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Introduction
One of the most fascinating things about organic chemistry is that an incredible number of molecules can be formed; with the simplest, smallest alkanes to larger molecules such as porphyrin rings, each of them are chemically interesting, and sometimes even structurally interesting. With the almost unlimited array of organic compounds, literally anything is possible for them. Of course, there are some compounds that are more interesting than others; in this article, we will explore three of such molecules, and attempt to explain the reasons for why they are chemically or structurally interesting. The emphasis will be on chemically interesting molecules, as structurally interesting molecules, such as synthetic molecular knots or the Nanokid, have already been covered extensively.
Anthroic Acid
In the figure above, the structure of an anthroic acid (which has an extremely long name in IUPAC nomenclature) is shown. At first, it may appear that the molecule is ‘just another’ ordinary organic molecule. From this figure, we can note two things; firstly, as no plane of symmetry exists for this molecule, and the carbon atoms bonded to H and Cl atoms are chiral, it is possible to form the molecule’s stereoisomer (below) by swapping the positions of the H and Cl atoms. The second thing we note is that there is a carboxylic acid group bonded to the benzene (somewhat reminiscent of benzoic acid), and thus the anthroic acid molecule is acidic (which should be obvious just by the name of the compound). However, when the stereoisomers’ (i.e. the molecule above and its isomer below where the H and Cl atoms swap positions) acidities are compared, they are different. Between the two molecules, there is completely no change toward the carboxylic acid substituent, yet the acidities are different? Why is this so?
The Chemistry: The answer to this question, as would be expected, lies in the switching of the positions of the chlorine and hydrogen atoms. In the stereoisomer which places the chloride substituents closer to the carboxylic acid, a stronger field effect occurs. Chlorines draw electron density toward themselves, making the ‘space’ around them more negatively-charged. The closer the negatively-charged space to the carboxylic acid, the less energetically favorable the loss of a proton, because the loss of a proton would result in a negatively-charged carboxylate. This negatively-charged carboxylate repels the negatively-charged space around the chloride, destabilizing the molecule. In contrast, the other stereoisomer has the chloride (and therefore the negatively-charged space) further away from the carboxylic acid substituent, thus minimizing the same destabilizing interactions.
Bridgeheads
Many molecules that have bridgeheads, of course, cannot be neglected if we are to discuss chemically and structurally interesting molecules. Above and below, we show two examples of notable molecules that have bridgeheads. The above figure shows the structure of adamantane, which already has a relatively interesting structure by itself; the structure is difficult to interpret, but has been generally considered as a fusion of three cyclohexane rings. Furthermore, the molecule is extremely stable; this is why it has the informal name ‘adamantane’, an allusion to the indestructible metal alloy seen in Marvel. Meanwhile, in the bottom figure we see the resonance forms of the norbornane cation. Norbornanes are chemically interesting due to the structure of the cyclohexane at the bottom of the molecule. While normal cyclohexanes typically assume other conformations, an interesting ‘boat’ conformation (shape) is assumed by the cyclohexane in norbornane. Furthermore, norbornane also possesses interesting chemistry as a carbocation, because the typical ‘cyclic’ structure is broken in order to stabilize the carbocation.
The Chemistry: There are several chemical issues we have to address, mainly pertaining to the structure of the norbornyl cation directly above. To ‘progress’ from one resonance form to another, sigma bonds must be broken and reformed, meaning that there is a delocalization of sigma bonds, as can be seen in the 2,6 bond in (b). Many arguments have been directed especially at the nonclassical cation (b), since some felt it was unlikely that partial bonds could be formed from delocalization of sigma bonds (which are not typically seen in organic chemistry). To avoid the possibility that the stable norbornyl cation (b) existed, it has been suggested that there are no resonance forms, and the three structures are instead conformations which the molecule rapidly interconverts between. However, strong evidence has been found by crystallization that the structure (b) indeed exists.
Additionally, cyclohexanes typically assume the chair conformation, since this is the conformation which is the most stable for them. The chair conformation lacks both angle strain and torsional strain, making it relatively stable as compared to other conformers. However, as we have noted above, the cyclohexane in norbornane ‘chooses’ to assume a ‘boat’ conformation instead. The reason for this is typically attributed to steric factors; if the cyclohexane ring in norbornane were in a ‘chair’ conformation, it would result in substituents being too close to each other, resulting in excessive steric interactions that destabilizes the molecule.
Cyclopropanes
Cyclopropanes may seem very ordinary and possess a simple structure, but they do possess interesting chemical properties. For one, we know that the bond angles of cyclopropanes are around 60 degrees, yet typical sp3 carbons prefer 109.5 degree angles of bonding. Despite the fact that cyclopropanes seem highly strained, since preferred angles of bonding are not attained, they are still relatively stable. Cyclopropanes have only a slightly higher strain than cyclobutanes, while the next of the cycloalkanes, cyclopentanes, have almost no strain at all (no angular strain, but there is still eclipsing strain). Given this, we would expect that cyclopropanes should have a much higher strain than cyclobutanes, but this is not the case. Many theories have been proposed for why this is so, but the bent bond theory is most widely accepted.
The Chemistry: While typical sp3 carbons do indeed have 109.5 degree angles preferred for bonding, cyclopropane’s carbons are sp5 hybridized; this means that the orbitals have 1/6 s character and 5/6 p character. The bond angles preferred for sp5 bonding is much lower (since p orbitals prefer 90 degree angles) and thus 60 degree bond angles are possible without a significant increase in the amount of angular strain experienced. Due to the bent bonding, the orbitals of the carbon atoms ‘curve’ slightly outwards, causing the electron density to lie away from the ring, with 109.5 degree angles of bonding. This creates something similar to a double bond, and evidence for conjugation by UV spectra of cyclopropane has served to support this.