Why the formation of a fog is observed when triethylamine is added? In the procedure for the synthesis of N-anisoyl-2-pyrrolidinone that was given to me, it is written that when triethylamine is added, a &quot;fog&quot; is observed inside the flask.

Triethylamine is added to neutralize the hydrochloric acid that is generated during the reaction, forming triethylammonium chloride.
I searched for similar procedures involving an acyl chloride and an amine, but the formation of a &quot;fog&quot; is never mentioned.
A quick search on the internet gave me no clue.
In my experience I have never witnessed the formation of a &quot;fog&quot; with this kind of reactions.
Since I cannot enter the lab due to the COVID-19 restrictions, I cannot perform the reaction now, can someone help me to understand if and possibly why in this case a &quot;fog&quot; is formed?
You are adding Et3N to a solution that contains HCl as a reaction product. That means there will be HCl vapour above the solution. The Et3N is fairly volatile so there will be Et3N vapour around the addition stream/droplets and this will react with the HCl vapour as it is added to give triethylammonium chloride. This gives you the fog.