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Analytical Chemistry

Short Chemistry Topics

Blogging time is tight today, but there are several interesting stories and follow-ups that I wanted to mention. For starters, I wrote here about a cyclohexane analog that’s fluorinated all on one side of the molecule. That gives you very odd properties, and it and its relatives could be really useful solvents and additives, but getting such molecules has been very painful. Now, though, a route to this compound and others has been found from the far more readily available fluoroaromatic starting materials. The Glorius group at Münster reports hydrogenation using a particular rhodium catalyst that actually gives you access to all sorts of cis-fluorinated saturated rings, which is very nice to see. I would enjoy knowing if there’s a similar route that might work on fluoropyridines or fluoropyrroles to give the saturated heterocycles – any takers?

Second, here’s a neat paper for the NMR aficionados out there. It presents “supersequences” for combining a long list of useful 1D and 2D NMR experiments into single pulse sequences, importantly using only one relaxation delay (which is where the time starts to pile up). What you have, then, is an extremely efficient NMR experiment that gives you piles of data all in one run. The SI for the paper presents over 200 proposed supersequences, combinations of NMR data collection that will make your head spin. Just to give you the idea, they demonstrate one sequence that combines proton-nitrogen HMQC, proton-carbon HSQC, proton-carbon HMBC, COSY, and NOESY all at the same time, delivering pretty much everything you could want in small-molecule NMR at a substantial savings in instrument time.

While I’m on the analytical side of things, I should also note that the European X-ray laser facility (XFEL) is now open and running its first experiments. This is the fastest thing of its kind in the world – as I understand it, it can take 27,000 frames per second, which is completely new territory as far as time-resolved x-ray structure work goes, 200 times the rate of the LCLS machine out at Stanford’s SLAC facility. But there’s a free-electron laser arms race going on – the good kind, not the we’re-all-going-to-die kind. The LCLS is planning an upgrade that would take it up to millions of pulses per second, which will reveal things about protein structure alone that I can’t even imagine. I recall writing a blog post nine years ago anticipating these machines coming on line, and by golly, here they are. Time flies!

Courtesy of Angewandte Chemie/Wiley

Moving back to a much smaller scale (anything is smaller scale than a free-electron laser), do you know about Janus filters? These are membranes whose two sides are functionalized differently, and they can be great at separating out oil/water mixtures and breaking up emulsified messes. The problem with the current ones is that they haven’t been able to deal with emulsions that have non-ionic surfactants in them, and that’s a large category. But now there’s a new system that can handle pretty much the whole range. One side of the filter has polydimethylsiloxane on it, and the other has a polysoap, ethylene glycol spacers with laurate groups on the end. When an emulsion hits the polysoap side, that’s the emulsion breaker. The surfactant gets pulled away from the oil droplets, which then start to coalesce and move down the membrane material until they hit the PDMS side of things, where they happily dive through. The results is that you have a milky mass of emulsion on one side, while the oil/organic component slowly drains out the other while you go do something else (see illustration at right). I want one.

6 comments on “Short Chemistry Topics”

  1. Mad Chemist says:

    As a synthetic chemist, the idea of getting that much NMR data in ONE RUN has me giddy.

    1. yuri says:

      The hours of my life I could claw back from time itself

  2. Peter Kenny says:

    The equatorial fluorine atoms of the hexafluorocyclohexane may actually be making it less polar although electrostatic repulsion would be expected to lead to strengthen the hydrogen bond basicity of the axial fluorine atoms. I’ve linked a study with some relevant measurements (e.g. for syn-2,4-difluoroadamantane) as the URL for this comment.

  3. tangent says:

    Man, my undergrad chemistry sequence sure did not give give what you could call a thorough survey, in the sense of turning major unknown unknowns into known unknowns.

  4. ab says:

    Highly fluorinated anything, especially in the context of solvents, does stress me out a little bit. Do these things EVER break down? Can they be metabolized? Is this a bio- or environmental-persistence problem in the making?

  5. David Edwards says:

    What I’d like to know is this … prior to this new work on what I presume is 1,2,3,4,5,6-hexafluorocyclohexane, did previous syntheses produce an equal mix of all the possible orientation combinations, or does there exist a mechanism favouring some combinations over others? Furthermore, how would you determine this?

    Answering this question on its own sounds to me like a research project that could take a decade to complete rigorously …

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