Nitration reaction out of control

The Incident

A preparation of 3-nitropropiophenone was being carried out at -20°C. The yield of this preparation was repeatedly poor, and it was decided to try to follow the reaction by hplc. However the hplc does takes some time to run (typically 30 min) and during this time the mixture thermally runaway and exited the flask, at an indicated tempereaute of ca.80°C.

It was suspected that either:

The reaction mixture inadvertantly warmed up while the hplc was being run... OR
The mixture was held too long and some side reaction set in.

The Investigation

Some initial DSCs were carried out (using high pressure gold plated crucibles).

DCS of 3-Nitropropiophenone - very large exotherm stated at 150°C
DSC of Reaction mixture
1. No thermal drift when held isothermally at -20°C
2. Moderate exotherm starts at -10°C

Conclusions

It is suspected from the data obtained that the runaway was probably due to the oxidation of the side chain. This reaction is always present in the background, and it is surmised that, on this occasion, the temperature was allowed to warm up somewhat while waiting for hplc data.

Subsequent preparations have been limited to the current batch size, and temperature and time control are more carefully monitored.