Gas production reaction (1)

Reactions (including post-processing) often produce gas and generate positive pressure inside the container

For pressure reaction vessels such as steel autoclaves and stainless steel PTFE tanks, even if they are equipped with safety rupture discs, the gas that exceeds the discharge capacity of the vessel cannot be released immediately and may easily cause the vessel to explode.

1.

1.

For reactions and post-treatments that are not taboo against humidity and air, and do not require pressure, they can generally be operated with open ports, so that the generated gas can be discharged at any time

2.

For reactions and post-treatments that are taboo against humidity and air and do not require pressure, nitrogen flow can be used.

3.

1) Balloon buffer form

If the gas generated during the reaction or post-treatment is 2 to 3L, air balloons can be used cautiously as a buffer

2) Liquid seal

In order to prevent the reaction from directly contacting the air, a liquid sealing device can be used to smoothly lead out the positive pressure gas generated

It is necessary to consider whether the liquid sealing medium reacts with the sealed gas.

4.

If gas is generated during the reaction, it is necessary to strictly control the smooth progress of the reaction by adjusting the temperature (not necessarily the lower the better), adjusting the dripping rate, and leaving enough space

Case 1

According to the literature, a researcher responded as follows:

At 0°C, sodium hydride (70.

After the feeding is completed, there is no obvious sign of exothermic reaction, and then heating to reflux, that is, a large number of bubbles are generated, and the reaction intensifies suddenly and the upper part of the condenser tube catches fire

Reason analysis: heating to reflux, the reaction erupts instantly, a large amount of hydrogen produced overflows through the outlet of the condenser tube, and encounters the electric spark in the mechanical stirring motor and causes a fire

Case 2

A R&D personnel reacted as follows:

Under stirring, NaH (100g, 4eq.
) was slowly added to the THF reaction solution of A (150g) and B (100g), cooled with an ice bath, during which a large amount of solid appeared in the bottle, and the system became very viscous.
Magnetic stirring Unable to stir, the R&D personnel assisted in shaking the reaction flask by hand to make the reaction system uniform
.
When all the NaH is added, the reactants A and B all form sodium salts, the system becomes thinner, and the magnetic stirrer suddenly rotates.
The system produces a large amount of gas (hydrogen) foam, causing flushing, and the reaction liquid enters the ice-water bath, catching fire and causing Burn the solvent THF in the reaction flask and the organic solvent hanging on the cabinet wall to wash the flask, and the fire increases
.
The R&D personnel closed the fume hood as soon as possible, and extinguished the fire with yellow sand and dry powder fire extinguishers with the assistance of everyone
.
The accident photo is shown in Figure 24-31
.

Figure 24-31 The scene after the accident

Reason analysis: The two substrates of the reaction have three active hydrogens.
The amount of NaH-requiring substances is large, and the amount is large, so more gas is produced
.
Because the initial reaction system is very viscous, as the temperature rises slowly, the reaction liquid becomes thinner, and the reaction suddenly erupts, resulting in instantaneous production of a large amount of hydrogen and flushing
.
The reaction liquid containing excessive NaH enters the ice-water bath and catches fire
.
Lesson: Large batches of NaH should be fed in batches, and the reaction system should be stirred evenly.
Make sure that every addition of NaH has to be basically reacted before it can continue to be added in small amounts
.

Related Links: Decomposition and explosion of polynitrogen compounds