Thermal decomposition of a diazido ester: Pyrolysis GC–MS and DFT study

• Elucidated thermal decomposition mechanism of 1,6-bis(azidoacetoyloxy)hexane (HDBAA).
• Validated the mechanism elucidated by pyrolysis GC–MS using density functional theory.
• HDBAA decomposes exothermically to 1,6-bis (iminoacetoyloxy) hexane imine (HDBIA).
• HDBIA decomposes at very high temperature to form diimines, diols and dienes.
• Diimine formation by CO2, diols by CO2 and HCN and dienes by CO2 and CH2NH elimination.

Azides are versatile compounds which find applications in organic synthetic reactions, biological methods as well as in propellants. The basic understanding of thermal decomposition mechanism of azides not only aid in assessing their suitability for use in these areas but also are also critical for hazard analysis and for developing suitable models for the risk analysis during their synthesis, handling and application. In the present paper, the thermal decomposition of a diazido ester 1,6-bis (azidoacetoyloxy) hexane (HDBAA) was investigated by thermogravimetric-differential scanning calorimetric studies. The mechanism of decomposition was elucidated using pyrolysis gas chromatography–mass spectrometric technique. At 230 °C, HDBAA, preferentially form the corresponding diimine by elimination of N2. The decomposition of the diazido ester was complete, at 500 °C yielding N2, CO, CH2NH and HCN with concurrent formation of diols and dienes. The experimental findings were rationalized through density functional theory (DFT) based computational analysis. DFT studies revealed that an initial activation energy of 155.1 kJ/mol is required for the elimination of the first N2 from HDBAA leading to the formation of an imine through 1,2-hydrogen shift. The elimination of the first N2 is a highly exothermic reaction which leads to spontaneous elimination of the second N2 to form 1,6-bis (iminoacetoyloxy) hexane (HDBIA). All other pyrolytic products were formed from HDBIA and possess higher computed activation energy as evidenced from their presence in the pyrogram at elevated temperature of 500 °C.