Bond angles 10 -annulene

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

NMR). Note the trans-trans-cis double bonds all bond angles can be 120 . [20]annulene presumably could become planar (it isn t quite) but since it is a 4n p electron system rather than a 4n + 2 system, it is not aromatic and the structure shows localized single and double bonds. 

X-ray analysis shows that the [18]annulene (Problem 9, p. 336, / = 9) is planar. The nmr spectrum shows two broad bands t 1.1 and t 11.8, peak area ratio 2 1. (a) Are these properties consistent with aromaticity Explain, (b) Would you have predicted aromaticity for this compound Explain. Hint Carefully draw a structural formula for the compound, keeping in mind bond angles and showing all hydrogen atoms.)... 

If this annulene with five as double bonds were planar, each internal angle would be 144°. Since a normal double bond has bond angles of 120°, this would be far from ideal. This compoimd can be made but it does not adopt a planar conformation and therefore is not aromatic even though it has ten jt electrons. 

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