FORMAL CHARGE AND RESONANCE

Formal Charge

• Not all atoms within a neutral molecule need be neutral
• The location of any charges is often important for understanding reactivity.
• Get into the habit of labelling the formal charges on any atoms.

You should be able to determine which atoms have formal charge based on comparing the structure with common, known neutral structures.  To do this you need to recognize the common neutral structures: C 4 bonds, N 3 bonds, 1 lone pair, O 2 bonds, 2 lone pairs, F 1 bond, 3 lone pairs. 

Consider the molecule H₂CO₂. There are two possible Lewis structures for this molecule. Each has the same number of bonds. We can determine which is better by determining which has the least formal charge. It takes energy to get a separation of charge in the molecule (as indicated by the formal charge) so the structure with the least formal charge should be lower in energy and thereby be the better Lewis structure.

The two possible Lewis structures are shown below. They are connected by a double headed arrow and placed in brackets. The non-zero formal charge on any atoms in the molecule have been written near the atom.

Resonance Forms

The Lewis structure for certain molecules or ions can be drawn in more than one way.

e.g. NO₂ ^- number of e^- = 5+ 2 ( 6 ) +1 = 18 e^- (or 9 pairs).

You would expect that the doubly bonded oxygen-nitrogen distance to be slightly less than the singly bonded distance. Actually, both N-O distances are equivalent. The true structure of the molecule is some combination of the two. Anytime you have more than one valid structure for a molecule or ion, you have what are known as resonance structures. If you have many possible resonance forms, you choose the most likely resonance form or forms by calculating the formal charge on each atom in each resonance form.

By convention, resonance forms are shown connected by "double-headed arrows" to stress the fact that they are not separate valence isomers in some sort of rapid equilibrium.

Rules for Resonance

• Resonance structures exist only on paper

Although we cannot use drawings on paper to adequately represent molecules with resonance, drawing resonance forms allows to make predictions and helps us understand the reactivity of organic chemical compounds

• In writing resonance structures we are only allowed to move electrons

We Can Never Change the Connectivity of the Atoms of the Compound

When Moving Electrons:

·        The electron(s) moved must remain in association with at least one of the atoms with which it (they) was associated with before the move

·        For 2-electron moves, you essentially make a double bond from an unshared pair, an unshared pair from a double bond, or move the position of a double bond

 Resonance Structures for Ozone

• All of the structures we write must be “proper” Lewis structures

• The energy of a molecule or ion that has resonance is always lower than might be predicted

• Equivalent resonance structures contribute equally to the overall structure

Are these resonance structures equivalent? YES

Note: We would expect that the bond lengths in the NO₃^- ion to be somewhat shorter than a single bond

• Non-equivalent forms do not contribute equally to the final structure

More Important Resonance Structures:

·        Have formal charges on all atoms are at or near 0

·        Structures which involve the generation and separation of charge contribute less than neutral structures, or structures with the same charge,

·        Structures with negative charges on electronegative atoms contribute more than structures with negative charges on more electropositive atoms.

• The more resonance forms you can write for a given molecule, the more stable the molecule

• Charge separation decreases stability