McMurry (Kap. 2,5)

Question 1

What are he 5 rules for drawing a resonance form?

Answer 1

1. Individual resonance forms are imaginary - not real.
2. Resonance forms differ only in their placement of (pi) electrons or non bonding electrons (lone pairs).
3. Different resonance forms of a substance don’t have to be equivalent.
4. Resonance forms obey normal rules of valency.
5. The resonance hybrid is more stable than any individual resonance form.

Question 2

What does the 1.rule mean?

Answer 2

1. Individual resonance forms are imaginary - not real.

The real structure is the resonance hybrid, not the individual resonance. The hybrid form does not switch between the different individual resonance forms.

Question 3

What does the 2.rule mean?

Answer 3

2. Resonance forms differ only in their placement of (pi) electrons or non bonding electrons (lone pairs).

The position or the hybridization of no atom changes - the only things that change are the placement of the (pi) bond/bonds or/and the nonbonding electrons in the different resonance forms.

The movement of electrons can be illustrated by curved arrows.

• (from lonepair to dobbelbinding).
• (fra dobbeltbinding til lonepair).

Question 4

What does the 3.rule mean?

Answer 4

3. Different resonance forms of a substance don’t have to be equivalent.

Sometimes, the two resonance forms are not equivalent (for example in acetyl coenzyme A anion, one of the C only has 3 bondings, therefore has a negative charge. An oxygen is double bonded to the other carbon. The resonance structure is the negative charge from the carbon forming a double bound between the 2 carbons, and the double bond of the oxygen forming a lonepair so the oxygen becomes negative charged).

We might expect the resonance hybrid to resemble the more stable resonance form (with the negative charge on Oxygen rather then C, because oxygen is more EN).

Question 5

What does the 4.rule say?

Answer 5

4. Resonance forms obey normal rules of valency.

If a molecule is able to (pull a lone pair down to make a double bond) but it will result in obstructing the octet rule, then it DOES NOT happen.

Because fx. C cannot have 5 bindings, therefore it cant happen in that instance (because it cant have 10 electrons, only 8).

Question 6

What does the 5.rule say?

Answer 6

5. The resonance hybrid is more stable than any individual resonance form.

Resonance leads to stability. The larger the number of resonance forms, the more stable the substance is (because its electrons are spread over a larger area of the molecule, and are closer to more nuclei). (resonance hybrid)