Angle strain 10 annulene

Aimulene offers a particularly significant test of the Hiickel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

Aromaticity in the larger (4A+2) annulenes depends on whether the molecule can adopt the necessary planar conformation. In the all-cis [10]annulene, the planar conformation requires an excessive amount of angle strain. The [10]annulene isomer with two trans double bonds cannot adopt a planar conformation either, because two hydrogen atoms interfere with each other. Neither of these [10]annulene isomers is aromatic, even though each has (4/V+2) pi electrons, with N = 2. If the interfering hydrogen atoms in the partially trans isomer are removed, the molecule can be planar. When these hydrogen atoms are replaced with a bond, the aromatic compound naphthalene results. 

A Hiickel aromatic system with ten electrons (n = 2) can be established with the 9-membered azonine ting system involving the eight pi-electrons of the double bonds and the lone pair on nitrogen. The aU-cis ring (7.10) is known but not stable because of the angle strain as described in the aza-annulenes (Chapter 6). Its proton NMR spectrum... 

As the example of [10]annulene indicates, large annulenes having some trans double bonds can have lower angle strain than the corresponding all-cis... 

Larger annulenes permit the incorporation of irons double bonds into the rings hence, isomeric annulenes warrant consideration beginning with the cyclodecapen-taenes. [10]Annulene should, by the Huckel rule, possess aromatic stabilization if it were planar. All the isomeric cyclodecapentaenes suffer serious strain that prevents the planar geometry from being adopted. The trans,cis,trans,cis,cis-isomer, which has minimal angle strain, suffers a severe nonbonded repulsion between the two internal hydrogens ... 

The all-ctf-isomer is required by geometry to have ring angles of 144° in a planar geometry, and is therefore destabilized enormously by the angle strain associated with a distortion of 24° from the normal trigonal geometry at each of ten carbon atoms. Two isomeric [10]annulenes, as well as other products, are formed by photolysis of m-9,10-dihydronaphthalene ... 

Explain the following (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic, (b) Although [16]annulene is not aromatic, it adds two electrons readily to form an aromatic dianion. 

---