Reaction feeding sequence for exothermic or gas production (1)

From the perspective of safety, the order of addition of the reaction can only be ignored when it is confirmed that there is no obvious reaction heat, no large amount of gas production, and a small amount (gram level) of reaction substrate

1.

For violent exothermic or gas-producing reactions, a high-energy reaction substrate should be added to a solvent (or another liquid reactant) that has a higher heat capacity and good stability

(1) Metal-organic compounds such as lithium, zinc, aluminum , and liquids such as borane and sodium amide dissolved in an inert solvent , although the danger is reduced by dilution, they are still dangerous goods

(2) All active solids, such as anhydrous aluminum trichloride , sodium hydride , lithium tetrahydrogen aluminum, etc.

(3) After the reaction is over, it is safer to slowly pour the reaction solution into the quenching solution under stirring

Unless it is known or proven that there will be no heat of dissolution or heat of reaction, the order of addition can be chosen arbitrarily, and convenience is the priority

Two, accident case

There are many accidents caused by incorrect feeding sequence, and most of the consequences are serious, causing flushing, burning or explosion, and may even cause personal injury, property damage or delay the completion of the project

(1) The solid needs to be added to the liquid

Case 1

A pharmaceutical factory scaled up a Friedel-Crafts alkylation reaction.

Reason analysis: the order of feeding is reversed, and the reaction heat cannot be rapidly diffused in the solid, resulting in rapid accumulation of reaction heat

Case 2

A R&D personnel exploded during the transposition of 2-carboxylic acid pyridine oxide, the reaction formula is as follows:

Nitrogen oxide as a substrate is a colloidal semi-solid, while phosphorus oxychloride is a liquid

I have done several batches of the same translocation reaction on a small scale, and they all went smoothly, but this time it was an amplification reaction and an explosion occurred

The reasons and lessons are the same as above

Case 3

The parties involved operate a 70g NaH reaction with DMSO as the solvent

Figure 24-34 Screenshot of the accident video

Figure 24-35 Photos of the scene after the incident

Reason analysis: First, the feeding sequence is reversed, the reaction heat cannot diffuse rapidly in the solid or semi-solid objects, and the latent heat of reaction accumulates quickly; the second is that the water content of ordinary bottled DMSO is large, and it reacts with NaH.
After reaching a certain temperature, the NaH is wrapped.
The mineral oil is melted, and the reaction is accelerated, so that a large amount of heat is generated instantly and an accident occurs
.

Lesson: For similar exothermic reactions, you should add solids to the anhydrous solvent, and always observe the system temperature.
Do not add all the solids at one time.
Make sure that a little bit is reacted every time you add a little bit to ensure that no reaction heat or reaction accumulates.
Latent heat
.
In addition, for reactions involving a large amount of NaH, DMSO should not be used as a solvent, nor can DMF be used as a solvent
.

Related link: Gas release in the separatory funnel