A game one can play with pericyclic reactions is to ask students to identify what type a given example is. So take for example the reaction below.
Archive for the ‘Interesting chemistry’ Category
Three-for-one: a pericyclic brain teaser.
Sunday, January 12th, 2014Does forming a Wheland intermediate disrupt all aromaticity?
Friday, December 6th, 2013Text books will announce that during aromatic electrophilic substitution, aromaticity is lost by the formation of a Wheland intermediate (and regained by eliminating a proton). Is that entirely true?
Multiple personalities of Magnesium.
Tuesday, November 5th, 2013The following is a short question in a problem sheet associated with introductory organic chemistry.
The world ash tree of the computer hardware industry… crystalline silicon from 1854.
Thursday, October 31st, 2013The element silicon best represents the digital era of the mid 20th century to the present; without its crystalline form, there would be no computers (or this blog). Although it was first prepared in pure amorphous (powder) form around 1823[1] by Berzelius, it was not until 1854 that Henri Sainte-Claire Deville made it in crystalline form, using metallic aluminium to isolate it. He described it [1] as having a “metallic luster”.
References
Six vs ten aromatic electrons?
Sunday, October 20th, 2013Homoaromaticity is a special case of aromaticity in which π-conjugation is interrupted by a single sp3 hybridized carbon atom (it is sometimes referred to as a suspended π-bond with no underlying σ-foundation). But consider the carbene shown below. This example comes from a recently published article[1] which was highlighted on Steve Bachrach’s blog. Here aromaticity has resulted from a slightly different phenomenon, whereby a 4π-electron planar (and hence nominally anti-aromatic) molecule is elevated to aromatic peerage by promoting the two carbene σ-electrons to have π-status.
References
- B. Chen, A.Y. Rogachev, D.A. Hrovat, R. Hoffmann, and W.T. Borden, "How to Make the σ<sup>0</sup>π<sup>2</sup> Singlet the Ground State of Carbenes", Journal of the American Chemical Society, vol. 135, pp. 13954-13964, 2013. https://doi.org/10.1021/ja407116e
A short non-bonding H…H interaction (continued)
Wednesday, October 2nd, 2013This is a continuation of the discussion started on Steve Bachrach’s blog about a molecule with a very short H…H interaction involving two Si-H groups with enforced proximity. It had been inferred from the X-ray structure[1] that the H…H distance was in the region of 1.50Å. It’s that cis-butene all over again! So is that H…H region a bond? Is it attractive or repulsive? Go read Steve’s blog first.
References
- J. Zong, J.T. Mague, and R.A. Pascal, "Exceptional Steric Congestion in an <i>in</i>,<i>in</i>-Bis(hydrosilane)", Journal of the American Chemical Society, vol. 135, pp. 13235-13237, 2013. https://doi.org/10.1021/ja407398w
An example of an extreme gauche effect: FSSF.
Saturday, September 21st, 2013The best known example of the gauche effect is 1,2-difluoroethane, which exhibits a relatively small preference of ~0.5 kcal/mol for this conformer over the anti orientation, which is also a minimum. But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must surely be the smallest molecule (only four atoms) which could be theoretically resolved into two enantiomers (possibly at say 273K?).‡