B.R.S.M. Help! I'm trapped in a molecule factory!

29Oct/114

And Now For Something Completely Different 2: Five Uses For Wax In The Lab

Note: In my opinion, it would be fairly ill advised to attempt any of these.

So, wax. According to wikipedia, a wax is just a plastic/malleable compound that melts slightly above room temperature to give a non viscous liquid. As this definition doesn't stipulate any chemical properties, there's a pretty large number of compounds which fall into this bracket, and so waxes from different sources can have very different chemical compositions. Wax from natural sources such as insects and animals tends to be composed of the esters of various fatty acids with long chain alcohols, whereas synthetic waxes tend to be simple mixtures of various long chain hydrocarbons. Earwax is different again.[1] Regardless of its provenance, wax can be a pretty useful substance, with myriad uses around the lab. Here are the first five I can think of;  no doubt there are others!

1. Handling hydrofluoric acid safely*

I'm sure most people who read this post are probably familiar with the nastiness of hydrofluoric acid. It's not the world's strongest acid, but as HF doesn't dissociate very much it's surprisingly lipid soluble and can therefore penetrate deeply into the skin. This causes all kinds of horrific injuries including severe burns, nerve damage and cardiac arrest (as it sequesters calcium ions, precipitating insoluble CaF2). Derek describes it vividly here. He also touches on perhaps its best known quality:  its legendary incompatibility with glass. This is largely due to the fact that, as one of my lecturers so eloquently said, 'fluoride sticks to silicon like shit to a blanket'. This reaction is actually quite fast for conc. (70%) HF, and also highly exothermic. There's a delightful youtube video here, courtesy of powerlabs.com. I think we can agree that this looks terrifying, even in a smallish test tube. That's why people do reactions with it in plastic vessels. Or at least I do. But what if you can't scavenge a suitable vessel and don't have the money to buy a bucket? Are your days too boring? According to the organic syntheses prep for fluorobenzene (from aniline, using the classic Balz-Schiemann reaction) an alternative is at hand:

"Fluoboric acid[sic] is made concurrently with the diazotization by the addition, in small amounts, of 1 kg. (16.2 moles) of boric acid (u.s.p. crystals) to 2150 g. (65 moles) of 60 per cent hydrofluoric acid (Note 3). The addition is carried out in two 3-l. flasks coated with wax (Note 4) which are shaken and kept cold by immersion in iced water. The temperature of the acid should not be allowed to rise above 20–25°.[2]"

To restate: you take two litres of conc. HF and place them in a vessel that they'll happily dissolve (in a violent, exothermic reaction), separated only by a thin film of wax. Then you add boric acid and shake.  If all goes to plan then, thanks to the wax, you'll be able to produce around 800g of pure fluorobenzene without needing to splash out on a new plastic bucket. Or skin grafts.

*only for a given value of safe.

 

2. Tricky Oxidations

Although one of the most egregious examples of appalling refereeing (and total bullshit) that I'm aware of in the chemical literature, La Clair's synthesis of hexacyclinol does contain some interesting chemistry including some unusual conditions for the oxidation of an aldehyde (a shame it probably all only happened in his head).[3]

"For this example, oxidation... was achieved by delivering Ag2O in wax (5%Ag2O in paraffin). Here, slow addition and transfer of Ag2O to the reaction medium was optimal in favoring aldehyde oxidation."

Sure it was, James.

3. Making Grubbs' catalyst a bit harder easier to handle.

"The ruthenium... complex developed by Grubbs is widely known for its applications in organic and polymer synthesis. Despite the obvious utility of Grubbs’ catalyst, its routine use is hindered by special storage requirements. Prolonged exposure to air and moisture deactivates the complex, and it is typically stored under inert atmosphere... Here, we report that Grubbs’ catalyst dispersed in paraffin is easily handled and retains its activity indefinitely with no special storage precautions."

