Basicity of aromatic amine

Basic nature of aromatic amine and effect of substituent on basicity

1. Basicityof Aromatic
Amines
Prepared by
Ms. SayyedMohsina

2. Basicitycanbedefinedastherelativeeasewithwhichanamine
willacceptprotonfromwater.Thusanilineisprotonatedtogivethe
aniliniumion.
The value of equilibrium constant K
bdetermines the concentration of OH
ions or the basicityof aniline.
BASICITY OF AROMATIC AMINES

3. Compared to ammonia (NH
3) or aliphatic amines (R—NH
2), the basicity
of an aryl amine is significantly reduced.
Thereducedbasicityofanilineisduetothenon-bondedelectronsonthe
nitrogenatomaredelocalisedintotheπ-systemofthebenzenering.Thus
thevalueofK
bforanilineisdecreased,makingitlessbasic.

4. Structures1and2aretheKekulestructuresthatcontributetoanybenzene
derivative.Structures3–5,showdelocalizingtheunsharedelectronpairof
thenitrogenovertheorthoandparapositionsofthering.This
delocalizationoftheelectronpairmakesitlessavailabletoaprotonand
delocalizationoftheelectronpairstabilizesaniline.
DELOCALISATION OF LONE PAIR OF ELECTRON ON
NITROGEN ATOM

5. LOWER STABILITY OF ANILINIUM ION
Theaniliniumionformedbyprotonationofanilineislessresonance-
stabilisedasitexistsonlyintwoKekuleforms.
Anilinestructurewhichexistsinfiveresonanceforms,ismoreresonance
stabilisedthantheaniliniumionwhichcanexistintwoformsonly.

6. Anelectronreleasinggroup(—NH
2,—OCH
3,—CH
3,)pusheselectrons
towardstheN-atomandmakesthenonbondedelectronpairmoreavailable
forprotonation.Hencesuchasubstitutentincreasesthebasicity.ThusK
b,
valueforp-methoxyanilineis2.0x10
-9
ascomparedtoaniline4.2x10
-10
.
Ontheotherhandanelectron-withdrawinggroup(—NO
3,—SO
3H,—
COOH,—X)pullelectronsawayfromnitrogenandthusmakesthe
nonbondedelectronpairlessavailableforprotonation.Therefore,sucha
substituentlowersthebasicity.ThusK
bvalueforp-nitroanilineis1.0x10
-13
ascomparedtoaniline,4.2x10
-10
.
EFFECT OF SUBSTITUENT ON BASICITY OF AMINE

7. Whenanalkylgroup(electronreleasinggroup)ispresentonN-atom
ofaniline,thebasicityofanilineisincreased.
WhenmorethanonebenzeneringarebondedtoN-atom,the
nonbondedelectronssuffergreaterdelocalisationandaremuchless
availableforprotonation.
Thepresenceoflargephenylgroupsinthemoleculealsoscreensthe
nonbondedelectronsonN-atomandthusreducesthebasicityfurther.
Forexample,Diphenylamineisonlyaveryweakbasebuttriphenyl
amineiscompletelydevoidofbasiccharacter.

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