Resonance and Delocalization - Organic Chemistry Concepts: An EFL Approach - page 58

QUESTIONS AND PROGRAMS

PROGRAM 9 Drawing Resonance Forms
A Drawing reasonable resonance forms for molecular structures helps us understand
the stability and reactivity of molecules.
Using curly arrows helps with this by making sure that no electrons are lost or gained.
Some guidelines are:

(1) All resonance forms have the same geometric arrangement of atoms.

(2) No single resonance form should have unlikely high energy. This means that:

(a) All resonance forms should meet the octet rule for atoms that have atomic
number 10 or less (C, H, O, N, F).

(b) All resonance forms should have an equal number of electron pairs.

(3) All resonance forms have the same overall charge.

(4) No single resonance form gives a true picture of the molecule. Their weighted
average together makes the resonance hybrid.

For example, reasonable resonance forms for the propanone molecule are:

The curly arrow shows movement of the π -bonded electron pair onto the electro-
negative oxygen. This is the same as drawing that bond in the ionic form. Note
the use of the double-headed arrow between resonance forms.

Try to draw other resonance forms for propanone.

B Other electron movements that you could have drawn for the π -bond are:

These are both relatively high energy resonance forms. The first form shows the
unlikely movement of the π -bonded electron pair away from the more electronegative
oxygen toward the carbon. The second form shows the equal splitting of the π -electron
pair to give radicals.
How much a resonance form affects the resonance hybrid depends on its relative
energy. Because these resonance forms have much higher energies, they have little
effect on the final resonance hybrid.

Now draw the resonance hybrid for propanone.