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Electronic Displacement Effects | Chemistry Class 11 - NEET

  • The displacement of electrons within the same molecule is known as electronic displacement. These effects affect the stability of a species or compound and it also affects the acidic & basic strength.

Electronic Displacement Effect is divided into two parts:
(i) Permanent effect

  • Inductive effect
  • Mesomeric (resonance) effect
  • Hyperconjugation

(ii) Temporary effect

  • Electromeric effect
  • Inductomeric effect

Inductive effect

  • It is an effect in which permanent polarization arises due to partial displacement of s-electrons along the carbon chain or partial displacement of sigma-bonded electrons toward more electronegative atoms in the carbon chain.
    Electronic Displacement Effects | Chemistry Class 11 - NEET

Electronic Displacement Effects | Chemistry Class 11 - NEET


  • Magnitude of partial positive charge: δ1 > δ2 > δ3 = δ- (net charge remains constant in a molecule having inductive effect)
  • Electronic Displacement Effects | Chemistry Class 11 - NEET⇒ -I effect of X, if X is more electronegative.
  • Electronic Displacement Effects | Chemistry Class 11 - NEET ⇒ +I effect of Y, if Y is less electronegative.
  •  Electronic Displacement Effects | Chemistry Class 11 - NEET (- I effect order)
  • O- < O < O  (-I effect order)
Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note: 
Order of -I effect: –OH< -OR
Because C is more electronegative than H. Hence attracts e- from O slightly.

Thus, Its tendency to attract electron increases.

➢ Characteristic of Inductive Effect

  • It is a permanent effect.
  • It is caused due to electronegativity difference.
  • It operates via an s-bonded electron.
  • It is a distance-dependent effect.
  • As distance increases, its effect decreases.
  • It can be neglected after the third carbon.
  • It has a destabilizing effect.

➢ Types of Inductive Effect

  • It is divided into 2 parts on the basis of electronegativity w.r.t. hydrogen atom:
    (i) + I effect
    (ii) - I effect

Electronic Displacement Effects | Chemistry Class 11 - NEET


  • If any atom or group having electronegativity greater than that of hydrogen, then it is considered as - I effect and vice-versa.Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Example:
    (i) CH3 – CH2 – Cl i.e. - I effect of Cl
    (ii) CH3 – CH2 = CH2 i.e. - I effect of – CH2 and + I effect of – CH3
    (iii) CH3 – CH2 – C ≡ CH i.e. - I effect of – C ≡ CH and + I effect of – CH2 – CH3
    (iv) I – Cl i.e. + I of – I
  • Order of -I effect showing group:Electronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEET
  • Order of+ I effect showing group:Electronic Displacement Effects | Chemistry Class 11 - NEET

Electronic Displacement Effects | Chemistry Class 11 - NEET

  • Bond Strength: CT3 > CD3 > CH3 
    Thus, Order of +I effect: T > D > H

Q. Why carbon - hydrogen bond is longer than C - T bond?

Ans. As the mass increases, vibration decreases as a result of which the heavier isotope will be more closer to the C-atom for a longer time. Therefore C - T bond is stronger. 

C - T > C - D > C - H which implies that C - H bond has longest bond length.

➢ Application of Inductive Effect

  • To compare the stability of intermediates.
    Intermediates: These are real separable species having measurable stability formed during the conversion of reactant to product. After bond cleavage and before bond formation.They are formed by homolytical and heterolytical cleavage.
    6 types of intermediates:
    (i) Free radical
    (ii) Carbocation
    (iii) Carbanion
    (iv) Carbene
    (v) Nitrene
    (vi) Benzyne

Electromeric Effect

  • It involves the complete transfer of electrons of a multiple bond to one of the bonded atom in presence of an electron attacking reagent. It is called the E effect.Electronic Displacement Effects | Chemistry Class 11 - NEET
  • This effect is temporary and takes place only in the presence of a reagent. As soon as the reagent is removed, the molecule reverts back to its original position.

