In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown?
Archive for the ‘Interesting chemistry’ Category
Conformational analysis of cyclotriborazane
Sunday, February 14th, 2010The conformation of cyclohexane
Thursday, January 28th, 2010Like benzene, its fully saturated version cyclohexane represents an icon of organic chemistry. By 1890, the structure of planar benzene was pretty much understood, but organic chemistry was still struggling somewhat to fully embrace three rather than two dimensions. A grand-old-man of organic chemistry at the time, Adolf von Baeyer, believed that cyclohexane too was flat, and what he said went. So when a young upstart named Hermann Sachse suggested it was not flat, and furthermore could exist in two forms, which we now call chair and boat, no-one believed him. His was a trigonometric proof, deriving from the tetrahedral angle of 109.47 at carbon, and producing what he termed strainless rings.
Blisteringly bent (quadruple) bonds
Saturday, January 23rd, 2010So ingrained is the habit to think of a bond as a simple straight line connecting two atoms, that we rarely ask ourselves if they are bent, and if so, by how much (and indeed, does it matter?). Well Hursthouse, Malik, and Sales, as long ago as 1978, asked just such a question about the unlikeliest of bonds, a quadruple Cr-Cr bond, found in the compound di-μ-trimethylsilylmethyl-bis-[(tri-methylphosphine) (trimethylsilylmethyI)chromium(II)(DOI: 10.1039/dt9780001314[cite]10.1039/dt9780001314[/cite]). They arrived at this conclusion by looking very carefully at how the overlaps with the Cr d-orbitals might be achieved.
Chemical intimacy: Ion pairs in carbocations
Monday, January 11th, 2010The scheme below illustrates one of the iconic reactions in organic chemistry. It is a modern representation of Meerwein’s famous experiment from which he inferred a carbocation intermediate, deduced from studying the rate of enantiomerization of isobornyl chloride when treated with the Lewis acid SnCl4.
Contriving aromaticity from S≡C Triple bonds
Friday, January 1st, 2010In the previous post, the molecule F3S-C≡SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal/mol, ν 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below:
Clar islands in a π Cloud
Wednesday, December 9th, 2009Clar islands are found not so much in an ocean, but in a type of molecule known as polycyclic aromatic hydrocarbons (PAH). One member of this class, graphene, is attracting a lot of attention recently as a potential material for use in computer chips. Clar coined the term in 1972 to explain the properties of PAHs, and the background is covered in a recent article by Fowler and co-workers (DOI: 10.1039/b604769f). The concept is illustrated by the following hydrocarbon:
The nature of the C≡S triple bond: part 3.
Sunday, December 6th, 2009In the previous two posts, a strategy for tuning the nature of the CS bond in the molecule HO-S≡C-H was developed, based largely on the lone pair of electrons identified on the carbon atom. By replacing the HO group by one with greater σ-electron withdrawing propensity, the stereo-electronic effect between the O-S bond and the carbon lone pair was enhanced, and in the process, the SC bond was strengthened. It is time to do a control experiment in the other direction. Now, the HO-S group is replaced by a H2B-S group. The B-S bond, boron being very much less electronegative than oxygen, should be a very poor σ-acceptor. In addition, whereas oxygen was a π-electron donor (acting to strengthen the S=C region), boron is a π-acceptor, and will also act in the opposite direction. So now, this group should serve to weaken the S-C bond.
The nature of the C≡S Triple bond: Part 2
Saturday, December 5th, 2009In my first post on this theme, an ELF (Electron localization function) analysis of the bonding in the molecule HO-S≡C-H (DOI: 10.1002/anie.200903969) was presented. This analysis identified a lone pair of electrons localized on the carbon (integrating in fact to almost exactly 2.0) in addition to electrons in the CC region. This picture seems to indicate that the triple bond splits into two well defined regions of electron density (synaptic basins). In a comment to this post, I also pointed out that an NBO analysis showed a large interaction energy between the carbon lone pair and the S-O σ* orbital, characteristic of anomeric effects (in eg sugars). This latter observation gives us a handle on possibly tweaking the effect. Thus if the S-O σ* orbital can be made a better electron acceptor, then its interaction with the lone pair could be enhanced.
The nature of the C≡S triple bond
Tuesday, December 1st, 2009Steve Bachrach has just blogged on a recent article (DOI: 10.1002/anie.200903969) claiming the isolation of a compound with a C≡S triple bond;