Henry Rzepa's Blog Chemistry with a twist

April 4, 2019

Smoke and mirrors. All is not what it seems with this Sn2 reaction!

Previously, I explored the Graham reaction to form a diazirine. The second phase of the reaction involved an Sn2′ displacement of N-Cl forming C-Cl. Here I ask how facile the simpler displacement of C-Cl by another chlorine might be and whether the mechanism is Sn2 or the alternative Sn1. The reason for posing this question is that as an Sn1 reaction, simply ionizing off the chlorine to form a diazacyclopropenium cation might be a very easy process. Why? Because the resulting cation is analogous to the cyclopropenium cation, famously proposed by Breslow as the first example of a 4n+2 aromatic ring for which the value of n is zero and not 1 as for benzene.[1] Another example of a famous “Sn1” reaction is the solvolysis of t-butyl chloride to form the very stable tertiary carbocation and chloride anion (except in fact that it is not an Sn1 reaction but an Sn2 one!)

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References

  1. R. Breslow, "SYNTHESIS OF THE s-TRIPHENYLCYCLOPROPENYL CATION", Journal of the American Chemical Society, vol. 79, pp. 5318-5318, 1957. http://dx.doi.org/10.1021/ja01576a067

August 25, 2018

Organocatalytic cyclopropanation of an enal: (computational) mechanistic understanding.

Symbiosis between computation and experiment is increasingly evident in pedagogic journals such as J. Chemical Education. Thus an example of original laboratory experiments[1],[2] that later became twinned with a computational counterpart.[3] So when I spotted this recent lab experiment[4] I felt another twinning approaching.

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References

  1. A. Burke, P. Dillon, K. Martin, and T.W. Hanks, "Catalytic Asymmetric Epoxidation Using a Fructose-Derived Catalyst", Journal of Chemical Education, vol. 77, pp. 271, 2000. http://dx.doi.org/10.1021/ed077p271
  2. J. Hanson, "Synthesis and Use of Jacobsen's Catalyst: Enantioselective Epoxidation in the Introductory Organic Laboratory", Journal of Chemical Education, vol. 78, pp. 1266, 2001. http://dx.doi.org/10.1021/ed078p1266
  3. K.K.(. Hii, H.S. Rzepa, and E.H. Smith, "Asymmetric Epoxidation: A Twinned Laboratory and Molecular Modeling Experiment for Upper-Level Organic Chemistry Students", Journal of Chemical Education, vol. 92, pp. 1385-1389, 2015. http://dx.doi.org/10.1021/ed500398e
  4. M. Meazza, A. Kowalczuk, S. Watkins, S. Holland, T.A. Logothetis, and R. Rios, "Organocatalytic Cyclopropanation of (E)-Dec-2-enal: Synthesis, Spectral Analysis and Mechanistic Understanding", Journal of Chemical Education, vol. 95, pp. 1832-1839, 2018. http://dx.doi.org/10.1021/acs.jchemed.7b00566

March 10, 2017

Ammonium tetraphenylborate and the mystery of its π-facial hydrogen bonding.

A few years back, I did a post about the Pirkle reagent[1] and the unusual π-facial hydrogen bonding structure[2] it exhibits. For the Pirkle reagent, this bonding manifests as a close contact between the acidic OH hydrogen and the edge of a phenyl ring; the hydrogen bond is off-centre from the middle of the aryl ring. Here I update the topic, with a new search of the CSD (Cambridge structure database), but this time looking at the positional preference of that bond and whether it is on or off-centre. 

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References

  1. H.S. Rzepa, M.L. Webb, A.M.Z. Slawin, and D.J. Williams, "? Facial hydrogen bonding in the chiral resolving agent (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol and its racemic modification", Journal of the Chemical Society, Chemical Communications, pp. 765, 1991. http://dx.doi.org/10.1039/c39910000765
  2. H.S. Rzepa, M.H. Smith, and M.L. Webb, "A crystallographic AM1 and PM3 SCF-MO investigation of strong OH ⋯π-alkene and alkyne hydrogen bonding interactions", J. Chem. Soc., Perkin Trans. 2, pp. 703-707, 1994. http://dx.doi.org/10.1039/P29940000703

April 14, 2016

Hydronium hydroxide: the why of pH 7.

Ammonium hydroxide (NH4+…OH) can be characterised quantum mechanically when stabilised by water bridges connecting the ion-pairs. It is a small step from there to hydronium hydroxide, or H3O+…OH. The measured concentrations [H3O+] ≡ [OH] give rise of course to the well-known pH 7 of pure water, and converting this ionization constant to a free energy indicates that the solvated ion-pair must be some ~19.1 kcal/mol higher in free energy than water itself. So can a quantum calculation reproduce pH7 for water?

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July 25, 2015

Intermolecular atom-atom bonds in crystals? The O…O case.

I recently followed this bloggers trail; link1link2 to arrive at this delightful short commentary on atom-atom bonds in crystals[1] by Jack Dunitz. Here he discusses that age-old question (to chemists), what is a bond? Even almost 100 years after Gilbert Lewis’ famous analysis,[2] we continue to ponder this question. Indeed, quite a debate on this topic broke out in a recent post here. My eye was caught by one example in Jack's article: "The close stacking of planar anions, as occurs in salts of croconic acid …far from producing a lowering of the crystal energy, this stacking interaction in itself leads to an increase by several thousand kJ mol−1 arising from Coulombic repulsion between the doubly negatively charged anions" I thought I might explore this point a bit further in this post.

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References

  1. J.D. Dunitz, "Intermolecular atom–atom bonds in crystals?", IUCrJ, vol. 2, pp. 157-158, 2015. http://dx.doi.org/10.1107/S2052252515002006
  2. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. http://dx.doi.org/10.1021/ja02261a002

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