➢ Types of Electromeric Effect

  • +E effect: If the electrons of the π-bond are transferred to that atom of the double bond to which the reagent gets finally attached, the effect is called the +E effect.
    Example: Addition of acids to alkenes.
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • -E Effect: If the electrons of the double bond are transferred to an atom of the double bonds other than the one to which the reagent gets finally attached the effect is called -E Effect.
    Example: Addition of Cyanide ion to the carbonyl group.

Table: Differences between Inductive and Electromeric Effect

Electronic Displacement Effects | Chemistry Class 11 - NEET

Resonating Structure

➢ Conjugated System 

  • Compounds having continuous unhybridized p-orbital parallel to each other such systems are known as conjugated system.
  • Types of the conjugated system:
    (i) p-bond alternate to p-bond
    Example: CH2 = CH - CH = CH
    Explantion:
    Electronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEET
    Electronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEETElectronic Displacement Effects | Chemistry Class 11 - NEET
    (ii) p-bond alternate to a positive charge
    Example: CH2 = CH - CH2
    Explanation:Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Examples:
    (i)Electronic Displacement Effects | Chemistry Class 11 - NEET
    (ii)Electronic Displacement Effects | Chemistry Class 11 - NEET
    (iii)Electronic Displacement Effects | Chemistry Class 11 - NEET
    (iv) CH2 = CH - BH2 Electronic Displacement Effects | Chemistry Class 11 - NEET Electronic Displacement Effects | Chemistry Class 11 - NEET Electronic Displacement Effects | Chemistry Class 11 - NEET
    (v)Electronic Displacement Effects | Chemistry Class 11 - NEET

➢ Resonance

  • Delocalisation of p-electrons in conjugation is known as resonance.
  • When one structure is not sufficient to explain each and every property (chemical & physical) then, a different structure has been drawn which is known as Resonating Structure (canonical structure).

  • All these structures contribute to the formation of a Real structure, known as Resonating Hybrid.

  • Electronic Displacement Effects | Chemistry Class 11 - NEET (Actual Structure)
    (resonating structures)        (Resonance hybrid)
    Electronic Displacement Effects | Chemistry Class 11 - NEET in this form Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Condition for showing resonance:
    (i) Molecule should be planar, nearly planar or a part of it is planar
    (ii) Molecule should possess conjugated system.

Q. Which are planar?

 (A) Electronic Displacement Effects | Chemistry Class 11 - NEET (B) Electronic Displacement Effects | Chemistry Class 11 - NEET (C) Electronic Displacement Effects | Chemistry Class 11 - NEET (D) Electronic Displacement Effects | Chemistry Class 11 - NEET
Sol. (C) and (D), Because all carbon atoms are sp2 hybridised.
Not (B) because it can be converted into Electronic Displacement Effects | Chemistry Class 11 - NEET

  • Resonance takes place due to the delocalization of p e-s.
    Example:
    (a) Electronic Displacement Effects | Chemistry Class 11 - NEET ⇒ Resonance
    (b) Electronic Displacement Effects | Chemistry Class 11 - NEET ⇒ Resonance absent
    (c)Electronic Displacement Effects | Chemistry Class 11 - NEET ⇒ Resonance
    (d)Electronic Displacement Effects | Chemistry Class 11 - NEET ⇒ Resonance
  • The position of the atoms remains the same, only delocalization of p e-s takes place.
  • Bond pair gets converted into lone pair and l.p. gets converted into b.p.Electronic Displacement Effects | Chemistry Class 11 - NEET
  • In Resonance, No. of unpaired e-s remains the same.
    CH2 = CH - CH = CH2 Electronic Displacement Effects | Chemistry Class 11 - NEET Electronic Displacement Effects | Chemistry Class 11 - NEET(They are not resonating structure)
Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note:
Electronic Displacement Effects | Chemistry Class 11 - NEET 
They are not resonating structures rather they are tautomers.

➢ Resonating Structure

  • Hypothetical structure existing on paper.
  • The energy difference b/w different resonating structure is very small.
  • All R.S. contribute towards the formation of resonance hybrid (Their contribution may be different).
  • A single R.S. can't explain each & every property of that particular compound.

