Conjugated dienes bond lengths

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

As already mentioned, no [6 + 4] cycloaddition is observed when tri-carbonyl-i 6-l,3,5-cycloheptatrienemolybdenum(0) is irradiated in presence of conjugated dienes. So an explanation of these experimental results has to be provided. One reason might be an unfavorable fit of the CuH14 n(CH3 ) and C, 4H18 - (CH3) hydrocarbons into the coordination sphere of molybdenum, arising from the increased Mo—C bond lengths. The other reason concerns the possible different stabilities of the intermediates of the photochemical [6 + 4] cycloaddition with molybdenum as the central metal. 

Further evidence for the special nature of conjugated dienes comes from] data on bond lengths (Table 14.2). If we compare the length of the carbon-1 carbon single bond in 1,3-butadiene (148 pm) to that in ethane (154 pm), we find that the 1,3-butadiene single bond is shorter by 6 pm. 

The left-hand part of both Structures 5.3 and 5.4 will contribute more to the resonance hybrid than the others. Benzene is thus a delocalized hydrocarbon whereas buta-1,3-diene with its alternating carbon-carbon bond lengths is a conjugated hydrocarbon. 

The assessment of aromatic character from structural criteria would appear to be a very valid approach. Aromatic -electron delocalization requires planarity of the aromatic molecule and leads to a typical carbon-carbon bond length intermediate between that of a single bond and a formal olefinic double bond. Indeed qualitative orders of the aromaticity of five-membered heteroaromatics have been derived from a consideration of the relative degree of diene character of the conjugated system as assessed from structural determinations of bond lengths (see, for example, Sections III,C, 1 and 5). 

A conjugated diene does not behave like two isolated alkenes. For example, the length of the single bond connecting the two alkene units is slightly shorter than expected for a typical single bond (1.48A). This shows that there is a certain amount of double-bond character present in this bond. Two sp hybridised carbons rather than two sp hybridised carbons. Therefore, an sp hybridised orbital from each carbon is used for the single bond. Since this hybridised orbital ha more s-character than an sp hybridised orbital, the bond is expected to be shorter. An alternative explanation is that the n orbitals of the two alkene systems can overlap to produce the partial double-bond character ... 

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