Acidic & Basic character – Tricks-Part 1

  • Generally, phenol derivatives & benzoic acid derivatives have similar order of acidic character whereas aniline derivatives have opposite order, however look out for exceptions to this trick as we discuss here.
  • Below, acidic character order & pka values of phenol & benzoic acid derivatives & Basic character order & pkb values of aniline derivatives are given.

TYPE 1.

p-Nitrophenol > p-Chlorophenol >  phenol > p-Cresol > p-Methoxy phenol

p-NitroBenzoic acid > p-ChloroBenzoic acid >  Benzoic acid > p-Toluic acid > p-Methoxy Benzoic acid

p-NitroAniline < p-ChloroAniline <  Aniline < p-Toluidine < p-MethoxyAniline 

TYPE 2…

o-BromoPhenol  > m-BromoPhenol > p-BromoPhenol  >  Phenol [10]

o-ChloroPhenol  [8.48] > m-ChloroPhenol [9.02] > p-ChloroPhenol [9.38]>  Phenol

o-FluoroPhenol  [8.81] > m-FluoroPhenol [9.28] > p-ChloroPhenol [9.95]>  Phenol

o-Bromobenzoic acid [3.1] > m-BromoBenzoic acid [3.93]> p-BromoBenzoic acid [4.1] >  Benzoic acid [4.17]

o-ChloroBenzoic acid [2.89]  > m-ChloroBenzoic acid [3.82]> p-ChloroBenzoic acid [3.98]>  Benzoic acid [4.17]

p-ChloroPhenol [9.38] > p-FluoroPhenol [9.95] > p-BromoPhenol > p-IodoPhenol > Phenol

p-ChloroBenzoic acid [3.98] > p-FluoroBenzoic acid [4.14] > p-BromoBenzoic acid [4.1] > p-IodoBenzoic acid > Benzoic acid

o-Bromoaniline [11.4] < m-Bromoaniline [10.49] < p-Bromoaniline [10.09] < Aniline 

TYPE 3….

Phenol > m-Cresol [10.08]  > p-Cresol [10.14] > o-Cresol [10.28]

O-Toluic acid [3.89] > Benzoic acid [4.17]> m-Toluic acid [4.28] > p-Toluic acid [4.35]

O-Toluidine < Aniline  < m-Toluidine  < p-Toluidine

TYPE 4…

m-Methoxyphenol [9.65]  > Phenol  > o-Methoxyphenol [9.98] > p-Methoxyphenol [10.21]

m-HydroxyPhenol (Benzene-1,3-diol) [9.44] > o-Hydroxyphenol (Benzene-1,2-diol) [9.48] > p-Hydroxyphenol (Benzene-1,4-diol) [9.96] > Phenol [10]

o-Methoxybenzoic acid = m- Methoxybenzoic acid > Benzoic acid  > p-Methoxybenzoic acid

2,6-diHydroxybenzoic acid [2.3] > o-HydroxyBenzoic acid [2.98] > m-HydroxyBenzoic acid [4.08] > Benzoic acid > p-HydroxyBenzoic acid [4.58]

p-Methoxyaniline > Aniline > o-MethoxyAniline > m-Methoxyaniline [o-isomer should be weakest due to Ortho effect, but in meta-isomer, there is only –I effect hence it becomes weakest base]

TYPE 5…..

p- NitroPhenol [7.15] > o-Nitrophenol [7.23] > m-Nitrophenol [8.40] > Phenol
[o-isomer must be strongest acid, but due to chelation (intramolecular H-bonding) b/w  adjacent–OH & – NO2, deprotonation becomes slightly difficult.]

o-NitroBenzoic acid > p-NitroBenzoic acid > m-NitroBenzoic acid > Benzoic acid

o-Nitroaniline < p-Nitroaniline  < m-Nitroaniline < Aniline

TYPE 6…..

