Aromaticity Flashcards
history of benzene
isolated from coal tar distillation
debated structure from C6H6
kekule proposed that the double bonds averaged (equilbrate).
found to be different to other polyenes in that they are unreactive
evidence against kekule
- does not equilibrate between structures
- equivalence of C-C bonds confirmed
- pi electrons are equally spread
- delocalised over all 6 atoms
- diffraction studies showed C-C bonds to be 140pm which is between single 154.1 and double 133.7
resonance strutcure
how to draw resonance structure
with a resonance arrow and two different benzene rings or with one benzene ring and a circle in the middle
if not a benzene ring, you can use a dotted line or use mechanism arrows
the more resonance structures?
the more stable and can make them more acidic
- for example charge carried by 3O is more stable, distributed change is more stable
Hückel’s rule
molecules that are stable due to delocalisation are aromatic
if its aromatic:
1. is the molecule a planar ring with conjugated pi electrons?
2. does the number of pi electrons = (4n + 2), where n is an integer?
if there are two rings, analyse them separately
hetero-aromatic
one of the carbons in the ring is replaced with something like an N
being aromatic enables stability, N, O etc will hybridise differently to allow this.
benzene plus X-X will break the bond and be non-aromatic but if it donated a hydrogen to one of the Xs, it can be aromatic again
phenol
a benzene ring with an alcohol group on top
they are strong acids and donate the H from the OH
they have many resonance structures, the charge us spread over 4 atoms and this makes it stable
para position - bottom of benzene ring
ortho - side of ring
analine
benzene ring with NH2 on top
NH2 accepts H and becomes NH3+
it is a weak base
the two electrons from N are incredibly spread out. it is not very reactive and is a weak base
