Page 47 - Mechanism and Theory in Organic Chemistry

strain is not sufficient to overcome the tendency toward planarity for an eight-
membered ring with 4n + 2 .rr electrons.
A number of larger cyclic conjugated systems have been prepared, many of
them by Sondheimer and co-workers.l~lO]Annulene and [12]annulene are
subject to considerable steric difficulties and probably are not planar, but the
larger molecules are big enough to accommodate hydrogen atoms inside the rings
and so can have trans double bonds and still be planar or nearly so. Likely con-
formations for some of these compounds are shown in Structures 39, 40, and 41.

[14] annulene
39

H
/H
\\\
H H
H 63 H
H
[16] annulene
40

H H
[ 181 annulene
4 1

Annulenes as large as the 30-membered ring have been prepared, and many
dehydroannulenes, whicK contain one or more triple bonds, are also known.

le F. Sondheimer, Accts. Chem. Res., 5, 81 (1972).