Basic Character of amines

  • Larger the value of Kb or Smaller the value of pKb (=-logKb), the stronger is the base.
  • Any factor which increases lone-pair availability towards protonation increase the basic character.
  • Electron releasing groups (ERG) increase e- density of bases or stabilises “cationic conjugate acids” & hence increase basic character.
  • Basic Character is proportional to Stability of corresponding conjugate acid.
  • TYPE 1 [STABILISATION OF CONJUGATE BASE]

    Guanidine is one of best nitrogenous bases & is almost as strong base as alkali metal hydroxide. Its conjugate base is highly resonance stabilised.

  • TYPE 2 [HYBRIDISATION]

  • sp3 hybrid orbital is more electropositive (due to less s character) & hence more basic than sp2 hybrid orbital, which in turn is more basic than sp hybrid orbital.
  • RNH2 > RCH=NR > RC#N
  • CH3CH2NH2 > CH3CONH2 > CH3CN
  • TYPE 3 [ALIPHATIC , ARALKYL , ARYL AMINE]

  • Aliphatic amines (pKb = 3 to 4.22) are stronger bases than ammonia (pKb =4.75) due to +I effect of alkyl groups which increases e- density at nitrogen atom.
  • In gaseous phase: R3N > R2NH > RNH2 > NH3 (governed by +I effect of Alkyl groups).
  • Arylalkyl amines have amine- groups indirectly attached to aromatic rings. Aralkyl amines are weaker bases than Aliphatic amines due to e- withdrawing nature (-I) of aryl groups.
  • Aryl amines have amine- groups directly attached to aromatic rings. These are weaker bases than arylalkyl amines due to involvement of lone pair of nitrogen in resonance with benzene ring which decreases the availabilty of lone pair of e on nitrogen. Triphenyl amine is almost neutral. [Arylalkyl amines are stronger bases than aryl amines as lone pair of nitrogen in arylalkylamines are not involved in resonance & hence available to donate.]
  • Aniline is a resonance hybrid of five structures whereas its protonated form (conjugate acid) anilinium ion is a resonance  hybrid of only two structures.
  • R2NH > RNH2 > ArCH2NH2 > NH3 > ArNHR > ArNH2 > Ar2NH > Ar3N
  • For example,
  • CH3CH2NH2 [3.29] > CH3NH2 [3.38] >
  • Benzylamine or Phenylmethanamine, C6H5CH2NH2  [4.70] >  NH3 [4.75]
  • C6H5-N(CH2CH3)2, N,N-Diethyl aniline [7.44] >
  • C6H5-NH-CH2CH3, N-Ethyl aniline [8.89] >
  • C6H5-N(CH3)2, N,N-Dimethyl aniline [8.92] >
  • C6H5-NH-CH3, N-Methylaniline [9.30] > Aniline or Benzenamine, C6H5NH2 [9.42] >
  • Diphenylamine, (C6H5)2NH [13.10] >
  • N-Phenylethanamide or Acetanilide, C6H5-NH-CO-CH3 [13.60]

    TYPE 4 [SOLVATION IN POLAR SOLVENT]

  • The ammonium cation (protonated form) of  aliphatic amine is solvation-stabilised by H2O molecules via Hydrogen-bonding.
  • Ammonium cation of primary amine (RNH3+) is maximum solvated of all types (primary, secondary & tertiary)  due to greater number of hydrogens directly attached to nitrogen.
  • Three effects are most useful to decide basic character of simple aliphatic amines (a) Inductive effect – which favors tertiary amine due to greater + I effect of three alkyl groups directly attached to nitrogen (b) Solvation effect- which favor primary amine due to greater solvation of its cation. (c) Steric factor – which causes tertiary amine to be less basic due to steric hindrance around nitrogen.
  • As a combined result of above three factors, in aq solution, the order of basic character in isomeric aliphatic amines are different in different cases-
  • (CH3)2NH [3.27] > CH3NH2 [3.38] > (CH3)3N [4.22] > NH3 [4.75]
  • (C2H5)2NH [3.0] >  (C2H5)3N [3.25] > C2H5NH2 [3.29] > NH3 [4.75]
  • Me2CH-NH2 > NH3 > (Me2CH)2NH > (Me2CH)3N
  • NH3 > Me3C-NH2 > (Me3C)2NH > (Me3C)3N
  • TYPE 5 [Basic character of ARYL AMINES] 

