27/6/2011

Parabens

Filed under: — Bravus @ 12:09 am

I wanted to say ‘Happy Birthday’ to a couple of good friends, and since I’m in Portugal I looked up how to do it in Portuguese1. One way to say it is ‘Parabens!’ – and my chemistry ears pricked up a bit. In chemistry the parabens are a family of compounds derived from parahydroxybenzoic acid. They are widely used as preservatives.

Well, I guess really it was my ‘teacher ears’ that pricked up, because a possible approach to teaching a chemistry lesson about the parabens seemed to arrive. As a lesson plan it would need to include a lot more action and involvement from the students, but I thought I’d share a little bit of the explanatory portion here, just for fun2.

In teaching, I might start with the ‘Parabens means Happy Bithday in Portuguese’ thing, just as a quirky mnemonic device, but then get into the explanation. I’d probably bring in some cosmetics, pharmaceuticals and other products and point out the ‘methylparaben’ and ‘propylparaben’ in the list of ingredients. The lesson would probably come as part of an organic chemistry topic in a high school chemistry course, at a point where students already understood chemical bonding and some of the conventions of how organic molecules are represented:

This structure is for the actual parahydroxybenzoic acid (italics indicate where the name comes from). The other chemicals in the family are made by adding methyl, ethyl, propyl and so on groups where the R is on the diagram above, and that’s a discussion for a later time.

So, what do we see in this structure, and why does it have the name it has? The first feature is the hexagonal ‘ring’ structure. This is known as the ‘benzene ring’, and the chemical compound benzene, which used to be used in things like drycleaning and decaffeinating coffee, would be just the ring without the extra things sticking out at the top and bottom. They even used to use it as aftershave – it smells… interesting, but I personally wouldn’t like to smell of it.

(Because there are so many carbons in organic chemistry, we save time and energy by not drawing them in diagrams, they are just ‘taken as read’. Everywhere the lines join there is a carbon atom.) The ring is shown here with alternating single and double bonds, three of each. The ring is made up of carbon atoms bonded to each other, with a ‘spare’ bond on each pointing outward. Remember, carbon can form a total of 4 covalent bonds, and if you look at each of the corners of the hexagon and imagine an extra line pointing outward, linked to a hydrogen atom you’ll see that there are 4 lines in total connected to it. In benzene itself, hydrogen atoms are connected to the ends of the ‘spare’ bonds.

But we know a few things about double and single bonds. We know that they come out at an angle, and we know that double bonds are shorter than single bonds. So if the benzene ring really was as it looks in this diagram, the hexagon would not be regular, it would have 3 short sides (the double bonds) and 3 longer sides (the single bonds). It would also be kind of ‘crownshaped’, going up and down, if we rotated it around and looked at it from the side.

When we actually do the measurements on the benzene ring, though (and how those measurements are done on something as tiny as a molecule is a story for another day), we find that it’s flat, not crownshaped, and all six bonds are the same length.

That means the picture we have above is not quite right: they’re not double bonds and single bonds, they’re sort of ‘one and a half’ bonds. We won’t get to talk about it here in high school, but at university you can look forward to the discussion of how the p electrons in the carbon atoms form new π molecular orbitals above and below the ring… anyway, that’s for later.

I said benzene ‘used to be used’ for quite a lot of purposes, including drycleaning, but it’s not used as much any more, because it’s quite carcinogenic (cancer causing). It turns out that those unusual bonds mean it’s very good at attacking DNA, and broken DNA is what causes cancers.

Some of the health concerns around the use of parabens arise from the idea that, because it contains a benzene ring, it might have some of the same bad properties. You’ll be doing some research for your assignment about the chemistry of those claims and the scientific evidence, and will be asked to take and support a position on whether parabens should be banned, or used for a narrower range of purposes.

OK, so we’ve got the ring, now what about the stuff hanging off it? We’ll start with the easy one first, at the bottom. This is a ‘hydroxy’ group – hence the ‘hydroxy’ in the name – and is just an oxygen atom bonded to a hydrogen atom and the ring. The hydroxy group is the characteristic group of the alcohols, and we’ve looked at its properties a bit already.

At the top of our diagram, across the ring from the hydroxy group, is a carboxylic acid group. This involves a carbon atom that is double-bonded to an oxygen atom and single-bonded to another oxygen atom that in turn is bonded to a hydrogen atom (in this diagram they’ve represented it with an R instead of an H because it’s also possible to add other groups on in place of the hydrogen). That last part looks a bit like the hydroxy group, and has some similar properties, but it’s not a hydroxy group, it’s part of the larger acid group. There’s more to say here about electronegativity and electron density, but we’ll get to that later.

The last part of the name we need to explain is the ‘para’. It’s a way of saying where the two groups are around the ring. We start from the biggest attached group, which in this case is the acid group at the top. You can see that we could then put the hydroxy group on any of the other 5 carbon atoms in the ring. We need to be able to say where it is, because different locations will give the molecule slightly different properties.

Going clockwise around the ring, if the hydroxy group was next to the acid group, on the right, that position is called ‘ortho’. It would be possible (maybe, depending on the space in the molecule) to make ‘orthohydroxybenzoic acid’. I’ll give you a minute to draw that in your book and name it. Moving to the next position, that position is called ‘meta’. And then, when the molecule is as it actually is here, which the two groups opposite each other, the position is called ‘para’.

(I’d probably tie ‘ortho’ to ‘orthodox’, ‘meta’ to meta discussions, but this is already too long!)

You might think we’d need to have 5 labels since there are 5 positions, but if you imagine the hydroxy group in that bottom left corner, you can always just flip the molecule around its vertical axis, and the group will be in the bottom right ‘meta’ position… so it turns out we only need 3 labels, since it is how close that is important, not which side.

In a real lesson there’d be lots more questions, discussion, linking to past lessons and students’ life experience, discussion of how to answer exam questions on the topic, digressions into the conventions of how diagrams are drawn and what they really mean, and so on. But hopefully, at least, this has given you some sense of the kinds of processes that go on in a teacher’s head when preparing a lesson… and why it’s so much more than just information transfer. (And, linked to that, why a teacher can do a much better job than a textbook, all other things being equal.)

Oh, and for further reading, here’s the (excellent) Wikipedia article on the parabens: http://en.wikipedia.org/wiki/Paraben

  1. I’m fascinated with foreign languages and tend to try to figure out things like the words on labels. The Romance languages share enough Latin roots with English that it’s usually possible to piece together what’s going on with a little work. I’m fascinated by why Portuguese and Spanish are so different from one another when the countries are right next door with a land border (which makes it more mystifying to me than French and English).
  2. …and because it’s 3 am here and there’s nothing else to do. My efforts at synching my sleep cycle with here, coupled with the looong flight meant that I got my 9 hours of sleep from 6 pm to 3 am.

 

 

 

 

 

2 responses to “Parabens”

  1. Stan says:

    if you need any extra vowels or the letter Z, I know someone who has a lot of extras and is willing to share. Contact Blossom.

  2. Mark says:

    Great lesson, Dave! If you were a teacher, I’d love to be your student.

    I wonder if the greek you learnt at PAC has helped you with your label gazing? 😉

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