Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a π-system transformed it from a formally 4n-antiaromatic π-cycle into a 4n+2 aromatic π-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner’s[cite]10.1039/C2SC21555A[/cite] trapping and reactions of t-butyl-hydroxycarbene, as described on Steve Bachrach’s blog. A point I often make is that chemistry is all about connections, and so here I will discuss such a connection.
Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene.
November 13th, 2013Multiple personalities of Magnesium.
November 5th, 2013The following is a short question in a problem sheet associated with introductory organic chemistry.
Kinetic vs thermodynamic enolization.
November 5th, 2013The concept of kinetic vs thermodynamic control of a reaction is often taught in the context of the enolisation of e.g. 1-methylcyclohexanone as induced by a base. The story goes that at low temperatures (-78°C), the rate of the sterically more hindered thermodynamic enolisation does not compete with the faster kinetic product but that at higher temperatures when an equilibrium is possible, the thermodynamically more stable tetrasubstituted enol is formed. I set out to see if this result can be modelled.
The world ash tree of the computer hardware industry… crystalline silicon from 1854.
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[cite]http://doi.org/10.1021/ed009p1386[/cite] 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 [cite]http://doi.org/10.1021/ed009p1386[/cite] as having a “metallic luster”.
Six vs ten aromatic electrons?
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[cite]10.1021/ja407116e[/cite] 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.
Internet Archaeology: Blasts from the past.
October 11th, 2013In 1993-1994, when the Web (synonymous in most minds now with the Internet) was still young, the pace of progress was so rapid that some wag worked out that one “web-year” was like a dog-year, worth about 7 years of normal human time. So in this respect, 1994 is now some 133 web-years ago. Long enough for an archaeological excavation.
A short non-bonding H…H interaction (continued)
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[cite]10.1021/ja407398w[/cite] 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.