Consider acetaldehyde (ethanal for progressive nomenclaturists). What conformation does it adopt, and why? This question was posed of me by a student at the end of a recent lecture of mine. Surely, an easy answer to give? Read on …
Archive for the ‘Interesting chemistry’ Category
The conformation of acetaldehyde: a simple molecule, a complex explanation?
Friday, February 8th, 2013σ-π-Conjugation: seeking evidence by a survey of crystal structures.
Sunday, February 3rd, 2013The electronic interaction between a single bond and an adjacent double bond is often called σ-π-conjugation (an older term for this is hyperconjugation), and the effect is often used to e.g. explain why more highly substituted carbocations are more stable than less substituted ones. This conjugation is more subtle in neutral molecules, but following my use of crystal structures to explore the so-called gauche effect (which originates from σ-σ-conjugation), I thought I would have a go here at seeing what the crystallographic evidence actually is for the σ-π-type.
How does one describe the wavefunction for the π-complex formed from PhNHOPh?
Friday, January 25th, 2013Although have dealt with the π-complex formed by protonation of PhNHOPh in several posts, there was one aspect that I had not really answered; what is the most appropriate description of its electronic nature? Here I do not so much provide an answer, as try to show how difficult getting an accurate answer might be.
Aromaticity in the benzidine-like π-complex formed from PhNHOPh.
Saturday, January 19th, 2013The transient π-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-π-electron aromatic ring (the phenoxide anion 2) with a four-π-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity?
The π-complex in the benzidine rearrangement: a molecular orbital analysis.
Friday, January 18th, 2013Michael Dewar[1] famously implicated a so-called π-complex in the benzidine rearrangement, back in the days when quantum mechanical calculations could not yet provide a quantitatively accurate reality check. Because this π-complex actually remains a relatively unusual species to encounter in day-to-day chemistry, I thought I would try to show in a simple way how it forms.
References
- M. Dewar, and H. McNicoll, "Mechanism of the benzidine rearrangement", Tetrahedron Letters, vol. 1, pp. 22-23, 1959. https://doi.org/10.1016/s0040-4039(01)82765-9
The Benzidine rearrangement. Computed kinetic isotope effects.
Friday, January 11th, 2013Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement[1]. This classic mechanism proceeds via bisprotonation of diphenyl hydrazine, but what happens next was the crux. Does this species rearrange directly to the C-C coupled intermediate (a concerted [5,5] sigmatropic reaction) or does it instead form a π-complex, as famously first suggested by Michael Dewar[2] [via TS(NN] and only then in a second step [via TS(CC)] form the C-C bond? Here I explore the isotope effects measured and calculated for this exact system.
References
- H.J. Shine, H. Zmuda, K.H. Park, H. Kwart, A.G. Horgan, and M. Brechbiel, "Benzidine rearrangements. 16. The use of heavy-atom kinetic isotope effects in solving the mechanism of the acid-catalyzed rearrangement of hydrazobenzene. The concerted pathway to benzidine and the nonconcerted pathway to diphenyline", Journal of the American Chemical Society, vol. 104, pp. 2501-2509, 1982. https://doi.org/10.1021/ja00373a028
- M. Dewar, and H. McNicoll, "Mechanism of the benzidine rearrangement", Tetrahedron Letters, vol. 1, pp. 22-23, 1959. https://doi.org/10.1016/s0040-4039(01)82765-9
NCI (non-covalent-interaction) analysis for some π-hydrogen bonded systems.
Tuesday, January 8th, 2013The mechanism of the Benzidine rearrangement.
Sunday, January 6th, 2013The benzidine rearrangement is claimed to be an example of the quite rare [5,5] sigmatropic migration[1], which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route[2]. The topic has been reviewed here[3].
References
- H.J. Shine, K.H. Park, M.L. Brownawell, and J. San Filippo, "Benzidine rearrangements. 19. The concerted nature of the one-proton rearrangement of 2,2'-dimethoxyhydrazobenzene", Journal of the American Chemical Society, vol. 106, pp. 7077-7082, 1984. https://doi.org/10.1021/ja00335a035
- H.J. Shine, L. Kupczyk-Subotkowska, and W. Subotkowski, "Heavy-atom kinetic isotope effects in the acid-catalyzed rearrangement of N-2-naphthyl-N'-phenylhydrazine. Rearrangement is shown to be a concerted process", Journal of the American Chemical Society, vol. 107, pp. 6674-6678, 1985. https://doi.org/10.1021/ja00309a041
- H.J. Shine, "Reflections on the π‐complex theory of benzidine rearrangements", Journal of Physical Organic Chemistry, vol. 2, pp. 491-506, 1989. https://doi.org/10.1002/poc.610020702