Q37E - Organic Chemistry (Mcmurry)

Question

Which of the following pairs of structures represent resonance forms?

a.

b.

c.

d.

Step-by-Step Solution

Answer

1. These are not resonance forms.
2. These are resonance forms.
3. These are resonance forms.
4. These are resonance forms.

Step 1: Resonance

Resonance is a representation of delocalized electrons within specific molecules or polyatomic ions when a single Lewis structure cannot conclude the bonding.

Resonance occurs when two double bonds are present consecutively or double bond followed by a single bond followed by another double bond, triple bond, positive charge, negative charge, or a free radical.

Step 2: Rules for structures of resonance form

Rule 1

Individual resonance forms are imaginary, which means they are not real. The real structure is a composite or resonance hybrid of all forms.

 

Rule 2

Resonance forms differ only in the placement of their  or nonbonding electrons. So,  the position or the hybridization of an atom doesn’t change.

 

Rule 3

Different resonance forms of the molecule don’t have to be equivalent. It can be converted into its anion by reaction with a strong base. The resultant anion can have resonance forms.

 

Rule 4

Resonance forms obey standard rules of valency. Like any other structure, a resonance form has to follow the octet rule.

 

Rule 5

 

The resonance hybrid is more stable than any particular resonance form. So, resonance leads to stability. The more the number of resonance structures, the more stability will be.

Step 3: (a) Structure comparison for resonance forms

Structural comparison

We will see if the rules are followed:

Rule 2: Resonance forms differ only in the placement of their $\mathrm{\pi }$ or nonbonding electrons. So, the position or the hybridization of an atom doesn’t change.

But here, the position of atoms is changed.

Hence these are not resonating forms.

Step 4:( b) Structure comparison for resonance forms:

Structural comparion

We will see if rules are followed:

Here all rules are followed:

Rule 2: Resonance forms differ only in the placement of their $\mathrm{\pi }$  or nonbonding electrons. So, the position or the hybridization of any atom doesn’t change. 

Here, position of only $\mathrm{\pi }$  or nonbonding electron is changed.

Rule 4: Resonance forms obey normal rules of valency. A resonance form like any other structure has to follow the octet rule.

Here, each atom is following octet rule.

So, it can be concluded these are resonating forms.

Step 5: (c) Structure comparison for resonance forms:

Structural comparison

We will see if the rules are followed:

Here all rules are followed:

Rule 2: Resonance forms differ only in the placement of their $\mathrm{\pi }$ or nonbonding electrons. So, the position or the hybridization of an atom doesn’t change.

Here, the position of only  $\mathrm{\pi }$ or nonbonding electron is changed.

Rule 4: Resonance forms obey normal rules of valency. Like any other structure, a resonance form must follow the octet rule.

Here, each atom follows the octet rule.

So, it can be concluded these are resonating forms.

Step 6: (d) Structure comparison for resonance forms

Structural comparison

We will see if the rules are followed

Here all rules are followed:

Rule 2: Resonance forms differ only in the placement of their $\mathrm{\pi }$ or nonbonding electrons. So, the position or the hybridization of an atom doesn’t change.

Here, the position of only  $\mathrm{\pi }$ or nonbonding electron is changed.

Rule 4: Resonance forms obey normal rules of valency. A resonance form, like any other structure, has to follow the octet rule.

Here, each atom follows the octet rule.

So, it can be concluded these are resonating forms.