AROMATICITY

1
AROMATICITY
Three Simple Rules
1. Continuous cyclic cloud of delocalized pi (?
) electrons which produce a ring current.
2. The ? cloud must contain (4n 2) pi
electrons (Huckels rule).
3. Ring must be planar.
2
Ring Current
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3
Ring Current
4
Ring Current
Conjugation (resonance) can be a clue to
presence of ring current.
Yes
No
5
Ring Current
Another view are all atoms included
in conjugation (resonance)?
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No
Yes
Yes
6
Ring Current
Be wary! More to ring current than conjugation!
And is not planar
Does not satisfy Huckel Rule
7
Huckel Rule
4n 2 2, 6, 10, 14 .
Which electrons do you count?
Count those that generate reasonable resonance
structures!
these you count
8
Huckel Rule
4n 2 2, 6, 10, 14 .
Which electrons do you count?
Count those that generate reasonable resonance
structures!
these you dont
9
Huckel Rule
4n 2 2, 6, 10, 14 .
Which electrons do you count?
Count those that generate reasonable resonance
structures!
dont count these either
10
Planarity

• p clouds will form if p orbitals overlap

• overlap favored by bond angles of 120º

Number of sp2 atoms that will fit a flat ring
without undue angle strain is
Hint
5 to 7.
11
Some Additional Hints

• 6-membered ring must have 3 double bonds.

• 5-membered heteroaromatic ring must have
• 2 double bonds and 1 lone pair of electrons.

• Break condensed rings into component parts.
• Both must be aromatic.