Resonance or Mesomeric effect

Summary

This transcript explains the concept of resonance structures and their importance in molecular stability. It covers the ozone molecule as an example of resonance hybrids, describing how electron delocalization stabilizes the molecule. Key principles for identifying stable resonance structures include fulfilling the octet rule, maximizing covalent bonds, and positioning formal charges on electronegative atoms. The video also explores the effects of substituents, with examples like phenol and aniline, showing how the resonance and inductive effects influence reactivity. Overall, the transcript highlights the role of resonance in enhancing molecular stability and reactivity in conjugated systems.

Takeaways

• 😀 Ozone (O₃) has 18 valence electrons, and its resonance structures depict electron delocalization between single and double bonds.
• 😀 Resonance structures are contributing forms that represent electron delocalization, which leads to a more stable molecule known as the resonance hybrid.
• 😀 The bond length in Ozone is 128 pm, which is intermediate between a single (148 pm) and a double bond (121 pm), showing electron delocalization.
• 😀 The real structure of Ozone is an intermediate of its resonance forms, which cannot be represented by a single Lewis structure.
• 😀 Formal charge distribution in molecules like Ozone affects their resonance stability, with charges being delocalized over the molecule.
• 😀 A resonance hybrid represents the most stable configuration of a molecule, with maximum delocalization of electrons and minimized energy.
• 😀 In molecules like Nitroaniline, electron delocalization through resonance increases stability and decreases basicity.
• 😀 The more resonance structures there are, the greater the electron delocalization, leading to a more stable molecule.
• 😀 Major resonance contributors are the most stable structures, typically having a filled octet, more covalent bonds, and charge on more electronegative atoms.
• 😀 Resonance energy is the difference in energy between the most stable canonical form and the resonance hybrid, indicating the molecule's stability.
• 😀 Substituents in conjugated systems can affect electron density via +M (electron-donating) and -M (electron-withdrawing) effects, influencing reactivity in electrophilic substitution reactions.

Q & A

What is the resonance structure of ozone, and how is it formed?

The resonance structure of ozone is formed by considering two contributing structures where the double bond alternates between the two oxygen atoms. The actual structure is a resonance hybrid, meaning it is an intermediate between these two structures. This delocalization of electrons stabilizes the molecule, and the bond lengths in ozone are found to be intermediate between a double bond and a single bond.

Why is the real structure of ozone considered a resonance hybrid?

The real structure of ozone is considered a resonance hybrid because the actual bonding is not fixed between two oxygen atoms. Instead, the bonding is a blend of two contributing structures, with electrons delocalized across the molecule. This results in equal bond lengths that are intermediate between those of a single bond and a double bond.

How does the delocalization of electrons affect the stability of molecules?

The delocalization of electrons, as seen in resonance structures, leads to greater stability. The more delocalized the pi electrons are over the entire molecule, the more stable the molecule becomes, as the charge is dispersed and the overall energy is minimized.

What makes one resonance structure a major contributor and another a minor contributor?

A major resonance contributor is one where the atoms fulfill the octet rule, the formal charges are minimized, and the structure closely resembles the actual resonance hybrid. Minor contributors, on the other hand, have less stability, often due to incomplete octets or higher formal charges.

Why is the second resonance structure of carbon monoxide considered a major contributor?

The second resonance structure of carbon monoxide is a major contributor because both carbon and oxygen fulfill the octet rule, with both atoms having a complete set of valence electrons. The first structure, where carbon does not fulfill the octet, is less stable.

What factors make a resonance structure more stable?

A resonance structure is more stable if it follows the octet rule, minimizes formal charges, has a greater number of covalent bonds, places negative formal charges on more electronegative atoms, and avoids charge separation. These features lower the overall energy of the molecule.

How does the electronegativity of an atom influence the stability of a resonance structure?

The stability of a resonance structure is enhanced when a negative formal charge is placed on the more electronegative atom. This is because electronegative atoms, like oxygen, are more capable of stabilizing negative charges due to their higher affinity for electrons.

What is resonance energy, and how is it related to the resonance hybrid?

Resonance energy is the energy difference between the most stable resonance structure (canonical form) and the resonance hybrid. The lower the energy difference, the more stable the molecule. It represents how much stabilization occurs due to the delocalization of electrons across different resonance structures.

What is the difference between +M (or +R) and -M (or -R) effects?

+M (or +R) effects refer to substituents that increase electron density in the conjugated system, stabilizing the molecule. Examples include -OH, -NH2, and -O groups. In contrast, -M (or -R) effects refer to substituents that withdraw electron density from the system, like -NO2, carbonyl groups, and -CN, making the molecule less stable.

Why are phenol and aniline more reactive than benzene towards electrophilic substitution?

Phenol and aniline are more reactive towards electrophilic substitution than benzene because the +R effect from the -OH and -NH2 groups increases the electron density on the benzene ring, making it more susceptible to attack by electrophiles.