So with my pupils having just finished their final organic paper I decided to base this post around connectivity – the attachment of atoms in a molecule. I introduce its importance in class by showing this brilliant video by Professor Dave which he made after spotting a poor structural representation of a molecule in the opening credits of Brian Cox’s tv programme. This video emphasises the importance of correct representations and after that I show no mercy and the red pen is out ready to start circling ! Mark schemes also see connectivity as an important chemical skill and you can see that when drawing an alcohol the examiners will penalise if the connectivity of the carbon to oxygen is not shown correctly.
Only last week I read a post by the eminent chemistry blogger Derek Lowe about a structure he spotted in a paper about piperazines. Now, although the connectivity is correct he points out the poor bond lengths and angles. Even more disappointing they do not show the stereochemistry of the hydroxy groups and that could mean the diagram could represent up to 8 different structures. I would like to think by the time our A level students sit their final organic paper they understand the importance of correct representation and if a career in chemistry beckons they would not let mistakes like this occur.
So outside of chemistry who really cares? Well this is a true story. Recently I was in a very trendy coffee shop with the organic chemist (check out his guest post for me here) who noticed that the barista had tattoos of the structures of caffeine and theobromine (chocolate). When the organic chemist mentioned the tattoos the barista became quiet and sullen. Turns out she thought he had spotted the mistake on the structure of caffeine, a methyl group CH3 was incorrectly tattooed as OH3. A permanent reminder that there is no room for mistakes with your structural representations.
This is my fiftieth post and I have been mulling over what to make it about – it’s my half century celebration and I never thought I would have kept the blog going so long. So with all the talk this week of naming the new elements I thought I’d go back in time to element number 50 and boy has it been around for a while ! It’s TIN – named for the Etruscan God Tinia and its symbol Sn ( always a good quiz element ) from the Latin stannum.
One of my favourite books is ‘Napoleons Buttons – 17 molecules that changed history” by Penny le Couteur. Unfortunately I lent it out to a pupil in school and it wasn’t returned, and now it can only be bought on Amazon for an extortionate price ! It told the story of Napoleon’s retreat from Moscow and how in the depth of winter the buttons on his men’s uniforms started to fall off. Apparently they were made of tin and when the temperature dropped it changed from the allotrope shiny beta tin to brittle alpha tin (apparently the transition temperature is 13 oC but small amounts of antimony or bismuth can prevent this happening). This campaign was seen as the turning point in the Napoleonic wars and if the hypothermia suffered by Napoleon’s troops had been avoided would European history have played out differently ?
But tin goes back much further than the 1800’s and was known to ancient civilisations. Tin does not occur as a native element but can be isolated by heating it’s ore in the presence of carbon. Tin melts at a relatively low temperature (230 oC) so it has been used widely in alloys in particular bronze. Bronze is a mixture of about 80% copper and 20% tin ( the composition changes depending on the type of bronze required). The Bronze Age started about 3000 BC ( linking the Stone Age with the Iron Age) and the combination of stone and bronze allowed for tools and weapons to be made. The earliest tin-alloy bronzes date to 4000BC in Iran and Iraq and as copper and tin mines are not found together it involved a lot of trade to make bronze. Bronze is hardwearing and there is no superficial oxidation due to a barrier of copper oxide. However, it soon was superseded by iron as although not as durable it was more accessible. Nowadays bronze is the reserve of church bells and medals but once upon a time it defined an era.
So forget the tinfoil, tin soldiers and even the tin man -this element deserves its accolades, it’s time for tin to take its place as a defining element of the Periodic table, a history shaper and game changer.