A simultaneous TG-DTG-DSC-quadrupole mass spectrometric study
Mechanistic aspects of the thermal decomposition of benzyl-triethyl-ammonium tetrafluoroborate (BTEATFB) employing simultaneous TG-DSC coupled with a quadrupole mass spectrometer are considered in this work. The experiments were conducted in an inert atmosphere of helium. The decomposition of BTEATFB proceeds through several competing mechanisms. While nucleophilic substitution reaction occurs through the formation of a tertiary amine and plays an important role in the initial stages of the decomposition, and the probability of Hoffman elimination also exists.
KeywordsBenzyl-triethyl Ammonium Tetrafluoroborate Decomposition Hoffman elimination Nucleophilic substitution
Among many other quaternary ammonium compounds: halides, hydroxides, and tetrafluoroborates of benzyl-triethyl- ammonium and benzyl-tri-n-butyl ammonium cations have been extensively used as phase-transfer catalysts [1, 2, 3, 4, 5, 6, 7]. Quaternary ammonium compounds such as choline are important in medical research . Inclusion in the synthesis mixture of various quaternary salts, particularly bulky alkyl ammonium cations, has introduced a new structure-directing parameter into the synthesis mechanism , so that new type of zeolites have been patented [10, 11, 12] to widen up the scope of zeolite chemistry and applications. Toxicological evaluation was performed on human colon carcinoma cell line (CaCo-2) of ionic liquids based on imidazolium, guanidinium, ammonium, phosphonium, pyridinium, and pyrrolidinium cations .
The present work envisages studying the thermal decomposition of benzyl-tri-ethyl ammonium tetrafluoroborate (BTEATFB) in an inert atmosphere of Helium, and has not been considered hitherto. This is a prelude to using this compound as an additive for the modification of the thermal decomposition of ammonium per chlorate, a widely used oxidizer in composite solid rocket propellant technology even today.
The pure compound BTEATFB was procured from M/s. Aldrich Chemicals. The thermal decomposition was studied using simultaneous TG-DSC coupled with Quadrupole Mass Spectrometer (QMS). Measurements were carried out at Netzsch’s laboratory, Germany, employing STA/QMS 409-403 systems.
A sample heating rate of 5 °C min−1, an inert atmosphere of Helium, and a gas flow rate of 75 mL min−1 were employed in this study. The QMS data acquisition was carried out in the scan mode, in the mass range of 10–200 amu.
Results and discussion
Assignment for the evolved mass fragments during thermal decomposition of benzyl-triethyl-ammonium tetrafluoroborate
CH3CH2N + H
CH2=NH + C2H5
As BTEATFB is analogous to tetra alkyl ammonium compounds, its thermal decomposition can be predicted to follow similar pattern as for NH4BF4.or for NR4BX4.
The formation of an intermediate amine in the thermal decomposition of quaternary ammonium compounds was reported by Haskins and Mitchell , Udupa , Nambiar et al. , and Prasad and Krishnamurthy . Similar observations were made in the thermal decomposition of tetra alkyl ammonium thiomolybdates  and thiotungastates .
A peak corresponding to m/z = 30 is good though not conclusive evidence for a straight chain primary amine.
In the thermal decomposition of BTEATFB, while at low temperatures simple displacement reactions appear to be predominant. At higher temperatures, more complex multi-center reactions are the probability. Observation of a strong peak corresponding to m/z = 28 is confirmative of the existence of Hofmann elimination process.
Dr. M. R. R. Prasad would like to profusely thank and acknowledge the experimental support extended by M/s. Netzsch Instruments, Germany in the present research work.
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