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__Acidity & Basicity__

**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)

(b)

(c)

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) CHF_{3} (b) CHCl_{3}

**Sol. **CHF_{3} > CHCl_{3}

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

After the removal of proton

(a) (b)

(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) CHF_{3} (b) CHCl_{3}

(c) CHBr_{3}

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

**Sol. **CHCl_{3} > CHBr_{3} > CHF_{3}

_{ }

**Ex.4 Compare the acidic strength of the following**

(a) CH (CN)_{3} (b) CH (NO_{2})_{3} (c) CHCl_{3}

**Sol. **After removing H^{ }

(Resonance) In its resonating structure, -ve charge will be on N)

(Resonance) (- In its resonating structure -ve charge will reside on O)

→ more effective Resonance

(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) CH _{2} = CH_{2} (c) CH_{3} - CH_{3}**

**Sol. **

(Stability of the conjugate base)

→ a > b > c (acidic strength)

**Ex.6 Compare the acidic strength of the following :**

**Sol. d > c > b > a**

** **

**Ex.7** **Compare the acidic strength of the following :**

(a) H_{2}O (b) H_{2}S (c) H_{2}Se (d) H_{2}Te

**Sol. **Conjugate base has the stability order:

→ H_{2}O < H_{2}S < H_{2}Se < H_{2}Te (acidic strength)

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

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

**Sol. **A**fter forming conjugate base of the above:**

c > d > b > a

**Ex.9 Compare the reactivity of the following compounds with 1 mole of AgNO _{3}**

(a)

(b)

(c)

(d)

**Sol. **After removing Cl^{-}

( ve charge is not on resonance least stable)

(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)

extent of +ve charge decreases stability increases.

**Ex.10 Compare the acidic strength:**

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

**Sol. **After making conjugate base

c > b > a > d

__BASIC STRENGTH__

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

**Ex.11 Compare the basic strength of following:**

**Sol. **

**Ex**.**12** **Compare the basic strength of the following**

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

**Sol. **, , ,

CH_{4} < NH_{3} < H_{2}O < HF

(acidic strength)

* Strong Acids have weak conjugate base.

* 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 or ?**

**Sol. **>

Which is more basic NH_{3} or

forming conjugate acid

__Comparison of Basicity of Ammonia and Alkyl Amines :__

**Ex.14 Compare the basic strength of the following NH _{3}, CH_{3}NH_{2}, (CH_{3})_{2}NH, (CH_{3})_{3}N**

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.

Stability order of conjugate acid

Therefore basic strength,

(CH_{3})_{3}N > (CH_{3})_{2}NH > CH_{3}NH_{2} > NH_{3}

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

In Aqueous solution or in polar solvent

(CH_{3})_{2}NH > CH_{3}NH_{2} > (CH_{3})_{3}N > NH_{3}

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° > NH_{3}

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)

(b)

(c)

(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:**

**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)

(b)

a < b

**Ex.18 **Compare the basicity of the following compounds:

(a) CH_{3} - CH_{2} - CH = CH -

(b)

(c)

(d)

**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, sp^{2}, sp^{3} hybridised carbon atoms.

b > c > d > a

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

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

Basicity order of Nitrogen follows the order:

N(sp^{3}) > N(sp^{2}) > N(sp)

(In this sp^{2}, 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)

(b)

(c)

**Sol. **In part (a) NO_{2} is at p-position Hence will attract e^{-} density by both -M and -I.

In part (b) NO_{2} 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.

Also

**Ex.21 Compare the basic strength of the following :**

(a)

(b)

(c)

(d)

**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) (b) (c) (d)

**Sol. **Do yourselves.

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

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

**S.I.R → **Steric inhibition of resonance

(a)

(b)

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