Organic Chemistry

Key characteristics

cause Lewis dot diagram^7 s often cannot represent the true electronic structure of a molecule,
resonance structures are often employed to approximate the true electronic structure. Res-
onance structures of the same molecule are connected with a double-headed arrow. While
organic chemists use resonance structures frequently, they are also used in inorganic struc-
tures, with nitrate^8 as an example.

10.3 Key characteristics.

The key elements of resonance are:

• Resonance occurs because of the overlap of orbital^9 s. Double bonds are made up of pi
  bonds^10 , formed from the overlap of 2p orbital^11 s. The electrons in these pi orbitals will
  be spread over more than two atoms, and hence are delocalized.


• Both paired and unshared electrons may be delocalized, but all the electrons must be
  conjugated in a pi system.


• If the orbitals do not overlap (such as in orthogonal^12 orbitals) the structures are not true
  resonance structures and do not mix.


• Molecules or species with resonance structures are generally considered to be more stable
  than those without them. The delocalization of the electrons lowers the orbital ener-
  gies, imparting this stability. The resonance in benzene gives rise to the property of
  aromaticity^13. The gain in stability is called theresonance energy.


• All resonance structures for the same molecule must have the same sigma framework
  (sigmabond^14 s form from the ”head on” overlapof hybridizedorbitals). Furthermore, they
  must be correct Lewis structure^15 s with the same number of electrons (and consequent
  charge^16 ) as well as the same number of unpaired electrons. Resonance structures with
  arbitrary separation of charge are unimportant, as are those with fewer covalent bonds.
  These unimportant resonance structures only contribute minimally (or not at all) to the
  overall bonding description; however, they are important in some cases such as for a
  carbonyl^17 group.


• Thehybrid structureis defined as the superposition of the resonance structures. A
  benzene ring is often shown with a circle inside a hexagon (in American texts) rather than
  alternating double bonds — the latter example misrepresents the electronic structure.
  Bonds with broken bond order^18 s are often displayed as double bonds with one solid and
  one dashed line.

7 https://en.wikipedia.org/wiki/Lewis%20dot%20diagram
8 https://en.wikipedia.org/wiki/nitrate
9 https://en.wikipedia.org/wiki/atomic%20orbital
10 https://en.wikipedia.org/wiki/pi%20bond
11 https://en.wikipedia.org/wiki/atomic%20orbital
12 https://en.wikipedia.org/wiki/orthogonal
13 https://en.wikipedia.org/wiki/aromaticity
14 https://en.wikipedia.org/wiki/Sigma%20bond
15 https://en.wikipedia.org/wiki/Lewis%20structure
16 https://en.wikipedia.org/wiki/Electric%20charge
17 https://en.wikipedia.org/wiki/Carbonyl
18 https://en.wikipedia.org/wiki/Bond%20order