1) Amines act as bases according to both Lewis and Bronsted-Lowry theories due to their ability to donate a lone pair of electrons or accept a proton. 2) The basicity of amines depends on factors such as the stability of the conjugate acid formed, inductive effects, and hydrogen bonding capabilities. In general, aliphatic amines are stronger bases than aromatic amines. 3) Within aliphatic amines, the order of basicity from strongest to weakest is typically tertiary > secondary > primary > ammonia in the gas phase. In aqueous solution, primary amines are stronger bases due to hydrogen bonding of the conjugate acid form.

1. Basicity of amines
Pt L R college of pharmacy
faridabad

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 an lone pair
electron, so they are Lewis bases.
R3N: + H → R3N-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 R3NHNO3
-
+
+
+ -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 = [RNH3
+][OH-
] /[RNH2][H2O]
 Since water is taken in large excess, its
conc” [H2O] remains constant
 the basically 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 –OCH3 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