Taber and Frankowski, J. Org. Chem., 2003, 68, 6047

 

Basically, you mix Grubbs' 2nd generation catalyst and molten paraffin wax in a glove box to give a 'homogeneous purple liquid' which can be cast, broken up and stored in the open air, even for two years without loss of activity. This also makes weighing out small quantities of the catalyst easier, and the paraffin can simply be columned out at the end of it all. The authors conclude:

"There are many catalysts in common usage in the organic synthesis laboratory that are much more air and moisture sensitive than the Grubbs catalyst. It is remarkable that dispersion in paraffin to protect them has not been more widely employed."

Is it, though?

 

4. Putting holes in things

Although wax is a fairly soft substance,  it's still a fair stand in when you run out of musket balls. In fact, it's perfectly possible to fire a candle through a barn door using an ordinary household musket, as this University of Nottingham video shows. As anyone who's ever landed badly when jumping into a swimming pool can attest, sometimes soft things can seem surprisingly firm...

 

5. Removing grease

My favourite use of wax, specifically in candle form, is playing pranks on new students. A postdoc in our group once told an MSci student he could remove grease from his crucial final product at the end of a 10 step total synthesis by placing the sample in a vacuum dedicator with a candle. After some fairly bemused looks from different members of the technical staff a red candle was found. Not only did he do it, it's also in his final year report experimental. Didn't work, though.

 

Addenda, appendices and references

1. If you'd like to see TLCs and NMRs of Dylan's earwax then check out this excellent post over at the now dead Tenderblog (user: tender, pass: button). This is just about my favourite chemistry blog post ever.

 

2. This prep dates from the final year of the Wiemar republic (i.e., 1933), when fluoboric acid was considered an equally valid spelling of fluoroboric acid.

 

3. I'm not going to exhaustively re-cover the hexacyclinol debacle here. A good summary was written by Derek over at In The Pipeline back when a relevant paper last came out, in 2009. The route itself, for those without literature access, can be enjoyed in this Wipf group meeting presentation and the relevent Tot. Syn. posts.

Dylan made me laugh out loud with this gem:

" The mystery grows when we look closer at the ACIE paper. This La Clair character operates out of the Xenobe Research Institute, PO Box 4073 in San Diego CA. Not to be confused with the Scientology outpost in low orbit around Mars. While this institution may look like one of those fake companies Enron established for tax-evasion purposes, Googling shows it’s affiliated with Scripps. The author notes that “a considerable portion of this work was conducted during 1999-2002 at Bionic Bros GmbH, Germany. J.J.L.C. acknowledges the assistance of five technicians.” A gigantic synthesis like this that appears as a one-author publication and merely acknowledges five technicians seems odd to me. My mental image of the Bionic Brothers is 5 guys in matching Adidas track suits with a ghetto blaster, breakdancing on a big piece of cardboard. Who are these people?"

A good question. According to the Wipf presentation, a Google search for the Bionic Bros address gives a yoga studio in Berlin, and as thechemblog remarked at the time, 'aside from Alexander Shulgin who the hell uses a P.O. box in a publication?'. According to this 2006 C & En article 'La Clair praises Porco's and Rychnovsky's reports and says the work is helping to elucidate the chemistry of these metabolites'.

I'll leave you to make up your own minds!

Photo from Nullius In Verba:

http://ashujo.blogspot.com/2006/08/hexacyclinol-soon-to-be-in-washing.html

 

Comments (4) Trackbacks (0)
  1. To add a bit: KH in wax — http://pubs.acs.org/doi/abs/10.1021/jo061420v

    • Ah, another Doug Taber paper! I hadn’t seen that one. Could be a good thing as 30wt% KH in oil is awful to handle, and I’ve still seen several people have fires with it.

  2. IIRC, JJ published a separate protocol during his Ph.D. that dealt with the silver oxide oxidation in wax, so it’s likely quite legit. Check his list of pubs. It’s actually one of the few reaction details of the hexacyclinol synth he was happy to elaborate on in depth.


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