Resonance & Mesomeric Effect
The delocalization of pi-electrons that stabilizes molecules.
Resonance is the phenomenon where a molecule cannot be represented by a single Lewis structure but is a hybrid of two or more structures. This involves the delocalization of $\pi$ (pi) electrons or lone pairs over a conjugated system.
1. Key Concepts
- Canonical Structures: The various imaginary Lewis structures drawn to represent the molecule (Resonating Structures).
- Resonance Hybrid: The real structure which is a weighted average of all canonical forms. It is more stable than any single form.
- Resonance Energy: The difference in energy between the Resonance Hybrid and the most stable canonical structure. Higher resonance energy means greater stability.
2. Conditions for Resonance (Conjugation)
Resonance occurs in systems where p-orbitals are parallel and adjacent, allowing electron overlap. Common conjugated systems include:
$CH_2=CH-CH=CH_2$
$CH_2=CH-\overset{+}{C}H_2$ (Allylic Carbocation)
$CH_2=CH-\ddot{Cl}:$ or $CH_2=CH-\overset{-}{C}H_2$
$CH_2=CH-\dot{C}H_2$
3. Rules for Stability of Resonating Structures
Not all canonical structures contribute equally to the hybrid. The stability order is decided by:
Rule 1: Number of Covalent Bonds
Structures with more covalent bonds are generally more stable.
Rule 2: Octet Rule
Structures where all atoms (especially C, N, O) have complete octets are more stable.
Rule 3: Charge Separation
Neutral structures are more stable than charged ones (charge separation requires energy). If charged, unlike charges should be close, and like charges far apart.
Rule 4: Electronegativity
Negative charge is more stable on an electronegative atom (e.g., O, N). Positive charge is more stable on an electropositive atom (e.g., C).
4. Mesomeric Effect (M or R Effect)
This refers to the permanent polarity produced in a molecule by the interaction of two $\pi$-bonds or a $\pi$-bond and a lone pair. It is essentially resonance involving functional groups.
+R (+M) Effect
Electron Releasing: The group pushes electrons towards the conjugated system/ring via lone pairs.
Identifying Feature:
Atom attached to the ring has at least one Lone Pair.
Activates Benzene Ring (Ortho/Para directing).
-R (-M) Effect
Electron Withdrawing: The group pulls electrons away from the conjugated system/ring.
Identifying Feature:
Atom attached to the ring has a Multiple Bond with a more electronegative atom.
Deactivates Benzene Ring (Meta directing).
Knowledge Check
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Explanation was too simple....thank you sir
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