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Our teacher had taught us that cis-trans-[10]annulene is non-aromatic. The reason he gave is due to repulsions between hydrogens, the structure is no longer planar

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But later he told us that [14]annulene is aromatic. But isn't there a repulsion between hydrogens in [14]annulene too.

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In fact, there are 4 hydrogens repelling each other, so why is it still planar and aromatic?

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    $\begingroup$ It is not quite planar and not quite aromatic. $\endgroup$ Commented Feb 17, 2023 at 12:03
  • $\begingroup$ Of possible interest regarding [10]-annulene. Go here. $\endgroup$
    – user55119
    Commented Feb 17, 2023 at 17:48

1 Answer 1

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If you have a molecular model kit, use it to see the difference.

In [10]annulene, the inward-directed hydrogen atoms are separated by six carbon atoms along the ring and thus, in your conformation, are forced onto the same side of the hexagon where steric hindrance becomes severe. Other confirmations prevent that, but overstress the sigma bonds within the ring.

In [14]annulene, the inward-directed hydrogens are separated by only four or five intervening carbons, so they are pushed towards each other along diagonals rather than sides of the hexagons. The difference in length of the diagonals versus the sides makes the steric hindrance less severe.

Even so, [14]annulene has only limited aromatic character. [18]annulene, with a still more sterically favorable arrangement of inward-directed hydrogens, is more strongly aromatic.

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