Acidity and Basicity Class 11 Notes | EduRev

Chemistry Class 11

JEE : Acidity and Basicity Class 11 Notes | EduRev

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Acidity & Basicity

Acidity and Basicity Class 11 Notes | EduRev

Note : More stable the conjugate base (i.e., ), more will be the forward reaction which results in more acidic nature of HA.

Ex.1 Compare the acidic strength of the following acids.

(a) C - C - C - COOH (b) C = C - C - COOH (c) C≡C-C-COOH

Sol. The acid whose conjugate base is most stable will be more acidic.

After forming conjugate base from the above acids.

(a) Acidity and Basicity Class 11 Notes | EduRev 

(b) Acidity and Basicity Class 11 Notes | EduRev 

(c) Acidity and Basicity Class 11 Notes | EduRev

It is clear that sp hybridised carbon being most electronegative will decrease e-density from O most effectively making the conjugate base most stable.

c > b > a (acidic strength)


Ex.2 Which is more acidic between the two:

(a) CHF3 (b) CHCl3

Sol. CHF3 > CHCl3

If we consider the -I effect of F and Cl But this effect will not be considered here

After the removal of proton

(a) Acidity and Basicity Class 11 Notes | EduRev (b) Acidity and Basicity Class 11 Notes | EduRev

(vacant d-orbital available where C will coordinate its electron) (pπ - dπ bonding)

→ a < b (acidic strength)


Ex.3 Compare the acidic strength of the following:

(a) CHF3 (b) CHCl3

(c) CHBr3 

(pπ - dπ bonding in Br is not as much as effective as in Cl due to large size of Br)

Sol. CHCl3 > CHBr3 > CHF3

 

Ex.4 Compare the acidic strength of the following

(a) CH (CN)3 (b) CH (NO2)3 (c) CHCl3

Sol. After removing H 

Acidity and Basicity Class 11 Notes | EduRev (Resonance) In its resonating structure, -ve charge will be on N)

Acidity and Basicity Class 11 Notes | EduRev (Resonance) (- In its resonating structure -ve charge will reside on O)
 

→ more effective Resonance

Acidity and Basicity Class 11 Notes | EduRev (pπ - dπ)

b > a > c

* -ve charge on O is more

stable than -ve charge on N as O is more electronegative than N.

* pπ - dπ Resonance < Actual Resonance


Ex.5 Compare the acidic strength of the following:

(a) CH≡CH (b) CH2 = CH2 (c) CH3 - CH3

Sol. Acidity and Basicity Class 11 Notes | EduRev

(Stability of the conjugate base)

→ a > b > c (acidic strength)


Ex.6 Compare the acidic strength of the following :

Acidity and Basicity Class 11 Notes | EduRev

Sol. d > c > b > a

 

Ex.7 Compare the acidic strength of the following :

(a) H2O (b) H2S (c) H2Se (d) H2Te

Sol. Conjugate base has the stability order:

Acidity and Basicity Class 11 Notes | EduRev

→ H2O < H2S < H2Se < H2Te (acidic strength)


Ex.8 Compare the acidic strength of the following compound:

 

(a) Acidity and Basicity Class 11 Notes | EduRev (b) Acidity and Basicity Class 11 Notes | EduRev (c) Acidity and Basicity Class 11 Notes | EduRev (d) Acidity and Basicity Class 11 Notes | EduRev
 

Sol. After forming conjugate base of the above:

 

Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev

c > d > b > a

Ex.9 Compare the reactivity of the following compounds with 1 mole of AgNO3

(a) Acidity and Basicity Class 11 Notes | EduRev 
(b) Acidity and Basicity Class 11 Notes | EduRev 
(c) Acidity and Basicity Class 11 Notes | EduRev
 (d) Acidity and Basicity Class 11 Notes | EduRev

Sol. After removing Cl-

Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev

( ve charge is not on resonance least stable)

Acidity and Basicity Class 11 Notes | EduRev

(most stable as L.P. of Cl will be coordinated to ve charge completing the octet of each atom and making the carbocation most stable)

Acidity and Basicity Class 11 Notes | EduRev

extent of +ve charge decreases stability increases.


Ex.10 Compare the acidic strength:

(a) Acidity and Basicity Class 11 Notes | EduRev (b) Acidity and Basicity Class 11 Notes | EduRev (c) Acidity and Basicity Class 11 Notes | EduRev (d) Acidity and Basicity Class 11 Notes | EduRev

Sol. After making conjugate base

 

Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev

c > b > a > d


BASIC STRENGTH

Acidity and Basicity Class 11 Notes | EduRev

Basic strength directly depends on the availability of lone pair for H. 

Ex.11 Compare the basic strength of following:

Sol. Acidity and Basicity Class 11 Notes | EduRev


Ex.12 Compare the basic strength of the following

(a) Acidity and Basicity Class 11 Notes | EduRev (b) Acidity and Basicity Class 11 Notes | EduRev (c) Acidity and Basicity Class 11 Notes | EduRev (d) Acidity and Basicity Class 11 Notes | EduRev

Sol. Acidity and Basicity Class 11 Notes | EduRevAcidity and Basicity Class 11 Notes | EduRevAcidity and Basicity Class 11 Notes | EduRevAcidity and Basicity Class 11 Notes | EduRev

CH4 < NH3 < H2O < HF

(acidic strength)

Acidity and Basicity Class 11 Notes | EduRev

* Strong Acids have weak conjugate base.

Acidity and Basicity Class 11 Notes | EduRev

* For the same period

less electronegativity, more nucleophilicity as more electronegative element has less tendency to give its electron pair.


