Posts Tagged ‘RSC Publishing’

The dawn of organic reaction mechanism: the prequel.

Sunday, November 13th, 2011

Following on from Armstrong’s almost electronic theory of chemistry in 1887-1890, and Beckmann’s radical idea around the same time that molecules undergoing transformations might do so via a reaction mechanism involving unseen intermediates (in his case, a transient enol of a ketone) I here describe how these concepts underwent further evolution in the early 1920s. My focus is on Edith Hilda Usherwood, who was then a PhD student at Imperial College working under the supervision of Martha Whitely.1

The doctoral degree itself had only been introduced into British universities in 1919,1 and so Usherwood was very much a forerunner of the modern system of training.The academic staff and students at Imperial totalled 30, making it one of the largest research schools in UK chemistry at the time. Usherwood’s project was on tautomers, or isomers of molecules which differ only in the position of a labile hydrogen atom. The then quite novel electron-pair symbolism introduced by G. N. Lewis’ in 1916 was adopted to represent two tautomeric equilibria (the supposed mobile or tautomeric hydrogens being enclosed in […])2

  1. [H]C:::N ⇔ C::N[H]
  2. [H]C:::CH ⇔ C::CH[H]

or in our more modern representation (in which lines replace colons, and charges are used to ensure the octet rule is adhered to when possible):

  1. H-C≡N ⇔ –C≡N+-H
  2. HC≡CH ⇔ :C=CH2

Modern structural techniques such as electron diffraction or microwave spectroscopies not yet existing, the problem was tackled using specific heat measurements as a function of temperature. This method suggested to Usherwood that for e.g. equilibrium 2, the concentration of iso-acetylene (we now call this vinylidene) was insignificant at ordinary temperatures, but it became appreciable between 200-300°C. Further evidence was claimed for the formation of the “unseen” vinylidene by observing ketene as a by-product of the oxidation of acetylene. This article very much set the trend of (an almost mandatory) speculation on the outcome of (nowadays much more complex) reactions by the need to formulate a reaction mechanism in which various (otherwise undetected but) plausible intermediates are involved.

Moving on some 90 years, and how might one approach such a problem nowadays? Well, I have oft argued on this blog that a good place to obtain an immediate reality check on a proposed mechanism is a calculation. It will come as no surprise that a very accurate calculation can be done on the systems shown above. For example, CCSD(T)/cc-pVTZ will yield a free energy for the equilibria with a pretty small error (< 1 kcal/mol). We use ΔG = -RT Ln K to inter-convert free energies and equilibrium constants. If we are generous and state that in order to observe an appreciable concentration of a minor species, the equilibrium constant can be no smaller than 10-3, its energy cannot be greater than 4 kcal/mol above the more abundant isomer. Our reality check will be to see if the free energy of vinylidene is indeed no more than 4 kcal/mol greater than acetylene. Well, CCSD(T)/cc-pVTZ predicts vinylidene is 41.3 kcal/mol higher @298K, reduced to 33.8 @2000K (and before you ask, these results took a total of perhaps 30 minutes to obtain).

In 1924, the concept of calculating the relative energies of two species using first principles was not even a glimmer on the horizon. The nature of mechanisms was slowly and often painfully established by recourse to experiments alone. And many of the unseen intermediates often remained just such, their existence only inferred indirectly from the models one constructed (of specify heats in Usherwood’s case). It is perhaps no great surprise that these models do not always stand the test of time. In this case, within a year of Usherwood’s publication, Partington was suggesting that the model for the specific heats of acetylene should have included allowance for polymer formation.3 The modern take, armed with the calculation I note above, might in fact side with Partington after all. As for the formation of ketene by oxidation, it is indeed known that (peracid) oxidation of an alkyne will produce ketene, but the modern mechanism (an interesting exercise in arrow pushing for a student) does not involve vinylidene intermediates.

I will add at this point that Hilda Usherwood was married to Christopher Ingold, and the pair of them subsequently published many of the seminal articles in what became known as physical organic chemistry. That legacy continues to this day with (as I noted above) the almost mandatory speculation about the mechanism of any new reaction. But it is only in the last five years or so that these speculations have started to be increasingly tested against reliably accurate computation. A new era is underway.

1 My post was inspired by reading W. H. Brock, “The case of the Poisonous Socks”, chapter 28, RSC Publishing, 2011, 978-1-84973-324-3.

2 These representations are taken from ref 1, p 225 (and including a correction of replacing C:C as drawn there by C::C). The original article apparently appeared in the proceedings of the British Association of 1924, which is not yet available online.

