2012: Advent Chemistry: Ethanoic Acid

by on December 3, 2012

The molecular structure of ethanoic acid

This is ethanoic acid, also known as acetic acid. It’s the part of vinegar that makes it taste and smell of vinegar. I’m rather fond of it, partly because I actually rather like the smell, partly because it’s a ‘weak’ acid that isn’t scary to work with, and partly because it can be used to make pretty homemade fireballs.

The molecule is a ‘Brønsted-Lowry acid’. That sounds horribly complicated, but it isn’t. All it means is that the molecule is acidic because it loses hydrogen. That H in the OH group? That can dissociate (split off) from the rest of the molecule as H+. H+ is the acidic bit of most things that you think of as acids – hydrochloric, nitric, sulphuric, and plenty of others.
The rest of an acid matters too, because it affects how quickly and easily H+ is lost, which is why some acids are stronger than others.

Ethanoic acid is a good example of charge delocalisation – that is, spreading electrons around to either fill in a gap (a positive charge) or cope with an extra (a negative charge). In this case, H+ has gone off and left the rest of the molecule holding an electron. That electron gets passed around between the two oxygen atoms, by means of bond shuffling.

You may have noticed that sharing out a negative charge between two oxygens is what happened in TNT yesterday, and you’re right. Oxygen is a fantastic atom for storing extra electrons – negative charges. Conversely, it really hates being positively charged – not having enough electrons. Hydrogen is the other way around. It would much rather be H+ than H.
This preference for being positively or negatively charged is called electronegativity. More electronegative things, like oxygen, want electrons more.

The other concept we’ve encountered today is that of functional groups. You’ll note that we haven’t discussed the left hand side of the molecule at all. That’s because it doesn’t do anything interesting. It just sits there while all the chemistry happens around the acid part. The acid is a functional group – it does stuff – and the CH3 isn’t.

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