Carbocation and Carbanions Class 11 Notes | EduRev

Chemistry Class 11

Class 11 : Carbocation and Carbanions Class 11 Notes | EduRev

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Carbocation

Carbocation and Carbanions Class 11 Notes | EduRev

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.

Structure :

(sp2), Triangular planar

Stability :

Its stability can be determined with the help of Inductive effect, Hyperconjugation and Resonance effect.

Stability of Carbocation:

Carbocation and Carbanions Class 11 Notes | EduRev Carbocation and Carbanions Class 11 Notes | EduRev

Carbocation and Carbanions Class 11 Notes | EduRev

Stability of carbocation can also be determined by Hyperconjugation (no bond Resonance).

Carbocation and Carbanions Class 11 Notes | EduRev

Carbocation and Carbanions Class 11 Notes | EduRev

9 α C - H bond                6αC - H bond             3α C - H bond

Allylic Carbocation:

Carbocation and Carbanions Class 11 Notes | EduRev

Benzylic Carbocation:

Carbocation and Carbanions Class 11 Notes | EduRev

Ex.8 Compare the stability of the following carbocation:

(a) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions Class 11 Notes | EduRev (d) Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions Class 11 Notes | EduRev (d) Carbocation and Carbanions 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.

Carbocation and Carbanions Class 11 Notes | EduRev

(Each atom has its full octet)

* Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions Class 11 Notes | EduRev

Sol → more s-character

→ more e.n.

→ attracts e-

→ reduces stability

b > a > c

Carbanion

1. It is a -ve charged species 2. It has octet of electrons. 3. diamagnetic

Structure :

If -ve charge is in Resonance then the hybridisation of carbanion is sp2(Triangular planar shape).

* If -ve charge is not in Resonance then the hybridisation of carbanion is sp3(pyramidal).

Stability :

Its stability can be determined with the help of

(1) Inductive effect

(2) Resonance effect


Ex.14 Carbocation and Carbanions Class 11 Notes | EduRev

a > b (stability)

* Stability of the carbanion is as follows

Carbocation and Carbanions Class 11 Notes | EduRev


Ex.15 Compare the stability of the following carbocation:

(a) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions Class 11 Notes | EduRev

Sol. c > a > b


Ex.16 Compare the stability of the following carbanion:

 

(a) Carbocation and Carbanions Class 11 Notes | EduRev 
(b) Carbocation and Carbanions Class 11 Notes | EduRev 
(c) Carbocation and Carbanions Class 11 Notes | EduRev

→ become more stable

Sol. b > a > c


Ex.17 Compare the stability of the following carbanion:

(a) Carbocation and Carbanions Class 11 Notes | EduRev (b) Carbocation and Carbanions Class 11 Notes | EduRev (c) Carbocation and Carbanions 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:

Carbocation and Carbanions 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:

Carbocation and Carbanions 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:

Carbocation and Carbanions Class 11 Notes | EduRev

we have already proved that

Carbocation and Carbanions Class 11 Notes | EduRev > Carbocation and Carbanions Class 11 Notes | EduRev > Carbocation and Carbanions Class 11 Notes | EduRev > Carbocation and Carbanions Class 11 Notes | EduRev (Stability)

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


Ex.24 Compare the stability of the following carbanion.

(a) Carbocation and Carbanions Class 11 Notes | EduRev 
(b) Carbocation and Carbanions Class 11 Notes | EduRev
 (c) Carbocation and Carbanions Class 11 Notes | EduRev 
(d) Carbocation and Carbanions 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) Carbocation and Carbanions Class 11 Notes | EduRev 
 (b) Carbocation and Carbanions Class 11 Notes | EduRev 
 (c) Carbocation and Carbanions Class 11 Notes | EduRev 
 (d) Carbocation and Carbanions Class 11 Notes | EduRev

Sol. a > b > c > d


Ex.26 Compare the stability of the following carbocation.

 

(a) Carbocation and Carbanions Class 11 Notes | EduRev 
(b) Carbocation and Carbanions Class 11 Notes | EduRev 
(c) Carbocation and Carbanions Class 11 Notes | EduRev 
(d) Carbocation and Carbanions Class 11 Notes | EduRev

Sol.  M (OH) > M (OCH3)

b > c > d > a


Ex.27 Compare the stability of the following carbocation

(a) Carbocation and Carbanions Class 11 Notes | EduRev 
(b) Carbocation and Carbanions Class 11 Notes | EduRev 
(c) Carbocation and Carbanions Class 11 Notes | EduRev

Sol. c > a > b


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

(a) Carbocation and Carbanions Class 11 Notes | EduRev 
(b) Carbocation and Carbanions Class 11 Notes | EduRev
 (c) C - C - C - OH

Sol. After formation of carbocation

Carbocation and Carbanions Class 11 Notes | EduRev Carbocation and Carbanions Class 11 Notes | EduRev, Carbocation and Carbanions 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.

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