The Odd Oxygen

There is a weird smattering of organic oxides whose molecules contain a foreign oxygen latched on to an otherwise familiar framework. I’ve written about DMSO before, which is essentially dimethyl sulfide with an extra oxygen atom along for the ride. N-Oxides fit into this group of compounds as well.

The structure of nitrous oxide, also known as laughing gas.

Nitrous oxide (also known as laughing gas).

Perhaps no molecule better typifies the class than nitrous oxide, N2O. Even the molecular formula sets off neural fireworks: that can’t be right. A central nitrogen flanked by N and O? Something’s wrong here. The central nitrogen seems overworked, while the oxygen seems to be missing a bond. Despite its bizarre structure, nitrous is surprisingly unreactive—I learned this recently while helping out a teacher friend with one of his students’ science fair projects. Thanks to its lack of reactivity at sane temperatures, detecting N2O is a pain.

Synthesizing nitrous, on the other hand, is quite easy. Upon heating, ammonium nitrate breaks down into N2O and two water molecules. The melting point of NH4NO3 is downright eye popping for an ionic compound: 170 ºC. Some rather unsafe methods actually produce nitrous from molten ammonium nitrate at high temperatures.

NH4NO3(l) → N2O(g) + 2 H2O(g)

One must be careful as the dissociation of ammonium nitrate into gaseous nitric acid and ammonia competes with N2O formation (“decomposition”).

NH4NO3(l) → HNO3(g) + NH3(g)

Dissociation is endothermic and decomposition exothermic, so heating can set up an interesting situation where the dissociation reaction can “quench” the heat released by decomposition. When dissociation is suppressed, the decomposition reaction can become a runaway exotherm. Although this document on the safe production of nitrous oxide from ammonium nitrate is written for chemical engineers—they even go to the trouble of writing out “gram-mole”!—it’s an illuminating read with some additional information about the synthesis of nitrous oxide. Continue reading →

Demo This!: Smash Glow Crystals

It’s time for a nod to one of my favorite chemistry-themed YouTube channels, NurdRage. NurdRage’s channel is basically a laundry list of awesome, little-known chemistry experiments…with a creepy yet soothing voice-changer voice to boot. One of his recent videos is especially cool: he synthesizes blue triboluminescent crystals from copper thiocyanate, pyridine, and PPh3, then grinds them up. The experiment is based on a recent JCE article from Marchetti et al. The reaction itself is straightfoward to perform: mix everything up, heat to dissolve, wait for cystallization, and wash! The copper salt, pyridine, and triphenylphosphine react to form the coordination complex (SCN)Cu(py)2(PPh3). Grinding of the crystals produces a stunning blue light.

NurdRage’s explanation of triboluminescence (the direct transduction of mechanical energy to light) is clear and concise—check it out below! Long story short, charge separation occurs in the crystal upon mechanical agitation, and recombination of the charges produces a blue light.

Demo This!: Trautz-Schorigin Reaction of Polyphenols in Green Tea

Periodically, I plan to cover a new demonstration from the recent chemical education literature in a feature I’m calling Demo This! Today’s featured demonstration comes from a recent J. Chem. Educ. article, which highlights the use of polyphenols in green tea for the luminescent Trautz-Schorigin reaction.

Background

Pyrogallol, or what we might call 1,2,3-trihydroxybenzene, undergoes an interesting set of transformations under oxidative conditions. In the presence of water, formaldehyde, base, and hydrogen peroxide, pyrogallol is oxidized and excited singlet oxygen is produced. Relaxation of singlet oxygen to its ground state produces red luminescence.

The Trautz-Schorigin Reaction: see if you can draw a mechanism for this beast!

The Trautz-Schorigin Reaction: see if you can draw a mechanism for this beast!

A quick literature search has revealed that this reaction has been understudied (or at least underpublished) over the years. See if you can draw a mechanism accounting for all the products! All manner of oxygen-containing species may be present under these harsh conditions, including superoxide anion and hydroperoxide anion.

This reaction can be slowed or prevented by treatment with boric acid (forming cyclic borate esters, which are resistant to oxidation) or by treatment with ascorbic acid, which can reduce the 1,2-keto intermediate back to pyrogallol and (in a separate reaction) react with singlet oxygen. Considering these quenching reagents, this demonstration has all the trappings of a “green,” easy-to-prepare experiment.

Experiment

This demo can be carried out either with the parent pyrogallol or with polyphenols found in green tea. Either way, set up is straightforward, and the article claims that the entire kit and kaboodle takes less than one hour. Assuming that a tea bag holds about 2 grams of tea leaves, infusing for ~3 minutes in 200 mL of hot water is long enough to push a sufficient quantity of polyphenols into the water. Paraformaldehyde and sodium carbonate are then added to the hot tea with stirring, and the solution is allowed to cool to room temperature in a water bath. The pH of the solution is checked using pH paper or indicator before adding hydrogen peroxide (it should be ~11). 50 mL of the pH 11 solution are transferred to an empty beaker, and the lights are killed. Finally, 50 mL of dilute (3%) hydrogen peroxide are added. Luminescence should be instantaneous, and lasts for 5-10 seconds.

Ascorbic acid completely shuts down the reaction, while boric acid only slows it down. These quenching reagents should be added just before the lights are killed, right before the addition of hydrogen peroxide.

Source

Panzarasa, G.; Sparnassi, K. J. Chem. Educ. 2012ASAP. doi: 10.1021/ed200810c