Ex.13 Which is more basic Acidity and Basicity Class 11 Notes | EduRev or Acidity and Basicity Class 11 Notes | EduRev?

Sol. Acidity and Basicity Class 11 Notes | EduRev Acidity and Basicity Class 11 Notes | EduRev

Which is more basic NH3 or

forming conjugate acid

Acidity and Basicity Class 11 Notes | EduRev

 Acidity and Basicity Class 11 Notes | EduRev


Comparison of Basicity of Ammonia and Alkyl Amines :

Ex.14 Compare the basic strength of the following NH3, CH3NH2, (CH3)2NH, (CH3)3N

Factors which affect the basicity of Amines

(1) steric effect (2) Inductive effect (3) solvation effect.

The base whose conjugate acid is more stable will be more acidic forming conjugate acid of the given base.

Acidity and Basicity Class 11 Notes | EduRev

Stability order of conjugate acid

Acidity and Basicity Class 11 Notes | EduRev

Therefore basic strength,

(CH3)3N > (CH3)2NH > CH3NH2 > NH3

(vapor phase or gaseous phase or in Non polar solvent)

In Aqueous solution or in polar solvent

(CH3)2NH > CH3NH2 > (CH3)3N > NH3

In aqueous solution, the conjugate acids form H-bonds (intermolecular) with water molecules and stabilise themselves. Conjugate acid of 1° amine which has largest no. of H-atoms form maximum H-bond with water and is most stable. Consequently 1° amine is most basic.

Due to steric effect 1° amine is considered more basic as compared to 3° amine as lone pair is hindered by three alkyl group and less available for H .

Considering the combined effect of the three (Inductive, solvation and steric effect) we can conclude that

2° > 1° > 3° > NH3

Aromatic amines are least basic as their lone pair is in conjugation and less available for protonation.


Ex.15 Compare the basic strength of the following:

(a)

 Acidity and Basicity Class 11 Notes | EduRev 

(b) Acidity and Basicity Class 11 Notes | EduRev 

(c) Acidity and Basicity Class 11 Notes | EduRev

(if L.P. will participate in Resonance, then molecule becomes aromatic)

Hence L.P. will have a greater tendency to take part in Resonance and will be less available for H+
This compound will be least basic.


Ex.16 Compare the basic strength of the following:

Acidity and Basicity Class 11 Notes | EduRev

Sol. sp hybridised carbon being most electronegative will attract e- density from nitrogen and will make it less available for H+. Hence basicity decreases.

c > b > a


Ex.17 Compare the basic strength:

(a) Acidity and Basicity Class 11 Notes | EduRev 
(b) Acidity and Basicity Class 11 Notes | EduRev

a < b


Ex.18 Compare the basicity of the following compounds:

(a) CH3 - CH2 - CH = CH - Acidity and Basicity Class 11 Notes | EduRev 
(b) Acidity and Basicity Class 11 Notes | EduRev

(c) Acidity and Basicity Class 11 Notes | EduRev 
(d) Acidity and Basicity Class 11 Notes | EduRev

Sol. In part (a) the lone pair of nitrogen is in resonance therefore will be less available for H making it least basic among all followed by sp, sp2, sp3 hybridised carbon atoms.

b > c > d > a


Ex.19 Compare the basicity of the numbered nitrogen atoms.

Acidity and Basicity Class 11 Notes | EduRev

Sol. The planarity of ring will be destroyed if L.P. will take part in Resonance.

Basicity order of Nitrogen follows the order:

N(sp3) > N(sp2) > N(sp)

Acidity and Basicity Class 11 Notes | EduRev

(In this sp2, l.p. is in Resonance with ring hence will be less available for H+ therefore it will be least basic)


Ex.20 Compare the basic strength of the following:

(a) Acidity and Basicity Class 11 Notes | EduRev 
(b) Acidity and Basicity Class 11 Notes | EduRev 

(c) Acidity and Basicity Class 11 Notes | EduRev


Sol. In part (a) NO2 is at p-position Hence will attract e- density by both -M and -I.

In part (b) NO2 is at m-position hence will attract e- density by -I only

There is no such effect in part (c)

→ Availability of L.P. on nitrogen in part (a) is minimum followed by b and then c.

c > b > a

Ortho effect :

The ortho substituted aniline are less basic than aniline and ortho substituted benzoic acids are more acidic than benzoic acid.

Ortho effect is valid only for benzoic acid and aniline.

e.g. Acidity and Basicity Class 11 Notes | EduRev 

Also Acidity and Basicity Class 11 Notes | EduRev 

Ex.21 Compare the basic strength of the following :

(a) Acidity and Basicity Class 11 Notes | EduRev 

(b) Acidity and Basicity Class 11 Notes | EduRev 

(c) Acidity and Basicity Class 11 Notes | EduRev

 (d) Acidity and Basicity Class 11 Notes | EduRev

Sol. a > b > d > c

* Due to ortho effect d > c

if c is less basic than d then it will be certainly less basic than b as b is more basic than d. 


Ex.22 Compare the basic strength of the following :

 

(a) Acidity and Basicity Class 11 Notes | EduRev (b) Acidity and Basicity Class 11 Notes | EduRev (c) Acidity and Basicity Class 11 Notes | EduRev (d) Acidity and Basicity Class 11 Notes | EduRev

Sol. Do yourselves.

S.I.P → Steric inhibition of Protonation (ortho effect)

 

Acidity and Basicity Class 11 Notes | EduRev

After protonation, repulsion increases therefore ortho substituted aniline is less basic than aniline.

S.I.R → Steric inhibition of resonance

(a) Acidity and Basicity Class 11 Notes | EduRev 

(b) Acidity and Basicity Class 11 Notes | EduRev

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