  • Inductive effect operates from ortho as well as meta & para-positions & inductive effect is strongest from ortho position (being nearest)
  • Resonance or mesomeric effect does not affect from groups attached at meta positions. However it does affect from both ortho and para-positions with equal intensity.
  • Alkyl groups release e- by hyperconjugation effect (+H) from ortho & para-positions.
  • The groups having lone pair at the key atom (e,g -OH, -OR, -O-CO-R, -NH2, -NHR, NR2, -NH-CO-R, -X etc.) exert +R effect(release e- by resonance) when conjugated (alternate or attached) to unsaturated system like aromatic ring e,g, Benzene ring.
  • Order of +R effect: -NH2 > -OR > -OH >-OCH3 > -OCOR > -F > -Cl
  • CH3OH is slightly stronger acid than water hence -OCH3 is slightly more e- donating than -OH.
  • For -Cl, -Br & -I , +R effect is almost ineffective due to poor overlap of 2p orbital of Carbon with 3p or 4p or 5p of Cl, Br or I.
  • The groups having Key atom multiply bonded to more powerful electronegative atoms (e.g -NO2, -CN, -CHO, -CO-R, -COCl, -CONH2, -CO-O-COR, -CO-OR, etc) exert -R  effect (withdraw e- by resonance) when conjugated (alternate or attached) to unsaturated system like aromatic ring e,g, Benzene ring.
  • Due to ortho effect, in most cases,-
  • “Ortho-isomer of Benzoic acid is strongest Acid as compared to Simple benzoic acid or meta & para-isomers  & Ortho- isomer of Aniline is Weakest Base as compared to Simple Aniline or meta & para-isomers.”

Toluidine, NH2-C6H4-CH3:  P- [8.92] > M- [9.31] > ANILINE [9.42] > O- [9.61]

[from p- position, -CH3 group releases electron by +I as well as +H hence making it strongest base, whereas in meta-isomer, methyl group releases electron only by +I]

Anisidines(Methoxyanilines), NH2-C6H4-OCH3: P- [8.82] > ANILINE > O- [9.51] > M- [9.80]

[ p-isomer is stronger base than aniline due to stronger +R effect of -OCH3; It is an exception of ortho effect. o-isomer should be weakest due to Ortho effect, but it can be rationalised that somehow in meta-isomer, -OCH3 group exerts only -I effect from meta position, hence it becomes weakest base.]

Nitroaniline, NH2-C6H4-NO2: ANILINE > M-[11.50] > P- [12.98] > O- [14.29]

[all nitroanilines are weakest bases than aniline, from meta-position there is -I effect of -NO2 group & in para-isomer, -NO2 group withdraws electron by both -I as well as -R]

Chloroaniline, NH2-C6H4-Cl:  ANILINE  > P- [9.82] > M-[10.66] > O-[11.36]

[although halo- group is e- donating by resonance, its -I effect predominates that decreases with distance of halogen from NH2 group, making all halo-anilines less basic than aniline. -I effect of halo-group destabilises conjugate acid i.e., anilinium ion.-I effect is least dominant in p-isomer, making it most basic of all halo-isomers.]

Bromoaniline, NH2-C6H4-Br :  ANILINE > P- [10.09] > M- [10.49] > O-[11.4]

Phenylenediamine, NH2-C6H4-NH2: P-[7.96] > M-[9.12] > ANILINE > O-[9.53]

  • TYPE 6 [MISCELLANEOUS]

  • C2H5NH2 > benzamide, C6H5CONH2 > acetamide,  CH3CONH2 [in benzamide, there is cross-conjugation]

 

 

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