Q. Draw the resonating structures:

Sol. (i)Electronic Displacement Effects | Chemistry Class 11 - NEET
(ii)Electronic Displacement Effects | Chemistry Class 11 - NEET 

  • Resonance hybrid: It is a real structure that explains all the properties of a compound formed by the contribution of different R.S.
    It has got maximum stability as compared to any R.S.
  • Resonance Energy: It is the difference b/w theoretical value of H.O.H & experimental value. Or it can be defined as the difference b/w more stable R.S. & R. H.
    The more resonance energy, the more stable will be the molecule.
    Resonance energy is an absolute term.
Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note:

Cyclohexane is thermodynamically more stable than benzene, even though the resonance energy of benzene is more.

Contribution of different R. S. towards resonance hybrid:

  • Non-polar R.S. contributes more than polar R.S.
    Example: 
    (a) CH2 = CH - CH = CH2 (b)  CH2-CH = CH - CH2 (c) CH2 - CH = CH - CH2 
    Order of stability of given R.S. ⇒ a > b = c
  • Polar R. S. with complete octet will contribute more as compared with the one with an incomplete octet
    CH3 - CH  - OCH(Incomplete octet)Electronic Displacement Effects | Chemistry Class 11 - NEET CH3 - CH = :O - CH(Complete octet)
  • In polar R. S. the -ve charge should be on more electro - ve atom & +ve charge should be on more electro +ve atom.
    Example: 
    (a) Electronic Displacement Effects | Chemistry Class 11 - NEET (more stable)
    (b) Electronic Displacement Effects | Chemistry Class 11 - NEET (less stable)
  • Compound with more covalent bonds will contribute more.
  • Unlike charges should be closer to each other whereas like charges should be isolated.
  • Extended conjugation contributes more than cross conjugation.
    Example:Electronic Displacement Effects | Chemistry Class 11 - NEET < Electronic Displacement Effects | Chemistry Class 11 - NEET(Cross conjugation < Extended conjugation)

➢ Fries Rule:

  • Compounds with more benzenoid structures are more stable. 
  • As the Resonance energy is greater than those in which lesser no. of benzenoid structures are present.
  • R. E. is Electronic Displacement Effects | Chemistry Class 11 - NEET < Electronic Displacement Effects | Chemistry Class 11 - NEET
  • If a double bond is participating in resonance then it will acquire a partial single bond character as a result of which bond length increases & bond strength decreases.
  • If a single bond is involved in resonance then it will acquire partial double bond character. As a result of which bond length decreases & bond strength increase.

Q. Find the order of Stability in the following:

(i) Electronic Displacement Effects | Chemistry Class 11 - NEET

(ii) (a) Electronic Displacement Effects | Chemistry Class 11 - NEET (b) Electronic Displacement Effects | Chemistry Class 11 - NEET(c) Electronic Displacement Effects | Chemistry Class 11 - NEET(d) Electronic Displacement Effects | Chemistry Class 11 - NEET

(iii)Electronic Displacement Effects | Chemistry Class 11 - NEET
(iv) (a)Electronic Displacement Effects | Chemistry Class 11 - NEET (b) CH2 - CH = F                         

Sol. (i) a = e > b = d > c
(ii)  a > b > c > d, c and are in complete
(iii) +CCl3 < +CF3, due to back bonding in +CF3

(iv) a > b (stability)

Electronic Displacement Effects | Chemistry Class 11 - NEET

Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note: 

When lone pair, as well as a double bond, is present in some atom then only p bond will be participating in resonance. Whereas lone pair remains sp2 hybridized orbital. When an atom has two or more than two lone pairs then only one lone pair will participate in resonance and the other one remains in sp2 hybridized orbital.

Mesomeric Effect (Resonance effect)

  • The mesomeric effect is valid only for the conjugated system.

