Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Chemistry Class 11

Class 11 : Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

The document Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev is a part of the Class 11 Course Chemistry Class 11.
All you need of Class 11 at this link: Class 11

Homolytic Bond Fission: Homolysis

The bond cleavage in which each bonded atom gets their own contribution.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

  • Cleavage takes place due to:
    HELP (H = Heat, E = Electricity, L = light, P = Peroxide)
  • Favoured when E.N. difference is less or zero.
  • Cleavage favoured in non polar solvent.

Hetrolytic Bond fission

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

  • It is formed when the electronegativity difference between the bonded atoms is more.
  • formation is favoured by polar solvent.

 

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

+ve charge of the solvent attracts the -ve pole of compound and the -ve pole of the solvent attracts +ve pole of compound and the bond breaks.

Intermediates of organic compounds

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Carbocation

Carbocation is the intermediate of carbon containing positive charge. It has six electrons in the valence shell.

Properties of Carbocation :

1. It is positively charged species.

2. It has sixtet of electrons i.e. diamagnetic.

3. It is formed by heterolysis.

4. It is generally formed due to polar solvent.


Geometry and Hybridization
Hybridization of C = sp2
Geometry of C = Trigonal Planar
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Classification of Carbocations
This classification will also be used for carbanions and carbon free radical and will be studied only in this section.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRevStability of Carbocations
There are three factors contributing to the stability of carbocations:
(a) Inductive Effect
(b) Hyperconjugation
(c) Resonance
Order of stability:
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev


Ex.8 Compare the stability of the following carbocation:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

→ more s character

→ more electronegativity

→ +ve charge on more electronegative element is symbol of unstability.

a > b > c


Ex.9 Compare the stability of the following compounds:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (d) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. d > c > b > a

 Being most electron attracting group it decreases the e- density from positively charged C-atom and decreases the charge density and makes the carbocation less stable.


Ex.10 Compare the stability of the following carbocation :

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (d) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. Due to greater size of Iodine, its L.P. will not be available for coordinate bond. Therefore L.P. would not stabilize carbocation.

In case of F due to its small size its lone pair can be easily coordinated to making it most stable.

a > b > c > d (Stability)

* By coordination the carbocation completes its octet and structure having complete octet of its atom is supposed to be most stable.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

(Each atom has its full octet)

* Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Note : In Resonating Structure of at least one C gets sixtet of e-s and hence less stable than coordinated compound.


Ex.11 Compare the stabilities of the following carbocation:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. N, O, F belongs to same period

→ In period Electronegativity of the atom is deciding factor

→ F being most electronegative, holds its e- pair very firmly.

→ Its L.P. will not be easily available for coordination.

→ Stability by it will be minimum.

a > b > c


Ex.12 Compare the following carbocation in order of their stability.

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. If periods of atoms which have to donate their electrons for coordination (for stability) is different then atomic size will be deciding factor. The atom whose size is greater will be unable to make its e- pair available for coordination.

b > a.

Ex.13 Compare the stability of the following compounds:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol → more s-character

→ more e.n.

→ attracts e-

→ reduces stability

b > a > c

Formation of Carbocations
Ionization of Carbon-Leaving Group Bond
In this method
(a) Bond between carbon and leaving group ionizes.
(b) Leaving group accepts the pair of electrons that were shared in the covalent bond.

Rate of formation of carbocation depends on
(a) The stability of carbocation formed.
(b) The nature of the leaving group. Weaker the base better the leaving group. This is because weaker leaving group implies a stable compound and its formation will therefore be favoured.
Addition of Proton to a π bond
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Rate of carbocation formation depends on
(a) Stability of carbocation formed.
(b) Strength of the electrophile.
Reactions of Carbocations
There are three important reactions of carbocations:
(a) Capture a Nucleophile
(b) Lose a proton to form a π bond.
(c) Rearrangement

Capture a Nucleophile

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Loose a proton to form a π bond
Ex. Form the products from the following reaction:
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol.

Step-1 : Protonation
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Step-2 : Formation of Carbocation
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Step-3 : Deprotonation
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
When carbocation deprotonation can lead to more than one product, all products are formed and the most stable product is the major product.

Carbocation Rearrangement

A carbocation can become more stable by rearrangement. Bonding electrons of carbocation may shift between adjacent atoms to form more stable carbocation. There are two kinds of shifts that take place in order to gain stability.
(a) Hydride Shift (b) Alkyl Shift


Carbanion

Carbanion is the intermediate of carbon containing negative charge. It has eight electrons in the valence shell.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Geometry and Hybridization
Hybridization of Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRevsp3

Geometry : Trigonal Pyramidal
Carbanion and ammonia are isoelectronic species having same structure

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Stability

ERG will increase the electron density at carbon and will make it unstable. EWG will decrease the electro density at carbon and will make it stable.

