s)
3
3
3
3
3
3
ꢀ
þ
#
þCH OBF Na ðlÞ þ 2CH OH
(1d)
3
3
3
Some authors obtained coordination compounds as mentioned in the Introduction. This is
true when sodium methoxide is mixed with solvent such as ethyl ether or anisole, it does
not dissolve. For example, our experiment showed that the reaction between BF –anisole
3
and sodium methoxide in anisole lead to no methyl borate liberated, while methyl borate
was obtained when BF ·anisole reacted with sodium methoxide methanol solution. So the
3
solvent is important to this reaction. Methanol is not just a solvent but supplies CH O–
3
group in this reaction.
3
.4.3. Heating and keeping. After addition, heating began and 30–40 min was needed to
achieve the set temperature. Recording for the figures started at this point. This period of
time corresponds to the conductivity rising stage and solid content rising stage. We specify
this stage as the reactions 2d–5d. The reactions 2d–4d mainly consumed the dissolved
CH OBF Na and generated solid NaF, which caused the solid content to rise quickly. As
3
3
for the rise of conductivity, there are two explanations. One is that dissolution of the solid
CH OBF Na leads to the rise of the conductivity. Another is that the generated intermediate
3
3
featured high conductivity leading to rise of conductivity. With regard to the F removing
rate of 2d, 3d, and 4d, there is no direct evidence, but we are inclined to consider (2d) is
faster than the others.
The conductivity began to decrease smoothly and the solid content kept constant in the
remaining 5 h. This period still corresponds to reactions 2d–4d and a physical process 5d.
The fact that the solid content is constant can be attributed to that solid dissolved (5d) and
the solid consumed is equal to the solid generated. The decrease of conductivity still has
two explanations. One is that the consumption of CH OBF Na is larger than the liberation
3
3

Boron trifluoride methanol
1539
of CH OBF Na from the solid, another is the consumption of the high conductivity inter-
3
3
mediate which led to the decrease of the conductivity. The long time of this period is due to
the rate decrease which resulted from the reduction of NaOCH and CH OBF Na after con-
3
3
3
sumption in the first 40 min.
If we suppose 5d is not affected by temperature, the fact that the reaction rate does not rise
much with temperature rise indicates that reactions 2d–4d are not thermodynamically con-
trolled but kinetically. In other words, this system is not sensitive to heating temperature
above 60 °C.
Although, some researchers [15] claimed to have prepared BF OCH and BF(OCH )
3 2,
2
3
and studied the property of these two compounds, we cannot isolate these two intermedi-
ates. Maybe there is equilibrium between these two compounds and B(OCH ) as follows:
3
3
2
BFðOCH3Þ ꢀ BF2OCH3 þ BðOCH3Þ
(6d)
2
3
The production of BF OCH (mp 41.2°, bp 85.2 °C) by passing BF to B(OCH ) was
2
3
3
3 3
repeated in our laboratory. This compound is solid in room temperature, but fumed if
exposed in air. Purification through distillation was difficult. It dissolved in methanol, and
methyl borate was obtained if distilled with methanol as described in (7d). So we think this
compound cannot be separated when methanol is present.
heat
3
BF2OCH3 þ 2CH3OH $ 2HBF3CH3OH þ BðOCH3Þ
(7d)
3
3
.4.4. Side reactions. There are two competing reactions which lead to side products, and
lead to completely different reaction products if conditions are changed. If BF –CH OH is
3
3
overdosed, reaction (8d) will occur, which explains reaction (3c), and explains why the
yield of methyl borate decreases sharply when the mole ratio is more than 1 : 3. If NaOCH3
is overdosed, reaction (9d) will occur, which accounts for the 1 : 4 ratio results.
CH
3
OH
BF ꢂ CH OH þ NaF ꢀ! NaBF þ CH OH
(8d)
(9d)
3
3
4
3
CH OH
3
NaOCH3 þ BðOCH3Þ3 ꢀ! Na½BðOCH3Þ ꢁ
4

1
540
. Conclusion
The reaction conditions for the reaction between BF and NaOCH were investigated.
L. Wuke et al.
4
3
3
When the mole ratio of BF and sodium methoxide is 1 : 3 and water bath is 55 °C or
3
above, the reaction between BF and sodium methoxide will mainly produce methyl borate
3
and sodium fluoride.
A mechanism of this reaction was proposed. The first step is an acid–base reaction and is
the controlling step. The reaction is kinetically controlled, and it is not sensitive to tempera-
ture. Although the initial F removing is quick, the complete removal is time consuming.
Acknowledgment
The authors are very grateful to the Tianjin University Innovation Fund for providing
financial support (project no. 1307).
References
[
[
[
[
1] C.J. Rodden, New Brunswick, A.R. Eberle, South Plainfield, New Jersey, US Patent 2789884 (1957).
2] A.L. Fluesmeier, Miami, FL, US Patent 3682590 (1972).
3] W. Changmei. Liaoning Chem. Ind., 29, 76 (2000).
4] R.F. Barnes, H. Diamond, P.R. Fields. Preparation of Methyl Borate from Boron Trifluoride, US Atomic
Energy Commission, Illinois, IL (1954).
[
[
[
[
[
5] M. Han, B. Zhao, W.-J. Zhang. Chem. Eng. (China), 35, 70 (2007).
6] M.F. Lappert. Chem. Rev., 56, 959 (1956).
7] H. Meerwein, W. Pannwitz. Journal für Praktische Chemie, 141, 123 (1934).
8] H. Bowlus, J.A. Nieuwland. J. Am. Chem. Soc., 53, 3835 (1931).
9] J.A. Nieuwland, R.R. Vogt, W.L. Foohey. J. Am. Chem. Soc., 52, 1018 (1930).
[
[
[
[
[
[
10] L.A. O’Leary, H.H. Wenzke. J. Am. Chem. Soc., 55, 2117 (1933).
11] G.L. Cunningham, The Preparation of Trimethyl Borate, US2830070 (1958).
12] N.N. Greenwood, R.L. Martin. Q. Rev. Chem. Soc., 8, 1 (1954).
13] J. Derouault, T. Dziembowska, M.T. Forel. J. Mol. Struct., 47, 59 (1978).
14] K.L. Servis, L. Jao. J. Phys. Chem., 76, 329 (1972).
15] J. Goubeau, K.E. Lücke. Justus Liebigs Annalen der Chemie, 575, 37 (1952).