➢ Types of Mesomeric Effect

  • It is divided into 2 parts: 
    (i) +M effect (-R)
    (ii) - M Effect (-R)
  • Consider the following conjugated system:
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Consider another conjugated system:
    Electronic Displacement Effects | Chemistry Class 11 - NEET

➢ Mesomeric Effect in the Compounds Having a Benzene Ring

  • If the movement of electrons is towards the ring, it will show +M effect. This effect increases the electron density over the benzene ring.
  • +M effect in phenol
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • +M effect in aniline
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • -M effect in Benzaldehyde
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • +M group increases the electron density of ring while - M decreases the electron density of benzene ring.
  • If NO2 is present on the ortho or para position then along with its -I effect, It will also show -M effect.
    Electronic Displacement Effects | Chemistry Class 11 - NEET
    Above compound have M of -OH and -M of NO2 group.
  • Electronic Displacement Effects | Chemistry Class 11 - NEET
    As we can easily see that -NO2 at meta position is not attracting e- density towards itself and that's why it will not show -M effect at m-position.

Q. Identify the compound showing +M or -M separately?

(a) Electronic Displacement Effects | Chemistry Class 11 - NEET(b) Electronic Displacement Effects | Chemistry Class 11 - NEET (c) Electronic Displacement Effects | Chemistry Class 11 - NEET

Sol. (a) -M (b) -M (c) +M

Hyperconjugation

  • Permanent polarisation caused by the displacement of s-electrons into p-molecular orbital is known as hyperconjugation.
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Hyper conjugation is called No-bond Resonance.
  • More the C - H bond, more will be the no bond resonating structure (Hyperconjugation).
    More the (C - H) bond, the more will be the stability of free radicals.Electronic Displacement Effects | Chemistry Class 11 - NEET

➢ Properties of Free Radical:

  • It is a neutral species.
  • It has one unpaired electron, therefore, why paramagnetic in nature.
  • Structure
    Electronic Displacement Effects | Chemistry Class 11 - NEET → methyl free Radical
    Electronic Displacement Effects | Chemistry Class 11 - NEET → ethyl free radical
  • Its hybridization is sp2 and triangular planar shape.
Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note: 
Unpaired electron is not counted while calculating the hybridization state. They stay perpendicular to the plane.

Electronic Displacement Effects | Chemistry Class 11 - NEET

➢ Stability of Free Radical

Its stability can be determined with the help of hyperconjugation as well as the Resonance effect.

  • Allylic Free Radical:
    Electronic Displacement Effects | Chemistry Class 11 - NEET (Free Radical is on next carbon to doubly bonded carbon atoms)
    Effect of Resonance > Hyperconjugation
    Electronic Displacement Effects | Chemistry Class 11 - NEET
  • Benzylic Free Radical:
    Electronic Displacement Effects | Chemistry Class 11 - NEET
    More Resonating structure, more will be the stability of the free Radical.
    Example:
    (i) Di-Benzylic free Radical:Electronic Displacement Effects | Chemistry Class 11 - NEET
    No. of Resonating structures = 7
    (ii) Tri-benzylic free Radical:Electronic Displacement Effects | Chemistry Class 11 - NEET 
    No. of Resonating structures = 10
  • Stability Order:
    Electronic Displacement Effects | Chemistry Class 11 - NEET

Solved Examples

Q.1 Compare the stability of the following free Radicals

(a)Electronic Displacement Effects | Chemistry Class 11 - NEET (b)Electronic Displacement Effects | Chemistry Class 11 - NEET (c) Electronic Displacement Effects | Chemistry Class 11 - NEET

Sol. b > a > c, Since the resonating structure is not possible for (c)

 
Q.2. Compare the Electronic Displacement Effects | Chemistry Class 11 - NEET bond energy of the following compounds.
(a) Electronic Displacement Effects | Chemistry Class 11 - NEET (b) Electronic Displacement Effects | Chemistry Class 11 - NEET (c) Electronic Displacement Effects | Chemistry Class 11 - NEET (d) Electronic Displacement Effects | Chemistry Class 11 - NEET

Sol. After forming free radical from the compound:
Due to 3° free radical, (a) will be most stable, therefore will have more tendency to come in this form, and C–H bond will break very readily because bond energy will be very less.

