I have a general question about laboratory techniques in organic chemistry…well, OK, let’s make it a specific question. I learn the mechanism for Fischer esterification, right (carboxylic acid + alcohol --> ester + water, catalyzed by protic acid)—not so hard. So I go into the lab thinking I’m just going to stick all these reactants together in the right amounts and voila, I get products. So why isn’t this the case? Why did I have to reflux it? And by the way, exactly what IS refluxing? (I mean, what is going on in the apparatus?)
Quick and Dirty…
The reaction won’t take place without heat. You can’t heat a closed system because the expanding gases will cause it to explode. But if you use an open system then all of your solvents will evaporate.
The condenser is just a cooling tube. The solvent evaporates in the flask, then moves into the condenser where it cools, reliquifies and drops back into the flask.
Oh, you’ll understand why heat is needed when you get to thermodynamics. If you already had thermodynamics then review
potential energy diagrams.
Former chemist here…
Basically, the reason you have to add heat to get a reaction to “go” is to overcome the energy of the bonds in the reactants. Although the products are favoured thermodynamically, there is an “energy barrier” that you have to overcome before you can get there.
The analogy I was taught was a rollercoaster on a track, high in the sky but in a small dip before the main drop. Obviously the rollercoaster “wants” to be right down at the ground, but it won’t fall down there until you give it a small push up to the crest of the little dip. Clear?
As for why you reflux, I think zigaretten has covered it - basically it’s a neat way of heating right up to the boiling point of the solvent without the risk of boiling the flask dry.
Oh and, org chem being what it is, you can still reflux for hours and come back to find either (a) unchanged reactants, or (b) a shattered reaction vessel sitting in a pool of noxious liquid.
But that’s what makes it fun, right?
… furthermore, you want to push the equilibrium to the right to produce more of the ester.
So, use an excess of either the acid or alcohol - whichever is cheaper. Also removing the water will tip the equilibrium to the right. This can be done by refluxing in a non-polar phase, such as toluene.
There are other tricks for removing the water, but I will let you work that out. 