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Saturday 28 November 2020

Cop That!

Copper. Slang for a police officer. A coin. That metal pans and wires are made of.

What’s so mysterious? A lot.

Copper is a catalyst. It’s variable oxidation states and low coordination numbers allow it to do funky things, bonding and unbonding with whatever floats its way.
Copper coins via Pikrepo.

Whisky still IProspectIE via wikimedia commons.
Copper is used in whisky stills, partly because it’s malleable and conducts the heat through the whisky really well, and partly because it seems to affect the flavour. One story, related to me by Ben from The Whisky Shop, was the story of Old Pulteney whisky, back in the days when the distillery was first setting up, the days when they decided to make the most important feature of the distillery – the still. It was made offsite and transported carefully to the distillery in the Highlands of Scotland, where they tried to take it into the building – but couldn’t. The manufacturers, so the story tells, had made a mistake and ordered a still too tall for the distillery. Still, never mind, they said, and they lopped off the top, making a low, squat-looking still pot that led to shorter refluxes, forever after blamed for the “dirty” flavour of the Old Pulteney spirit.

There’s more about the mysteries of the science of whisky in our article on the topic!

Lemons. Evan Amos via Wikipedia Commons.
Copper also played a role in the undiscovery of the cure for scurvy. Between the 15th and 18th centuries, around 2 million people – mostly sailors – died of scurvy. This horrible, slow-acting disease caused by a deficiency in vitamin C sees you lose appetite, become lethargic, and develop aching joints before fever, swelling, and skin cracking set in, finally ending in cardiac arrest. Although citrus fruits had been known to help since ancient times, conflicting approaches and non-scientific theories prevailed, and it wasn’t until 1747 that James Lind proved it both treated and prevented the disease in the first ever clinical trial. However, nobody knew what it was in the citrus fruits. And so, accordingly, sailors packed up bagloads of juice, storing it in air and piping it through copper – which catalysed the breakdown of the vitamin, so rendering it useless. For years, Lind’s result was contested, until this was discovered!
Sonogashira reaction? © TWDK.

More recently, copper has been implicated in the mechanism for the Sonogashira reaction[1]. Chemists have been trying to work out the mechanism for this reaction, first discovered in 1975, where palladium catalyses coupling between organic halides and alkynes. The main theory for this reaction is that alladium inserts into weak bonds like carbon-halide bonds, then later swaps or deinserts, creating new pairs. However, some chemists think small amounts of copper contaminating the reaction are essential for making it work. It’s difficult to prove them right or wrong, as copper impurities in palladium tend to be extremely hard to get rid of.

There’s more about the Sonogashira reaction and other weird chemistry in our article on ambient chemistry!

It could do more. Researchers have found that carbon beds studded with copper atoms form 3-4 atom clusters under electrical pulses and then catalyse the formation of ethanol from water and carbon dioxide – the opposite reaction, in fact, to the one that takes place when ethanol burns. Ethanol is industrially useful – as a fuel, as a food, as a solvent – and so researchers are keenly looking into how this may work[2].

why don't all references have links?

[1] Ljungdahl, Thomas, et al. Two competing mechanisms for the copper-free Sonogashira cross-coupling reaction. Organometallics 27.11 (2008): 2490-2498.
[2] H. Xu et al. Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically dispersed copper. Nature Energy. Vol. 5, Published online July 27, 2020, p. 623. doi: 10.1038/s41560-020-0666-x.

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