Pulling on a bond to break it – II

I’ve talked about pulling on covalent bonds to break them  before and even offered a calculation to convert the force necessary to break a bond into kiloCalories and vice versa — see https://luysii.wordpress.com/2010/09/15/how-strongly-do-you-have-to-pull-on-a-covalent-bond-to-break-it/.  No one has piped up to tell me I’m wrong (so far)

The latest example pulls apart a 1,2,3 triazole ring formed by classic click chemistry by embedding the ring in a polymer and effectively pulling on it  [ Science vol. 333 pp. 1582 – 1583, 1606 – 1609 ’11 ].  Heat won’t do it — even 300 C for days won’t break 1,2,3 triazoles. 27 June ’14 — This paper is probably fraudulent according to the Editor of Science

So you’ve got a polymer containing a triazole in its middle in a test tube — how do you pull on it?  With sound amazingly enough.  This brings up the whole fascinating topic of sonoluminescence.  Here’s some background.

     [ Nature vol. 418 pp. 381 – 382, 394 – 397 ’02 ] Sonoluminescence is the process by which a gas bubble in a liquid exposed to a strong standing sound wave collapses and emits light.  Where does the bubble come from?  What drives it to expand its volume by 1000 fold?  The bubble collapses shrinking to a radius corresponding to solid state densities.  The compression drives up the temperature of the gas inside the bubble.  The bubble interior is thought to reach 20,000 Kelvin.  The gas becomes partially ionized, and the recombination of electrons and ions leads to light emisssion.

     As the bubble expands, gas dissolved in the liquid enters the bubble.  The gasses react at these temperature.  N2 dissociates into nitrogen radicals, and then reacts with water (or oxygen?) to form NH and NO.  These highly soluble gases redissolve in the surrounding water when the bubble cools down.  

      They don’t say it directly, but since sound in liquid is alternating bands of density travelling through the liquid, the bubble expands in the low pressure part of the wave.  
 

      Single bubble sonoluminescence is characterized by the emission of picoSecond flashes of light.  It results from nonlinear pulsations of an isolated vapor gas bubble in an acoustic field.   At appropriate acoustic intensities, the bubble can emit 50 – 500 picoSecond flashes of light with clocklike regularity. 

      The featureless spectra of the light from 2000 – 8000 with increasing intensity toward the ultraviolet suggested the existence of extraordinary temperatures as high as 20,000 K in the bubble (black body effective temperatures). 

So just sound can produce some fairly amazing temperatures — hence collisional forces in solution.    Even so, the authors don’t think the process splitting the ring is thermal, because it depends on the lengths of the polymer chains attached to each side of the triazole ring — something that wouldn’t be expected from a purely thermal process — they think the polymer chains are pulling the triazole apart.

Great stuff — hard to see it being used in synthesis even though the authors provide excellent evidence that the azide is being reformed as the triazole breaks apart.

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