topic 2: resonance structures

Some molecules have structures where electrons cannot be shown in a single drawing, which is a limitation in valence bond theory.
In these cases, multiple structures are drawn that contribute to the final overall structure, differing in the position of $pi$ bonds and/or lone pairs, as well as charges (if any).
Such a structure is said to be delocalized (electrons and charges) and is represented by different resonance structures.
Resonance structures are connected by a double-headed arrow.
The $pi$ bond, lone pair, and charge are delocalized, and valence bond theory cannot describe where the $pi$ bond and one lone pair are located.
Multiple resonance structures are drawn, and MO theory is used to better illustrate delocalization; delocalization stabilizes the compound.

A curved arrow is used to imagine how electron pairs "move" to convert from one resonance structure to another.
Curved arrows show how electron pairs move from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow.
Resonance structures are not real; they are imaginary structures used to illustrate the molecule.
The compound does not "exist" like the structure on the left or the structure on the right but as a weighted combination of all possible resonance forms, and electrons do not really "move" between structures.

Resonance structures are needed when a combination of lone pairs, $pi$ -bonds, and/or charges are immediately next to each other in a molecule.
Double-headed curved arrows are used to indicate the movement of two electrons (electron pair).
Arrows are NOT used to show how charges move; they are only used to show how electron pairs move.
If lone pairs are not drawn in, add them, or start the arrow at an atom that has a lone pair, or start the arrow at a minus charge.

Redraw all atoms and sigma bonds in the same position.
Only move $pi$ electrons and lone pair electrons (note that electrons must remain on the atom it was originally part of) using curved arrows to illustrate.
All resonance forms must have the same net charge.
Never violate the octet rule for 2nd-row atoms.
A +2 or -2 charge is highly unlikely.
Resonance forms may be equivalent (degenerate) but do not have to be.
Not all resonance forms have to contribute equally to the overall resonance hybrid.
Typically, do not show resonance forms that split a $pi$ bond into a plus charge and a minus charge (but may do so in some cases to help explain reactivity).
If you make a bond to an atom with a full octet, you must also break a bond.

Apply the following guidelines in ORDER:

Structures with a maximum number of atoms with a full octet are preferred.
Charges, if any, should be placed on an atom with compatible electronegativity (negative charge on a more electronegative atom, positive charge on a less electronegative atom).
Structures with minimum charge separation are preferred.

Draw all likely resonance structures and identify major resonance contributors.
Examples are provided to illustrate the drawing of resonance structures and the identification of major, minor, and degenerate resonance structures.
Major resonance contributors = B and G (degenerate)
A and F are also degenerate
Major resonance contributors = I and K (not quite degenerate though)

A structure with resonance forms does not alternate between the forms (individual resonance forms are imaginary).
Instead, it is a hybrid of all resonance forms (weighted average), so the overall single structure is called a resonance hybrid.
Anywhere a $pi$ -bond is present is now a dotted bond; anywhere a charge is located is now a partial charge (will not be used in this class).

The major resonance contributor best describes the structure of the molecule.
Minor resonance forms best describe reactivity.
If we can’t draw/see all resonance structures, we do not see the whole picture to understand structure or reactivity in molecules.

Must consider all resonance structures to determine hybridization.
Each atom must have the correct number of $p$ -orbitals to satisfy any resonance structure.
If an atom looks $sp$ or $sp^2$ in any resonance form, it must be $sp$ or $sp^2$ in all.

For a given compound, the tasks include drawing a degenerate resonance structure, drawing a 'better' (major) resonance contributor (2 options), drawing a 'worse' (minor) resonance contributor, determining which cation is more stable and more reactive, and determining the hybridization of a carbanion.