2012: Advent Chemistry: Ethanol

by on December 4, 2012

Today’s exciting entry is ethanol:

A line drawing of an ethanol, using the thick and dotted line conventions to show 3D structure

Ethanol is of course the alcohol commonly known as, well, ‘alcohol’. It’s found in whisky, rum, cider, ale, mead, wine, vodka, gin, and so on and so forth. It has an intoxicating effect on most animals including humans and is legal in most countries, although forbidden in some religions.

The ethanol molecule looks a lot like ethanoic acid, the compound we looked at yesterday. In some ways it is quite a lot like it – in particular, that OH can dissociate to O and H+. Unlike ethanoic acid, however, there’s nowhere for a negative charge to go. The oxygen is stuck with it. That makes dissociation really slow and so ethanol is a rubbish acid – worse than water. Its main use in chemistry is as a solvent and for getting undergraduates drunk on fumes.

Solvents get a whole classification system of their own. Ethanol is a polar, protic solvent. ‘Polar’ means that it has a negative bit and a positive bit. The molecule has a direction, like a magnet. ‘Protic’ means it can give up a proton to the surrounding environment. How can it do that?
Well, we already established that H+ can be, and is, lost from a whole range of molecules. H, the hydrogen atom, is one proton and one electron. To give it a positive charge, you take away the electron, leaving just a proton. An H+ ion is a proton, and ‘protic’ means ‘a bit Brønsted-Lowry acidic’.
The reason solvents need to be classified is that different things dissolve in different solvents, and it’s therefore useful to be able to predict which solvents will work and which won’t, especially if you’re trying to purify a mixture.

A quick word is called for about the way the picture is drawn. This is the international convention for drawing 3D molecules in 2D media. Ordinary lines are level with the page or screen. Dotted lines should be thought of as going away from you at an angle, into the screen, and bold/wedge lines as coming out of the screen, towards you. For ethanol it doesn’t matter which bit goes in which direction, because it can all rotate around, but that’s not true of everything.

One Response to “Ethanol”

  • ZenGwen says:

    This is neat, but the convention discussed in the last paragraph is very cool. I can totally picture Ethanol as one of those models built with science class sets. 😀

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