Thus, Bond Energy Order: a<b<c<d

Electronic Displacement Effects | Chemistry Class 11 - NEET  View Answer

Note:

Electronic Displacement Effects | Chemistry Class 11 - NEET

Electronic Displacement Effects | Chemistry Class 11 - NEET

Q.3 Compare the potential energy of the following compounds.
(a) Electronic Displacement Effects | Chemistry Class 11 - NEET (b) Electronic Displacement Effects | Chemistry Class 11 - NEET (c) Electronic Displacement Effects | Chemistry Class 11 - NEET (d) Electronic Displacement Effects | Chemistry Class 11 - NEET

Sol. If the compound after being in free Radical form is very stable (i.e., less energy) it means it would have possessed more energy initially i.e. its potential energy will be the most. 
Potential energy ≈ stability of free Radical

Thus, Potential Energy Order: a < b < c < d


Q.4 (i) Compare the bond energies of the C - H bond at a, b, c, d, e, and f position in the compound below.

Electronic Displacement Effects | Chemistry Class 11 - NEET

(ii) In the above compound compare 2° benzylic allylic stability at two given positions:

Electronic Displacement Effects | Chemistry Class 11 - NEET

Sol. (i) Stability order of free Radical that might be formed after removal of H (Homolytically) from the given carbon: e > b > a > f > c = d

Thus, C - H bond energy order: e < b < a < f < c = d

(ii) Inspite of Resonance, three (C - H) bond are available for no bond Resonance.

Electronic Displacement Effects | Chemistry Class 11 - NEET

Therefore, 2° benzylic allylic corresponding to structure (a) will be extra stable than (b) which have only two (C - H) bond for Hyperconjugation.

Q.5 Compare the stability of the following free Radicals.

Electronic Displacement Effects | Chemistry Class 11 - NEET Electronic Displacement Effects | Chemistry Class 11 - NEET Electronic Displacement Effects | Chemistry Class 11 - NEET  Electronic Displacement Effects | Chemistry Class 11 - NEET

 

Sol.  Electronic Displacement Effects | Chemistry Class 11 - NEET
Electronic Displacement Effects | Chemistry Class 11 - NEET

Electronic Displacement Effects | Chemistry Class 11 - NEET 
Thus, Stability of the following free Radical: c > b > a > d


Q.6 Compare the potential energy of the following compounds.
(a) CH3 - CH(b) CH2 = CH(c) CH≡CH

Sol. After making free Radical of the above compounds:
(a)Electronic Displacement Effects | Chemistry Class 11 - NEET,
(b)Electronic Displacement Effects | Chemistry Class 11 - NEET
(c)Electronic Displacement Effects | Chemistry Class 11 - NEET

Therefore,Electronic Displacement Effects | Chemistry Class 11 - NEET

Thus, Potential Energy Order: a > b > c

The document Electronic Displacement Effects | Chemistry Class 11 - NEET is a part of the NEET Course Chemistry Class 11.
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FAQs on Electronic Displacement Effects - Chemistry Class 11 - NEET

1. What is the inductive effect?
Ans. The inductive effect is a phenomenon in chemistry where the distribution of electrons in a molecule or a functional group is modified by the presence of nearby atoms or groups. It occurs due to the electronegativity difference between atoms, resulting in the polarization of electron density towards the more electronegative atom.
2. What is the difference between the inductive effect and the electromeric effect?
Ans. The inductive effect is a permanent effect observed in molecules due to the electronegativity difference between atoms, leading to the polarization of electron density. On the other hand, the electromeric effect is a temporary effect seen in molecules due to the movement of a shared pair of electrons towards one atom, resulting in the formation of a new bond.
3. How does hyperconjugation affect the stability of molecules?
Ans. Hyperconjugation is a phenomenon in which the overlap of a sigma bond with an adjacent empty or partially filled p-orbital results in the delocalization of electrons. This delocalization increases the stability of molecules by dispersing the electron density, thus reducing the electron repulsions.
4. What are resonating structures?
Ans. Resonating structures, also known as resonance structures, are different representations of a molecule or an ion that can be obtained by moving electrons within the molecule. These structures are used to describe the delocalization of electrons and the stability of the molecule.
5. How does the mesomeric effect (resonance effect) influence the properties of organic compounds?
Ans. The mesomeric effect, also known as the resonance effect, is a phenomenon observed in organic compounds where the electron density is delocalized over a conjugated system of atoms. This effect can influence the chemical reactivity, stability, and electronic properties of the compounds, leading to variations in their physical and chemical behavior.
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