3 Brock, in ref 1, p226, suggests that Usherwood stood her ground on this one, and won her case by showing that Partington’s evidence for polymerization was valid for only a small part of the temperature range she had investigated. I have not managed to track down the original sources for this exchange.

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Henry Armstrong: almost an electronic theory of chemistry!

Monday, November 7th, 2011

Henry Armstrong studied at the Royal College of Chemistry from 1865-7 and spent his subsequent career as an organic chemist at the Central College of the Imperial college of Science and technology until he retired in 1912. He spent the rest of his long life railing against the state of modern chemistry, saving much of his vitriol against (inter alia) the absurdity of ions, electronic theory in chemistry, quantum mechanics and nuclear bombardment in physics. He snarled at Robinson’s and Ingold’s new invention (ca 1926-1930) of electronic arrow pushing with the put down “bent arrows never hit their marks“.1  He was dismissed as an “old fogy, stuck in a time warp about 1894.”1 So why on earth would I want to write about him? Read on…

He did worthy (nowadays this could mean dull) chemistry on e.g. naphthalenes, but I want to focus on two articles from the period 1887-1890 (10.1039/CT8875100258  and 10.1039/PL8900600095). Let me set the scene by reminding of an earlier post showing the structure of a bis(stilbyl)ketone, dated 1921. The two aromatic groups (yes, they really are such) are drawn in the manner we would nowadays draw cyclohexane. This practice in fact continued in texts and articles for perhaps 30 more years! Not much sign of electronic accounting there then! And by a professor at Imperial College no less, where Armstrong had been.

Aromatic molecule, circa 1921

So when would you date the diagrams below? So called Clar2 representations, originating from the 1950s? The one on the bottom below cites Clar and dates from 2010, DOI: 10.3390/sym2031653, but the one above it comes from Armstrong’s 1890 article!

Two representations of pyrene, 2010 and 1890.

Clar representations are used to count electrons (as coming in six packs). But there is little doubt that Armstrong’s use of a “C” (or inner circle, which is exactly what it is) means six as well. The evidence I present below, taken from his 1887 article.

Armstrongs six pack

  1. He counts the six carbons as having a total of 24 what he calls affinities (definition: An attraction or force between particles that causes them to combine), or four per carbon. Let us make life easy and equate affinity=electron (remember, the electron itself was not yet discovered or named!). He disposes of 12 affinities/electrons to form what we now call six carbon-carbon σ bonds, and a further six for the  six C-H bonds.
  2. He is left with exactly six affinities/electrons, which he presupposes to act upon each other, in the manner of resultants (the old term for vectors). In fact, he replaces these six vectors by a circle (the inner circle) in his second article of 1890.
  3. He invents delocalization in all but name when he states that any one atom has an influence on other atoms not contiguous to it in the ring (he really did have o/m/p directing influence in mind here).
  4. He compares the introduction of a substituent (R, which comes from the old name Radicle) perturbing the distribution of the affinity to how electric charges perturb each other. So, the affinity behaves as if it might have electrical (from which the name electron came of course) properties? And it might be described by a vector?
  5. Remember, this is a scientist who in later life did not believe in electronic theories of chemistry? Really? Well, again in 1890:

Is this an affinity (=electronic) theory of chemistry?

  1. Here, he is refining his vector representation of affinities, saying that these vectors in effect define a circle, an inner circle no less. One that can be disrupted  (Robinson some 30 years later wrote of how the cycle of six electrons are able to form a group that resists disruption) when an additive compound is formed (his examples are all electrophiles, what we now call electrophilic addition) such that the remaining carbons become merely unsaturated. There seems little doubt he is describing what we now call a Wheland Intermediate.
  2. Is this really a man who did not believe in electronic theories of chemistry? What about that concluding paragraph then? The laws of substitution require a knowledge of the inner structure of (what we now call the aromatic) hydrocarbons?
  3. And that such speculations may suggest fresh lines of experimental inquiry? This all sounds very much like the modern use of quantum mechanics and its electronic eigenvectors to describe the probability distribution of electrons (remember, Armstrong did not approve of this either) to probe the inner structure of molecules and to suggest new experiments.

We have a real mystery. Armstrong got so very close to a modern theory of chemistry. Was he asleep when Stoney named the electron around 1891 and Thomson discovered it in 1897? If only he had followed his own advice! Ah well, just as well he was ignored in the 20th century when he preached against it all.

  1. W. H. Brock, “The case of the Poisonous Socks”, chapter 20, RSC Publishing, 2011, 978-1-84973-324-3
  2. Clar, E. The Aromatic Sextet; Wiley: New York, NY, USA, 1972.

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