substituents effects on the bacicities of amines.ppt

nnn

1. Basicity of amines

2. 
All the three classes of amines (1°,2°&
3°) because of presence of a lone pair of
electrons on N atom which can be
donated to others thus amines behaves
as Lewis base

Amines are Lowry-Bronsted bases as
they accepts a proton thus amines acts
as both bases & nucleophiles

3. According to the Lewis acid-base
concept, amines can donate a lone pair
electron, so they are Lewis bases.
R3N: + H R
→
3N-H
Also, Brønsted-Lowry bases can accept
a proton to form substituted
ammonium ions.
R3N:+H2O ⇌ R3N-H +OH
So, amines are bases according to
both the Lewis and the Brønsted-
Lowry theories
+
+
+
_

4. Basic character

Reaction with water amines are
stronger bases than water

RNH2 + H2O RNH3OH
+ -

5. Reaction with acids

All types of amines react with mineral
acids such as HCl, HNO3, H2SO4 to
form soluble salts

RNH2 + HCl RNH3Cl
-

R2NH + H2SO4 [R2NH2]2SO4

R3N + HNO3 R3NHNO 3
-
+
+
+
-
Alkylammonium
chloride
Dialkylammonium
sulphate
Trialkylammonium nitrate

6. Structure-basicity
relationship of amines

The basicity of any amine depends
upon the ease with which it accepts a
proton to form ammonium cation
(conjugate acid) or stability of
ammonium cation forms after
accepting proton

The basic strenth of an amine is
determined by its basicity constant, Kb

7. RNH2 + H2O RNH3 + OH
-
According to law of mass of action,
equilibrium constant K expressed as

Keq = [RNH
3
+
][OH
-
] /[RNH2][H2O]
Since water is taken in large excess,
its conc” [H2O] remains constant
the basicity constant Kb = Keq [H2O]
Evidantly, greater the value of Kb,
stronger is the base
pKb values is the negative logarithm
of basicity constant, Kb
pKb = -logKb

8. 
Evidently, smaller the value of pKb,
stronger is the base

9. Basicity of aliphatic amines

Aliphatic amines are stronger bases
than ammoina due to electron-
donating alkyl groups

As the electron density on N-atom
increases thus it can donate lone pair
of electrons more easily than
ammonia

Thus basicity of amines decreases in
the order in gaseous phase

3°> 2°> 1° > NH3

10. 
But in aqueous sol” basicity of amines
depends upon 3 factors

1) +I effect of alky groups

2) Extent of H-bonding with water
molecules

3) Steric effects of alkyl groups

11. H-bonding with water
molecules & steric repulsion

Greater the no of H atom on N atom, more
stable is ammonium cation,

Ammonium cations formed from 1° amines
is most stable since it has 3 H-atom to
make H-bonding with water & have least
steric repulsion & most stability

Same Ammonium cations formed from 2°
amines is less stable since it has 2 H-atom
to make H-bonding with water & have less
steric repulsion

12. Ammonium cations formed from 3°
amines is least stable since it has only
1 H-atom to make H-bonding with
water & more steric repulsion to H-
bonding & stability further decreases
NH3 > 1° >2°> 3° order of stability on basis of H-bonding & steric effect

13. 
CH3 there is no stearic hindrance to
H-bonding

Stability due to H-bonding
predominates over stability due to +I
effect of CH3

1° amine is stronger base than 3°
amine

decreasing strength of
methylamines
(CH3)2NH
2°
amines
pKb 3.27
>
CH3NH2
1°
amines
pKb 3.38
>
(CH3)3N
3° amines
pKb 4.22
>
NH3
1° amines
pKb 4.75

14. 
If alkyl group is bigger than CH3
group i.e. ethyl, propyl. There will be
some stearic hindrance to H-bonding

Stability due to +I effect
predominates over stability due to H-
bonding

Decreasing strength of ethylamine
(CH3CH2)2NH
2° amines
pKb 3.00
>
(CH3CH2)3N
3° amines
pKb 3.25
>
CH3CH2NH2
1° amines
pKb 3.29
>
NH3
1° amines
pKb 4.75

15. Aromatic amines

These are the derivatives of aromatic hydrocarbon in which
a hydrogen of benzene ring has been replaced by amino
group.

All such compound in which an amino or substituted amino
group is bonded directly to an aromatic ring are termed as
aromatic amines
N-
methyaniline
n,n-
dimetylaniline

16. Basicity of aromatic amines
Aromatic amines are less basic than
ammonia & aliphatic amines
In ammonia & aliphatic amines,
delocalization of lone pair of e on
−
N-atom by resonance is not possible.
e density on N-atom is increased by
−
e donating inductive effect of alkyl
−
groups

17. 1) Due to resonance in
aniline

As a result of resonance, lone pair of
electrons on N-atom gets delocalized
over benzene ring & e is less easily
−
available for protonation.

Aromatic amines are weaker bases
than NH3

18. 2) Lower stability of
anilinium ion than aniline

Anilinium ion is resonance hybrid of
only two structures VI & VII
VI VII
Aniline is more stable than anilinium ion. Aniline has little affinity to
combine with a proton to form anilinium ion

19. Effect of substituents on
basicity of aromatic amines

Electron-donating groups increases the
basicity

EDG releases e , stabilizes conjugate acid
−
(cation) & increases the basic strength eg.
CH3, OCH3, OH, NH2

Electron-withdrawing groups withdraws
electrons, destabilizes conjugate acid thus
decreases basic strength eg. NO2 , CN, X
(halogens)

20. The base-strengthening effect of the EDG &
base-weakening effect of EWG is more
marked at p-position than at m-position

21. 
O-substituted anilines are usually
weaker bases than anilines without
effect of EDG & EWD called ORTHO-
EFFECT

Probably due to combination of steric
& electronic factors

22. When the substituent has a
strong +R effect & weak –I
effect

When –OCH 3 is present at m-position it
exerts only –I effect & resonance effect
does not operate at m-position so e-
density decreases at N-atom hence m-
methoxyaniline is weaker base than
aniline.

O-methoxyaniline is weaker base than
aniline due to ortho-effect.

p-methoxyaniline is stronger base than
aniline due to +R effect of the OCH3 group