Bond angle cyclooctatetraene

Cyclooctatetraene (or simply cyclooctatetraene) is a bright-yellow, nonplanar, nonaromatic compound (Section 21-9A). Apparently the resonance energy of a planar structure is insufficient to overcome the unfavorable angle strain the planar structure would have with its C-C-C bond angles of 135°. Cyclooctatetraene normally assumes a tub structure with alternating single and double bonds ... 

When cyclooctatetraene accepts two electrons, it becomes a An + 2) n electron aromatic ion. Cyclooctatetraenyl dianion is planar with a carbon-carbon bond angle of 135° (a regular octagon). 

Valence bond theory, in the terms defined by Pauling, is not able to account for the 4n+2 rule, and the properties of cyclobutadiene and cyclooctatetraene. It has been suggested that the problem with these molecules is the strain associated with the bond angles in the planar structures.10 However, this was shown to be incorrect by the observation that the addition of two electrons to cyclooctatetraene leads to the planar dianion. It is only recently that it has been recognized that cyclic permutations must be included in order to properly treat cyclic systems via valence bond theory.11 One of Pauling s few failures in structural theory is his nonrecognition of the problems associated with the 4n molecules. 

Cyclobutadiene has two pairs of rr electrons, and cyclooctatetraene has four pairs of tt electrons. Unlike benzene, these compounds are not aromatic because they have an even number of pairs of rr electrons. There is an additional reason why cyclooctatetraene is not aromatic—it is not planar but, instead, tub-shaped. Earlier, we saw that, for an eight-membered ring to be planar, it must have bond angles of 135° (Chapter 2, Problem 28), and we know that sp carbons have 120° bond angles. Therefore, if cyclooctatetraene were planar, it would have considerable angle strain. Because cyclobutadiene and cyclooctatetraene are not aromatic, they do not have the unusual stability of aromatic compounds. 

The structure of cyclooctatetraene is well known to be that of a tub. This allows the C-C-C bond angles to be about 122° and near to the normal value for a polyene. There is a conjugation around the ring, which would be more effective if the ring were planar. But since the molecule has 8 n electrons, it does not obey HiickeTs rule for aromaticity. 

Cyclooctatetraene (COT) is also antiaromatic when planar. To avoid the antiaromaticity and severe bond angle strain, the ring pucRers into a tub shaped conformation (shown in the margin). The barrier to "flipping" the tub is 13.7 Real/mol. Interestingly, addition of two electrons or removal of two electrons would create aromatic systems, and indeed both reactions are Rnown and lead to planar structures. 

Cyclooctatetraene is quite stable at room temperature. However, close examination of the stmcture reveals that it is not planar. This can be explained by optimization of bond angles. 

Use MO theory to constmct an argument not based on bond angle optimization that explains why cyclooctatetraene is not planar. 

Application of an extension of the above treatment to cyclooctatetraene 239 and other cyclic polyenes50 yields a satisfactory determination of the twist angles r between conjugated pairs of their double bonds, in excellent agreement with those derived by other methods. 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

This geometry precludes the possibility of two equivalent VB structures, as for benzene, because, as you will see if you try to make a ball-and-stick model, 25b is highly strained and not energetically equivalent to 25a at all. Thus we can conclude that the delocalization energy of cyclooctatetraene is not large enough to overcome the angle strain that would develop if the molecule were to become planar and allow the tt electrons to form equivalent tt bonds between all of the pairs of adjacent carbons. 

Make a model of cyclooctatetraene in the tub conformation. Draw this conformation, and estimate the angle between thep orbitals of adjacent pi bonds. 

Neutral ligands can also bond in a variety of ways. Cyclooctatetraene can act as an alkene (Tj2), a diene (T)4), a triene (T]6), or a tetraene (r 8), and the reactivity of the ligand changes accordingly. These are all ft complexes with the metal above or below the black portion of the ring and with the thick bond to the metal at right angles to the alkene plane. 

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