Resonance and Aromaticity

In summary, the speaker has two doubts regarding resonance and aromaticity. The first doubt pertains to the hybridization of the nitrogen atom in aniline, which can vary in different resonating structures. The speaker is unsure how to determine if the molecule is planar or not based on the hybridization of the nitrogen atom. The second doubt is about the planarity requirement for aromatic molecules and whether it applies to all atoms or just those involved in delocalization of electrons. The expert explains that only the delocalized system of electrons needs to be planar for a molecule to be considered aromatic.
  • #1
Ashu2912
107
1
I have two doubts in my course regarding resonance and aromaticity...

(1) In various resonating structures, the hybridization of an atom may vary. Eg:- in aniline's nitrogen atom, it is sp3 as well as sp2 in different resonating structures. Since the actual molecule is a weighted mean of all the resonating structures, the hybridization of the nitrogen atom must be somewhere between sp2 and sp3 that is, the hybrid orbitals have sp3 as well as sp2 character. Now, I want to know if the molecule is planar or not. If the hybridization of the nitrogen atom was purely sp3, the molecule would be non-planar and if it was purely sp2. So how do I decide it's planarity?

(2) When we check for aromaticity, the molecule must be planar. However, is this 'planar' condition only for the atoms involved in the delocalization of electrons, or for all atoms?
 
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  • #2
Ashu2912 said:
I have two doubts in my course regarding resonance and aromaticity...

(1) In various resonating structures, the hybridization of an atom may vary. Eg:- in aniline's nitrogen atom, it is sp3 as well as sp2 in different resonating structures. Since the actual molecule is a weighted mean of all the resonating structures, the hybridization of the nitrogen atom must be somewhere between sp2 and sp3 that is, the hybrid orbitals have sp3 as well as sp2 character. Now, I want to know if the molecule is planar or not. If the hybridization of the nitrogen atom was purely sp3, the molecule would be non-planar and if it was purely sp2. So how do I decide it's planarity?

The only resonance structures you can draw that have the aniline's nitrogen lone pair involved in delocalization are structures where you generate a separation of charge... positive on the nitrogen and negative at the ortho and para positions. You can argue that the nitrogen might direct an electrophile to those locations since the transition state would be favorable but the transition state wouldn't be aromatic. Even so, why are you obsessed regarding planarity? Do you think the nitrogen of aniline does not lie in the plane of the aromatic ring? It does but that doesn't prove that it's lone pair is involved in the aromatic system (it's not, BTW). There are enough pi electrons (6) in the aromatic system without the need for aniline's pair.

(2) When we check for aromaticity, the molecule must be planar. However, is this 'planar' condition only for the atoms involved in the delocalization of electrons, or for all atoms?

Only the delocalized system of electrons directly involved in the aromatic system need be planar. t-Butyl benzene is aromatic but is not planar everywhere.
 

1. What is resonance in organic chemistry?

Resonance in organic chemistry refers to the delocalization of electrons in a molecule, resulting in the stabilization of its structure. This occurs when there are multiple ways to draw the Lewis structure of a molecule, and the actual structure is a combination of these different forms.

2. How is resonance represented in Lewis structures?

Resonance is represented in Lewis structures using curved arrows to indicate the movement of electrons between different atoms or bonds. The different resonance structures are connected by these arrows, showing the delocalization of electrons in the molecule.

3. What is the significance of resonance in aromatic compounds?

Resonance is a key factor in determining the stability and unique properties of aromatic compounds. The delocalization of electrons in the ring structure of aromatic compounds results in a more stable molecule, making them less reactive and more resistant to chemical changes.

4. How does the concept of aromaticity differ from resonance?

Aromaticity refers to the specific type of resonance found in cyclic, planar molecules with a high degree of stability. It is a more specific and stringent concept than resonance, as not all molecules with resonance are considered aromatic.

5. Can molecules with resonance be polar or nonpolar?

Yes, molecules with resonance can be polar or nonpolar depending on the distribution of the electron density. While resonance can stabilize a molecule, it does not necessarily make it nonpolar. The overall polarity of a molecule is determined by the individual polarities of its bonds and the shape of the molecule.

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