C4. Resonance

Question 1

Describe the lewis model

Answer 1

• Does not model delocalised electron behaviour
• Does not explain relative bond strengths (σ vs π)

Question 2

Describe the atomic orbital/ hybrid orbital theory

Answer 2

• Good at predicting shapes and bonding
• Explains bond strengths
• Still predicts localised bonding

Question 3

Describe the resonance theory

Answer 3

• Can be applied to Lewis model or AO model
• Predicts delocalised bonding

Question 4

What does the resonance theory state

Answer 4

This states that there can be various possible structures for a molecule, and that no individual one of them is correct, but all contribute to the “true” structure

Question 5

what is the limitation of the lewis structure

Answer 5

The Lewis structure is unable to accurately describe the structure of
the carbonate anion, and many others

Question 6

what does a resonance arrow look like?

Answer 6

double headed arrow

Question 7

curly arrow vs radical arrow?

Answer 7

curly arrow- movement of a pair of electrons
radical arrow- movement of a single electron

Question 8

are resonance structures all equal?

Answer 8

no

Question 9

How does each resonance form contribute? (which is most stable form and therefore most “true” form)

Answer 9

-structures with more covalent bonds are more stable
-structures with separation charge are less stable
-structures where a negative charge is on an electronegative atom are more stable than those where it is on a electropositive atom

Question 10

why are tertiary carbocations more stable than primary carbocations?

Answer 10

methyl groups are electron donating therefore in tertiary carbocation groups there is more electron donating to the carbocation through sigma bond, increasing stability

Question 11

In terms of ion size, how do you determine the major resonance structure?

Answer 11

the bigger ion is more stable so it will be the major resonance structure

Question 12

in terms of octet rule, how do you determine the major resonance structure?

Answer 12

where octet rule is fulfilled, it is a major resonance structure. It is better to have a positive charge on oxygen rather than carbon. Structures where a negative charge is on an electronegative atom are more stable than those where it is on a electropositive atom.

Question 13

what is the formula to calculate the number of lone pairs around the carbon central atom?

Answer 13

LP= (Ve (valence electrons) - 8n)/2
n= number of non- central atoms

Question 14

In terms of number of covalent bonds, how do you determine the more stable resonance structure?

Answer 14

structures with more covalent bonds are more stable

Question 15

In terms of charge separation, how do you determine the more stable resonance structure?

Answer 15

structures with charge separation are less stable

Question 17

the larger the number of resonance structures…

Answer 17

the more stable these structure, the more stable the resonance hybrid. Resonance stabilizes molecules, ions and radicals

Question 18

when drawing resonance structures, what can be the source (electron rich species?

Answer 18

-negative charge
-lone pair
-bond (sigma or pi)

Question 19

when drawing resonance structures, what can be the sink (electron deficient species)?

Answer 19

-positive charge
-partial positive charge
-atom happy to accept electrons (e.g electronegative -O, N, Halogen)

Question 20

Compounds containing conjugated multiple bonds absorb light at wavelengths longer than what?

Answer 20

200nm (i.e in the visible). The longer the conjugated system, the longer the wavelength absorbed.

Question 20

why do polyenes with many conjugated double bonds appear coloured?

Answer 20

they absorb light in the visible region of the spectrum

Question 21

If a substance absorbs visible light, what colour will it appear?

Answer 21

the colour it will appear is the complementary colour to that absorbed. This is shown on the colour wheel. (colour wheel on one note)