0-Aminophenol [9.71] > m-Aminophenol [9.87] > Phenol > p-Aminophenol[10.3]

Benzoic acid [4.17] > m-Aminobenzoic acid [4.79] > p-AminoBenzoic acid [4.92] > o-Aminobenzoic acid (Anthranilic acid) [4.98]

Type 7..[HYDROGEN BONDING]

pKa1 (phthalic acid or Benzene-1,2-dicarboxylic acid) < pKa1(Terephthalic acid or Benzene-1,4-dicarboxylic acid) < pKa1 (Isophthalic acid or Benzene-1,3-dicarboxylic acid)

pKa1 (maleic acid) < pKa1(fumaric acid); pKa2 (maleic acid) > pKa2(fumaric acid)

Type 8..[SIR EFFECT]

2,6-diMethylbenzoic acid [3.21] > 2-t-ButylBenzoic acid [3.46] > o-Toluic acid [3.91]

3,5-diMethyl-4-CyanoPhenol > 2,6-diMethyl-4-CyanoPhenol   ;
[Here, “NO” SIR effect as Cyano is a linear group so, second is less acidic due to closeness of both methyl group to –OH hence greater +I decreases acidic character.]

2,6-diMethyl-4-NitroPhenol >  3,5-diMethyl-4-NitroPhenol
[In second, due to SIR effect,  Nitro group goes out of plane & hence unable to withdraw electron by –R effect.]

2,4-diNitroBenzoic acid > 2-NitroBenzoic acid > 3,5-diNitroBenzoic acid

TYPE 9..[GENERAL ORDER]

RCO2H > RCO3H (peroxycarboxylic acids)

RSO3H > RCOOH > H2CO3 [6.3] > C6H5OH [10] > CH3OH [15.5] > H2O [15.7]> ROH > RC#CH > NH3 > RCH=CH2 > RCH3 > H2

RSH >ROH

TYPE 10..[GENERAL EXCEPTIONS]

Formic acid [3.75] > Benzoic acid [4.17] > Acetic acid [4.74] [IIT 85]

C6H5-CH2-CO-OH > CH2=CH-CH2-CO-OH

CH#C-CO-OH >  CH2=CH-CO-OH > CH3CH2COOH

CH#C-CH2-CO-OH [3.22] >  CH2=CH-CH2-CO-OH [4.35]

Type 11..(GASEOUS PHASE/FIELD EFFECT)

t-BUTANOL > n-BUTANOL

PROPANOIC ACID > ACETIC ACID

Type 12..[GENERAL]

pKa value of CF3-CO-OH is zero (0.18) & it is the strongest organic acid known.[(10^5 times stronger than acetic acid.]

Formic acid is 10 times stronger than Acetic Acid.

N3H [2.810^(-5)] > CH3COOH [1.810^(-5)]

Beta-KETOESTERS are stronger acids than Alcohols & Esters.

NC-CH2-CO-OH > CH3-CO-CH2-CO-OH

CH3COOH > HO-CH2=CH2-OH > HO-OH > CH3CH2OH [IIT-80]

OXIMES(>C=N-OH) > HYDROXYL AMINE (NH2OH)

Type 14..[ACID-BASE REACTIONS]

Acid-base equilibriums move towards formation of weaker acids & weaker bases.

2,4,6-triNitroPhenol or Picric acid  [0.38] does dissolve in aq NaHCO3

Carboxylic acids dissolves in aq NaHCO3 whereas Phenol does not.

4-ChloroPhenol dissolves in NaOH (aq.) whereas 4-Chloro-1-methylBenzene does not.

  • Electron releasing groups (ERG) increase e- density of bases or stabilises “cationic conjugate acids” & hence increase basic character.
  • EWG increase stabilisation of “anionic” conjugate bases & hence increase acidic character.
  • Acidic Character is proportional to Stability of corresponding conjugate acid.
  • Basic Character is proportional to Stability of corresponding conjugate base.
  • 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 exert 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.”
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