The phlogiston theory reigned for quite a while. It held that all combustible resources contain phlogiston, a substance without color, odor, taste, or mass* that is liberated in burning. Once burned, the “dephlogisticated” substance was held to be in its “true” form, the calx.


The theory made good sense for hundreds of years and had its proponents still in the early 19th century. (*One irritating detail, however, is that phlogiston must have negative mass, which does not feel quite right to most people.) Then came the discovery of the element oxygen and everything changed. Again.

This is the beauty of science: That it evolves in an auto-corrective fashion. Those of you interested in the philosophy of science should check out the works of Karl Popper and Thomas Kuhn.

Where was I? Phlogiston, yes. What fundamental concepts in today’s chemistry do you think will eventually crumble and fall just like the phlogiston theory did? It is a tough question. One would like to believe that everything we now know, we know for sure. (Side note: In astronomy, as a layman, I would vote for dark matter and dark energy.)

Let me throw out a just a couple – and feel free to add or comment on:

1. Memory of chirality

Chiral memory

Some reactions, annoyingly, that we know proceed via achiral intermediates (such as carbo-cations, carbo-anions, carbenes and so on) still produce chiral products. As if nature somehow remembered that the starting material was chiral. An excellent write-up with numerous examples is found here.

2. Lewis structures

Lewis structures

All chemists use Lewis structures to represent chemical structures. Easy. But is it fool-proof? Actually, no. Try for yourself to draw an acceptable Lewis structure for chlorine dioxide (ClO2), keeping track of all electrons. It does not compute, does it? Even Linus Pauling – with two Nobel Prizes in his back pocket – had a tough time with this one.


8 Responses to Phlogiston-ish theories in modern chemistry — Which are they?

  1. Anon says:

    The theory that natural product synthesis is a useful pursuit.

  2. milkshake says:

    Once during quantum mechanic class our professor (non-chemist) joked that only chemists know where their electrons are and where exactly they are moving.

    Octet rule, 18-electron rule, orbital symmetry rules, Baldwin rules, etc are just useful mnemonics that work as a shorthand. They are not on fundamentally wrong track or upside-down theories like phlogiston.

    I don’t think you will find any equivalent of phlogiston in chemistry simply because it is a mature field and one where is easy to test theoretical concepts with experiments. In the SN1 example you gave, it is easy enough to take the system that produces only partially -racemized product, and look at the effect of solvent polarity and temeperaature (and additives that aid separation of ion pairs, as well as alternate leaving groups). What you get out of it is a deeper mechanistic understanding which will be more predictive than the starting presumption based on SN1 mechanism.

    • drfreddy says:

      “Mature field.” That’s a bold statement. 🙂 Just a quick note on chiral memory (check the linked slides for details): Some reactions that are PROVEN to proceed via achiral intermediates still give non-racemic products. It is a relatively rare occurrence – but still – quite disturbing. The million variations in which we aren’t fully sure of the exact nature of the intermediates/transition states are excluded from this discussion. Chiral memory is: [Known chiral starting material]—> [Known achiral intermediate] —> [Known chiral product]. The phenomenon exists, but we can’t explain it. In any case, calling it “memory” (decorating molecules with human attributes) is cute, but it raises more questions than it answers. Oh, another thing that I forgot to bring up while posting the above: Alchemy. The eternal quest for a gold synthesis. We laughed at it for a long time before (in the 90’s – was it?) some nuclear chemists actually made a couple of gold atoms from lead (if my memory serves me right). I’m just saying: Be careful with “truths” in life and in science. The only absolute truth (to a non-religious person) is the existence of self, according to Descartes. This blog, for all we know, could be a mirage!

      • Ted Rahedral says:

        A cold day in December, 1956. Physics is dumped on its head by experimental proof of parity violation. To this day we still have no good answer to the origin of asymmetry in the universe.

        I’m sure we still have a lot of learning to do 🙂

        • drfreddy says:

          You are overlooking the “hidden mirror sector“! 🙂 Seriously, the way I see it, chemistry is a branch of physics (because it just is so – come on). And physics, in turn, is a branch of philosophy. We are all trying to unlock truths about the universe, only we’re using slightly different methods. Our goals are the same. In any case… I wouldn’t dare to call chemistry a “mature field” at this point, because physics – on which chemistry is all based – keeps changing all the time at the most fundamental levels. Let us hope that Stephen Hawking publishes a full paper on the Theory of Everything soon, so we can get some rest.

  3. CatalysisAlex says:

    Nice read. I just unpacked my book boxes after moving and what do I find? “The Norton History of Chemistry”. Looks like I have my distraction set for the next few weeks….
    On another note: I agree, that one should be careful with bold statements. I remember reading a nature article on the theory of time travel and how it actually should be possible….. Did the authors watch too much StarTrek? Perhaps, but it had impact on some serious research….

  4. Chemjobber says:

    I submit the “microwaves do something special” theory of microwave-assisted organic synthesis.

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