The energy difference DG
between protonated and nonprotonated forms,
as shown in the following diagram, is higher for arylamines than it is for
alkylamines. This is why arylamines are less basic.
Alkylammonium ion, RNH 2
Arylammonium ion, ArNH 2
Alkylamine, RNH 2
Arylamine, ArNH 2
∆G o (alkyl)
∆G o (aryl)
Resonance
stabilization
+
+
Effect of substituents on the basicity of aniline The effect of substituent(s) on the basicity of aniline is summarized below. Electron-donating
substituents (Y ¼ À ÀCH 3 , À ÀNH 2 , ÀOCH 3 ) activate the ring, and increase the
basicity of aniline, whereas electron-withdrawing substituents (Y ¼ ÀCl,
À ÀNO 2 , À ÀCN) deactivate the ring, and decrease the basicity.
Y
NH 2
Substituent Y
pK a
Effect on reactivity
Stronger base
À ÀNH 2
6.15
Activating
ÀOCH 3
5.34
Activating
À ÀCH 3
5.08
Activating
ÀH
4.63
ÀCl
3.98
Deactivating
À ÀCN
1.74
Deactivating
Weaker base
À ÀNO 2
1.00
Deactivating
Preparation of aniline
Reduction of nitrobenzene Aniline can be prepared from nitrobenzene
by either chemical reduction using acid and metal or catalytic hydrogenation using molecular hydrogen.
NO 2
NH 3 Cl
NH 2
Fe, dil HCl
30 o C
+
_
Na 2 CO 3
Chemical reduction
NO 2
NH 2
H 2 , Pt
Ethanol
Catalytic hydrogenation
136
CH4 ORGANIC FUNCTIONAL GROUPS