Order of Stability

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Formation of Carbanion
There are two methods for the formation of carbanion:
(A) Partial formation via Grignard Reagent
(B) Formation from Carbonyl Compounds
We will discuss these methods in the subsequent section

Formation via Grignard Reagent
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
(i) Carbanion is never formed as an intermediate that can be isolated in the case of Grignard Reagent. It directly participates in the reaction.
(ii) Dry ether is used in this formation as it is inert to Grignard reagent. For the formation of Grignard reagent from aryl halides, we use tetrahedrofuran (THF) as solvent.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Formation from Carbonyl Compounds

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
There are three reasons for the easy formation of carbanion from carbonyl compounds
(a) Resonance - stabili zation of carbanion which is the conjugate base of carbonyl compound.
(b) Hyperconjugation makes the αC-H bond acidic.
(c) –I of increases the acidic strength of C–H bond.
Reactions of Carbanion
The reactions of carbanion are very fast as electropositive carbon carries negative charge.
Grignard Reagent as a Base
In this reaction it captures acidic hydrogen.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

In (d), the reaction with terminal alkyne also takes place as sp-hydridized carbon is highly electronegative and therefore H attached to it is fairly acidic.

Grignard Reagent as a Nucleophile
Grignard Reagent reacts with carbonyl compounds to form alcohols. This is a very important method for the synthesis of alcohols.

Mechanism:
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
H3Ois never added along with the first step as Grignard Reagent will react with H⊕ to give  as in previous reaction.
Aldol Condensation
This reaction is shown by carbonyl compounds containing atleast one αH in presence of dilute base such as dilute NaOH.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Mechanism:
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev


Ex.14 Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

a > b (stability)

* Stability of the carbanion is as follows

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev


Ex.15 Compare the stability of the following carbocation:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. c > a > b


Ex.16 Compare the stability of the following carbanion:

 

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

→ become more stable

Sol. b > a > c


Ex.17 Compare the stability of the following carbanion:

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. a > b > c


Ex.18 Arrange the following anion in order of their stability:

(a) Cl-, (b) Br- (c) F- (d) I(maximum size)

→ maximum dispersion of -ve charge

→ max stability

Sol. d > b > a > c


Ex.19 Compare the stability of the following:

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev


Ex.20 Compare the acidic strength:

(a) HCl (b) HF (C) HBr (D) HI

Sol. Acidic strength ≈ stability of the anion formed (conjugate base)

As we know, I> Br- > Cl- > F-

→ HI > HBr > HCl > HF


Ex.21 Compare the Acidic strength of the following:

(a) NH3 (b) pH3 (c) AsH3 (d) SbH3 (e) BiH3

Sol. Anion formed from these acids are:

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

→ acidic strength e > d > c > b > a


Ex.22 Compare the acidic strength of the following compounds:

CH4, NH3, H2O, HF

Sol. The conjugate base of the given acid is as follows:

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

we have already proved that

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev > Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev > Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev > Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev (Stability)

→ HF > H2O > NH3 > CH4 (acidic strength)


Ex.24 Compare the stability of the following carbanion.

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
 (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(d) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. d > c > b > a

* M or -M is not distance dependent


Ex.25 compare the stability of the following carbocation

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
 (b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
 (c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
 (d) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. a > b > c > d


Ex.26 Compare the stability of the following carbocation.

 

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(d) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol.  M (OH) > M (OCH3)

b > c > d > a


Ex.27 Compare the stability of the following carbocation

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(c) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Sol. c > a > b


Ex.28 Compare order of dehydration of the following alcohols :

(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev 
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
 (c) C - C - C - OH

Sol. After formation of carbocation

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev, Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
 

Since 3° carbocation is most stable therefore it will show greatest tendency to lose water as after loss of water it comes in stable form.

Carbon Free Radical
Carbon Free Radical is the intermediate of carbon having an odd electron. It is neutral and has seven electrons in the valence shell. It is highly reactive as it requires only one electron to complete its octet and therefore, is short-lived.
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Geometry and Hybridization

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRevThe hybridization of carbon free radical was proposed after experimental verification of structure of different radicals.
It was proposed that when ERG are placed on C, it has sp2 hybridization and when EWG are placed on C, it has sp3 hybridization.

Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Stability
ERG increase stability while EWG decrease stability.
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

Formation of Carbon Free Radical
Carbon Free Radicals are formed by homolytic cleavage of bonds. They are formed:
(a) at high temperature in the gas phase
(b) in non-polar solvents
(c) by ultraviolet light
(d) by addition of other radicals

Reactions of Carbon Free Radical
The most common reactions in which free radical is involved are:
(a) Halogenation of alkanes.
(b) Addition of HBr in the presence of peroxides to alkenes. (Anti-Markonikov Rule)
(c) α-Halogenation of alkenes.
(d) Wurtz Reaction
(e) Decarboxylation reaction

Free Radical Halogenation of Alkanes
The typical reactions of free radical are chain reaction mechanisms. There are three steps in a chain reaction mechanism: initiation, propagation and termination.
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
Mechanism
(I) Chain Initiation
There are two choices:
(a) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
(b) Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
(I) (b) will take place as it is energetically feasible.
(II) Chain Propagation
(a)
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev
(b)
Homolytic and Heterolytic Bond Fission and Electronic Displacement Effect Class 11 Notes | EduRev

(III) Chain Termination
The various free radicals in the progress of reaction are:
Cl•, CH3•. There are ways in which free radicals from different chains may combine to terminate the reaction.
CH3• + CI → CH3 - CI
CH3• + CH3• → CH3 - CH3
CI• + CI• → CI2

There is a side-product CH3 - CHthat is formed in this reaction. Besides these, a number of other side-products such as CH2Cl2, CHCl3 and CCl4 have also been observed. These get formed since carbon free radical is highly reactive. It randomly reacts with other species that make it stabler.




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