Page 44 - Mechanism and Theory in Organic Chemistry

Aromaticity 33

the last pair will be the only occupants of a doubly degenerate level and so one
electron will go into each orbital with spins parallel. Diagrams 34 and 35 show
the level filling for 4n + 2 and 4n electrons, respectively.

4n + 2 electrons 4n electrons
(odd number of pairs) (even number of pairs)

Because open-shell molecules are ordinarily highly reactive, the 4n electron rings
should be chemically unstable.
When the .rr electron energy of a conjugated C,H,ing is calculated from
__ _-
--
the energy-level diagra~~jt found to bs differe~t-fr~m the .rr en_eusySY~alculated
is
'----
for --..--- an open-chain CnH,+, p~&neir from the-~ energyo~bh~enemolecules.
The difference is termed k.~sona_n_ce egr-./ The simple Hiickel molecular
the 4n systems are destabilized\1° Theory thus provides an energy criterion for I
orbital theory yields -energies,
but more careful calculations show that
the 4n + 2 rings are stabilized compared with the open-chain analog, whereas
classification of cyclic conjugated systems as aromatic or antiaromatic: -A_
molecule is aromatic if it ~hermodypamically_lnare_s~ahle than expected for
tsopen-chmog, and it isa-rrciar~matic if it is thrnodynamically less
_
__
compound showisg_nei&~ stabi4dioII mx_&stahilization would be
--
csd as nonaromatic.
-AGcond criterion of aromaticity comes from analysis of the influence of a
magnetic field on the .rr electron cloud. Theory suggests thatimaagnetic field
perpendicular to the ring plane will cause the electrons to behave as thou&ihey
were cifciirating around the ring and generating their own small magnetic field,
which will be superimposed on the applied field.ll The term ring current is com-
monly applied to this phenomenon.
The directian ofthis induced magnetic field depends on the number of rr
electrons. In 4n _+ 2 rings ii is in thcdirection opposite-tothe applied field (the
rings are d&mqmticJ, wkreas in 4n rings it is in the same direction as the applied
field (paramagnetic - rings). --- ~i~urel.2 1 illustrates these two situations and shows
how the inducedfields affect the total field at protons attached to the ring. Be-
cause the magnetic lines of force must make closed loops, the induced field is in
opposite directions for protons inside and outside the ring. Note that in 4n + 2

lo See, for example, (a) M. J. S. Dewar and G. J. Gleicher, J. Amer. Chem. Soc., 87, 685 (1965); for
discussion of the various approaches to ?r electron molecular orbital theory, see (b) Streitwieser,
Molecular Orbital Theory for Organic Chemists, and (c) L. Salem, The Molecular Orbital Theory of Conju-
gated Systems, W. A. Benjamin, Menlo Park, Calif., 1966.
" J. A. Pople and K. G. Untch, J. Amer. Chem. Soc., 88, 481 1 (1966).