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Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

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Resonance

Delocalisation of p-electrons in conjugation is known as resonance.

 

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (Actual Structure)

(resonating structures)        (Resonance hybrid)

 

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE in this form Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Condition for showing resonance

1. Molecule should be planar, nearly planar or a part of it is planar

Q.1 Which are planar?

 

(A) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (B) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE *(C) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE *(D) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Because all carbon atoms are sp2 hybridised.

2. Molecule should possess conjugated system.

Conjugated system :-

Continuous unhybridised p-orbital parallel to each-other.

Types of conjugated system:-

(1) p-bond alternate to p-bond

CH2 = CH - CH = CH2

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(2) p-bond alternate to charge

CH2 = CH - CH2 

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Eg. Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEEResonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEEResonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEEResonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEEResonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Eg. Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE
 

 

Eg. Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(4) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

(5) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

(6) CH2 = CH - BH2 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

(7) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

1. Resonance takes place due to delocalization of p e-s.

(a) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonance
(b) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonance absent

(c) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonance
(d) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonance

2. Position of the atoms remains the same, only delocalization of p e-s takes place.

 

Note:-Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE [They are not resonating structure rather they are tautomers].

 

3. Bond pair gets converted into lone pair and l.p. gets converted into b.p.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

4. In Resonance No. of unpaired e-s remains the same.

CH2 = CH - CH = CH2 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(They are not resonating structure)

Resonating structure :

(1) Hypothetical strtucture exist on paper.

(2) The energy difference b/w different resonating structure is very small.

(3) All R. S. contribute towards the formation of resonance hybrid (Their contribution may be different).

(4) A single R. S. can't explain each & every property of that particular compound.

Draw the resonating structures : -

Q.1 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Q.2 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE -m of NO2 group

Resonance hybrid : -

It is a real structure which explains all the properties of a compound formed by the contribution of different R. S.. It has got maximum stability as compared to R. S. 

Resonance Energy : -

It is the difference b/w theoretical value of H.O.H & experimental value.

Or

It is the difference b/w more stable R.S. & R. H.

* More the resonance energy, more stable will be the molecule.

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

* Resonance energy is a absolute term.

Contribution of different R. S. towards resonance hybrid

(1) Non-polar R. S. contribute more than polar R. S.

(a) CH2 = CH - CH = CH2 (b)  CH2-CH = CH - CH2 (c) CH2 - CH = CH - CH2 

a > b = c stability

(2) Polar R. S. with complete octet will contribute more as compared with the one with incomplete octet

CH3 - CH  - OCH3 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE CH3 - CH = :O  - CH3

Incomplete octet                     Complete octet

(3) In polar R. S. the -ve charge should be on more electro - ve atom & +ve charge should be on more electro +ve atom.

(a) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (more stable )

(b) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (less stable )

 

(4) Compound with more covalent bonds will contribute more.

(5) Unlike charges should be closer to each other whereas like charges should be isolated.

(6) Extended conjugation contribute more than cross conjugation.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE < Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Cross conjugation < Extended conjugation

Fries Rule :-

Compound with more benzenoid structure are more stable as their Resonance energy is greater than those in which lesser no. of benzenoid structures are present.

R. E. is Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE < Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

* If double bond is participating in resonance then it will acquire partial single bond character as a result of which bond length increase & 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.1 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

a = e > b = d > c

 

Q.2 
(a) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE 
(b) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(c) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE(incomplete)
(d) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE(incomplete)

a > b > c > d

Q.3 

           Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE
Stability Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Q.4 (a)Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (b) CH2 - CH = F                          a > b (stability)

* Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Note:-When lone pair as well as double bond is present in some atom then only p bond will be participating in resonance. Whereas lone pair remains sp2 hybridised orbital. When an atom has two or more then two lone pair then only one lone pair will participate in resonance and the other one remains in sp2 hybridised orbital.


Hyper conjugation

Permanent polarisation caused by displacement of s-electrons into p-molecular orbital is known as hyperconjugation.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Hyper conjugation is called No bond Resonance.

* More the C - H bond, more will be the no bond resonating structure (Hyper conjugation)

More the (C - H) bond, more will be the stability of free radical.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Properties of Free Radical:

1. It is a neutral species.

2. It has one un-paired electron that's why paramagnetic in nature.

Structure :

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE → methyl free Radical

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE → ethyl free radical

3. Its hydridisation is sp2 and triangular planar shape.

Note : unpaired electron is not counted while calculating the hybridisation state.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (unpaired electron stay perpendicular to the plane)

 

Stability of free Radical :

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

Allylic Free Radical

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(Free Radical is on next carbon to doubly bonded carbon atoms)

Effect of Resonance > Hyper conjugation

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Benzylic Free Radical

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

* More Resonating structure, more will be the stability of the free Radical.

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (di-benzylic free Radical)

No. of Resonating structures = 7

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (Tri-benzylic free Radical)

No. of Resonating structures = 10

Stability Order :

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE


Ex.1 Compare the stability of the following free Radicals

(a)

 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE 

(b) 

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE 

(c)

 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(Therefore this resonating structure is not possible)

Sol. b > a > c

 

Ex.2 (a) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (b) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (c) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE (d) Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Compare the Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE bond energy of the above compounds.

Sol. After forming free radical from the compound

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE             Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE        Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE                 Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

            (3°)                             (2°)                          (1°)                     methyl free radical

            (a)                              (b)                            (c)                             (d)

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

* Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

* Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Ex.3 Compare the potential energy of the following compounds (above compounds)

Sol. If 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.

a < b < c < d

* Potential energy ≈ stability of free Radical

Ex.4 Compare the bond energies of C - H bond

(at a, b, c, d, e and f position)

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

e > b > a > f > c = d

Stability order of free Radical that might be formed after removal of H (Homolytically) from the given carbon.

→ e < b < a < f < c = d

(C - H bond energies)

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

 

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

while drawing the resonating structure of the above,

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

(Here inspite of Resonance three (C - H) bond are available for no bond Resonance.

→ Therefore extra stable than Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE which have only two (C - H) bond for Hyper conjugation.

 

Therefore 2° benzylic allylic corresponding to structure (a) is more stable than that of structure (b)

Ex.5 Compare the stability of the following free Radical

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE  Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

 

Sol.  

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE
Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE  ⇒  c > b > a > d

Ex.6 Compare the potential energy of CH3 - CH3, CH2 = CH2,  CH≡CH

Sol. After making free Radical of the above compounds:

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE, Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

Resonating Structures & Hyperconjugation Notes | Study Additional Documents and Tests for JEE - JEE

a > b > c

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