Inductive and Mesomeric Effects: Mesomeric or Resonance Effect Part 2

Doorsteptutor material for CTET is prepared by world's top subject experts: Get detailed illustrated notes covering entire syllabus: point-by-point for high retention.

Mesomeric or Resonance Effect

  • In a normal bond, the electrons are localized between the constituent atoms. However, if double and single bonds are present alternately in a molecule, it is called conjugation e. g. in 1,3-butadiene, the double bonds are conjugated
  • Similarly, if the double, single and a lone pair are present, alternatively, it is also called conjugation e. g. vinyl chloride.
  • The presence of conjugation alters the properties of the compound and there is a difference in the actual and expected properties.
  • To explain the anomalous properties of such conjugated compounds, Robinson and Ingold gave the concept of mesomerism, which was subsequently modified and developed by Pauling as the theory of resonance which you already know.
Mesomeric or Resonance Effect
  • The theory of resonance states that when a molecule can be represented by two or more classical structural (or electronic) formula, all of which can explain some but not all the properties, then the molecule has neither of these structures (called contributing or canonical or limiting structures) but is a hybrid of all these contributing structures.
  • Note that the electrons are not necessarily present where one would expect them but are rather delocalized over the entire molecule which gives it extra stability expressed in terms of delocalization energy or resonance energy.
  • Also, normally, we find chlorine withdrawing electrons towards itself by -I effect but here we find that the same chlorine has got a positive charge and is involved in a double bond. Does this mean that chlorine has lost its -I effect?
  • No, this is not true as -I effect is a permanent effect. In fact, in addition to the – I effect, it now also has an electron donating Mesomeric or resonance effect (called + M or + R effect) .
Mesomeric or Resonance Effect
  • Since the two effects are operating in opposite directions, one of them will overwhelm the other. Remember there was no such possibility if halogen՚s lone pair was not conjugated. For example, in the following case, the Cl is not conjugated to the double bond and hence in the following molecule, Cl is exerting its only – I effect.
  • The atoms/groups like Cl in which lone pair (or electrons of negative charge) is in conjugation with double or triple bond are electron donating and gain a formal positive charge in the resonating structure in the process and are known to exert + M/ + R Effect.
  • Consider the case of – NO2 joined to a conjugated system, where the nitro withdraws the conjugated electrons and gives rise to polarization as shown below
Mesomeric or Resonance Effect

The atoms/groups like – NO2 which are in conjugation with double or triple bond and are electron withdrawing and gain a formal negative charge in the resonating structure in the process and are known to exert -M/-R Effect.

  • The resonance effect also alters the electron density distribution in the molecule significantly and its direction may be different from the normal inductive effect. In case the two effects are operating in opposite directions, the relative strengths of the two effects will determine which will dominate.
  • Mesomeric/resonance effect introduces total delocalization of charges while inductive effect introduces partial polarization, hence, in general .
  • But there are exceptions to it like when halogens are attached to a conjugated system like benzene, (negative inductive effect is greater than resonance effect) .

MCQs

1. Identify the correct sequence according to electronegativity

a)

b)

c)

d)

e) In the sequence is the most stable one and CH3′ is the least stable one, as the stability of the anions increases on moving towards the right of the periodic table

Answer: D

2. Which of the following is a polar compound?

a)

b)

c)

d)

Answer: B

#mesomericorresonanceeffect

Developed by: