2013: Advent Chemistry: Refluxing

by on December 6, 2013
Reflux equipment

Reflux equipment

As a general rule, heating a reaction up makes it go faster. Many reactions work better if you boil the solvent they’re happening in. Actually, many reactions happen fastest of all in the vapour that comes off the boiling solvent. The reaction takes place in the steam, after it’s escaped from your flask.

There is a clever alchemist’s trick – you may have noticed that a great many of these lab techniques are just clever tricks – for minimising how much of your product escapes once it’s been synthesised in the vapour above your flask. You attach a condensing tube to the top of your flask, and let the mixture reflux.

A condensing tube is a fundamentally simple idea, though no doubt tricky to make. It consists of a straight tube, encased in a second tube. The outer tube has little pipes at top and bottom, sticking out sideways, and the inner tube is open at both ends – it’s basically a chimney. You connect it to your reacting vessel (your flask) at the bottom, so that the vapour is forced to go up the inner tube to escape. The bottom pipe of the outer tube you plumb into cold running water, generally by pushing a thin rubber tube onto it and also onto the tap. A second rubber tube takes the water out of the top and into the sink. This isn’t strictly speaking necessary for the condenser to function but does cut down on the cleaning afterwards.

Putting the water in from the bottom forces it to make a complete layer of ever-renewed cold water around the inner tube. If you put it in from the top, it would just run shallowly down one side.

The jacket of cold water cools the entire inside of the condenser, which proves itself to be aptly named, as the vapour from your flask condenses onto the glass just like steam onto a mirror, and runs back down into the flask, carrying reactants and products with it. Then it heats up (because of course your flask is still being heated) and goes round again, giving the reaction another chance to progress.

The result, in practice, is a gently bubbling flask with a condenser gently dripping into it, which is called refluxing, and generally has to happen for at least an hour. It is not remarkably entertaining. Your best bet is to team up with your lab-mates to keep an eye on each other’s reactions and take it in turns for a lunch break.

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