Thermal Stability and Explosive Hazard Assessment of Diazo Compounds and Diazo Transfer Reagents

Despite their wide use in academia as metal-carbene precursors,
diazo compounds are often avoided in industry owing to concerns over
their instability, exothermic decomposition, and potential explosive
behavior. The stability of sulfonyl azides and other diazo transfer
reagents is relatively well understood, but there is little reliable
data available for diazo compounds. This work first collates available
sensitivity and thermal analysis data for diazo transfer reagents
and diazo compounds to act as an accessible reference resource. Thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC), and accelerating
rate calorimetry (ARC) data for the model donor/acceptor diazo compound
ethyl (phenyl)­diazoacetate are presented. We also present a rigorous
DSC dataset with 43 other diazo compounds, enabling direct comparison
to other energetic materials to provide a clear reference work to
the academic and industrial chemistry communities. Interestingly,
there is a wide range of onset temperatures (Tonset) for this series of compounds, which varied between 75
and 160 °C. The thermal stability variation depends on the electronic
effect of substituents and the amount of charge delocalization. A
statistical model is demonstrated to predict the thermal stability
of differently substituted phenyl diazoacetates. A maximum recommended
process temperature (TD24) to avoid decomposition
is estimated for selected diazo compounds. The average enthalpy of
decomposition (ΔHD) for diazo compounds
without other energetic functional groups is −102 kJ mol–1. Several diazo transfer reagents are analyzed using
the same DSC protocol and found to have higher thermal stability,
which is in general agreement with the reported values. For sulfonyl
azide reagents, an average ΔHD of
−201 kJ mol–1 is observed. High-quality thermal
data from ARC experiments shows the initiation of decomposition for
ethyl (phenyl)­diazoacetate to be 60 °C, compared to that of 100
°C for the common diazo transfer reagent p-acetamidobenzenesulfonyl
azide (p-ABSA). The Yoshida correlation is applied
to DSC data for each diazo compound to provide an indication of both
their impact sensitivity (IS) and explosivity. As a neat substance,
none of the diazo compounds tested are predicted to be explosive,
but many (particularly donor/acceptor diazo compounds) are predicted
to be impact-sensitive. It is therefore recommended that manipulation,
agitation, and other processing of neat diazo compounds are conducted
with due care to avoid impacts, particularly in large quantities.
The full dataset is presented to inform chemists of the nature and
magnitude of hazards when using diazo compounds and diazo transfer
reagents. Given the demonstrated potential for rapid heat generation
and gas evolution, adequate temperature control and cautious addition
of reagents that begin a reaction are strongly recommended when conducting
reactions with diazo compounds.