Running Reactions Unattended!!

[....if you must.....]


    Organic reactions have never been famous for their speed - in fact, they are among the slowest reactions in the chemical world.  It is common to reflux a reaction for 12 or more hours, heat a flask in an oil or sand bath for three days (stirring the whole time), and otherwise consume huge amounts of time and energy waiting for a reaction to reach completion.

     That being said, it is still best, as much as possible, not to have to leave a heated reaction running overnight when no one can attend it.  If a drain becomes plugged, a flood can ensue in which water can (and has...) run all the way from the fourth floor to the basement, soaking every office or room through which it passes, along with any equipment that may be in a room and not appreciate being dowsed.  Obviously, a fairly new, tight building would never allow this, but, for now, we have to work with the old, leaky building we have.

     NOTE: An excellent habit to get into, which is mandatory for unattended reactions, but not a bad idea for all reactions, is to place a small crystallizing dish under your reaction flask.  That way, if the flask breaks (which happens more than you'd think if the magnetic stirring bar loses its lock and starts dancing all over the inside of the flask) or another disaster occurs, at least you won't lose your reaction contents all over the lab bench or hood.

What if a reaction needs to be heated all night?

If it appears that a reaction must be allowed to proceed overnight, verify that it really needs to be heated/refluxed.  Reactions with no running water almost never cause problems.  If your reaction must be refluxed all night, here's a set of guidelines for doing this with a minimal chance of drowning our downstairs neighbors.

Instead of using a faucet for the cooling water source and the sink for the drain, use one of our submersible water pumps in a 5-gal bucket obtained for the purpose.  Fill the bucket full of ice-water, and the submersible  pump will keep the condenser very cold for quite a long time.  Also, there's no chance of the sink's drain becoming plugged.   Ask a senior researcher how to set this up.

Make sure all rubber tubing is new-looking, supple, and free of cracks and other signs of wear that could turn into a tear, if the building's water pressure were to increase suddenly and without warning (which happens depressingly often).

Use the small metal clip-type clamps made for the purpose to clamp each hose end to the nipple in the glassware to which it's attached.  It doesn't make the hose popping off *impossible*, but it's pretty close.

Run the cooing water at the slowest practical speed.  Don't have it so slow that it may shut off if the building's pressure changes (vide supra), but not so fast that it may pop off all by itself.

Make sure to use a magnetic stirrer to aid stirring, NOT boiling chips.  These become clogged and useless incredible quickly, then one good bump can send your reaction mixture all over the place.

  • Also clamp all glassware together, using the multi-color Keck clips you've seen around.  Note:  The cheapest Keck clip is ca. $3.00, so don't treat them like they're disposable.
  • Do not use rubber speta to close any standard-taper (or any other, for that matter) joint in a flask which is refluxing, unattended or not.  Ether solvents, in particular (diethyl ether, THF, DME) cause rubber to swell, such that septa usually will pop out of the joint, exposing the reaction contents to the atmosphere and allowing the boiling solvent to escape, overheating the contents in an unprotected atmosphere.
  • Even though you may be a real pro who's done this a lot, please have someone else check over your setup before you leave it for the night.  Sometimes a fresh pair of eyes can see something so obvious that you miss it completely.
  • Make sure that your system is at thermal equilibration before leaving it.  That is, it's refluxing gently but firmly, and there's no indication that anything is going to change for the worse during the night.
  • Please a piece of paper with your name and phone number and the last time you saw the reaction in front of the reaction, and weigh it down.  That way, if something unfortunate happens, they know where to look.

What if it doesn't need to be heated, but still must run all night,
especially under an inert atmosphere?

  • Sometimes a reaction must stir for 24, 36h, or longer, even though heating is unnecessary.
    • If it can be open to the atmosphere, the setup is trivial.
    • If an inert atmosphere (N2, Ar) must be maintained, a gas bubbler is used to indicate a slight positive pressure of the gas.  Ask a senior researcher how to do this.
    • As with cooling hoses, all inert gas lines should be safety-wired.
  • All other precautions spelled out for hot reactions should also be carried out for room-temperature processes.

What if a reaction needs to be cooled all night?

  • This depends on whether stirring is required.1   If  none is required, you can just stopper the flask and let it sit in the refrigerator.  The reaction we usually use to prepare β-lactones is an ideal example of this.
  • If it must be stirred, make arrangements with the faculty in charge of the walk-in freezer to leave a reaction stirring in the freezer all night.

Clearly, by taking the right precautions, setting things up slowly and carefully, you can put the hours of the night to work for you.  But, PLEASE, be sure to be meticulous in your setup, double check everything, ask someone to look over your setup to be certain, and if you have the slightest doubt as to the safety or wisdom of running a reaction all night, DON'T DO IT!! 

To get permission to run a questionable reaction, make sure and ask ME - not a senior researcher, not a postdoc, not another faculty member - just me!

1. It's amazing how often people stir reactions that do not need it!  If the reaction is a solution; that it, if there are no solids present, and no solids are expected to form, why stir it?



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