Comprehensive Notes on Acid-Base Properties of Organic Substances

Acid-Base Properties: Acetic Acid vs. Ethanol

• Acidity Comparison: Acetic acid is acidic, whereas Ethanol is neutral.

• Litmus Test Observation: Acetic acid can turn blue litmus red; Ethanol cannot.

• Factors Governing Acidity: The acidity of an organic compound ($AH$) depends upon:

  • (A) The strength of the bond between $A$ and $H$.

  • (B) The stability of the conjugate base, $A^-$.

• Bond Strength Analysis ($A-H$):

  • Structure of Acetic Acid: $CH_3-C(=O)-O-H$

  • Structure of Ethanol: $CH_3-CH_2-O-H$

  • Both substances possess an $-OH$ bond. When dissolved in water, Acetic acid releases $H_3O^+$ ions:

    • $CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+$

    • The $CH_3COO^-$ is the conjugate base (Carboxylate Anion).

  • Ethanol theoretically dissociates to produce $H_3O^+$ and an ethoxide anion, but to a negligible degree compared to acid:

    • $CH_3-CH_2-OH + H_2O \rightleftharpoons CH_3-CH_2-O^- + H_3O^+$

• Electronegativity and Electron Withdrawal:

  • Oxygen in the $-OH$ group pulls the bonding electron pair away from the $H$ atom.

  • This effect occurs in both groups, but the $-COOH$ group contains an additional electron-withdrawing $C=O$ group. The carbonyl group pulls electrons from the $-OH$ bond with more force.

  • Ethanol Structure: Ethanol possesses an electron-releasing group (ethyl group) which increases the negative charge on the oxygen, making the alkoxide ion unstable.

  • Conclusion: Acids containing electron-withdrawing groups are stronger acids than those without them.

• Stability of Conjugate Base:

  • Resonance Effect: Resonance exists in the Acetate ion ($CH_3COO^-$) of Acetic acid, which is not possible in the ethoxide ion ($CH_3-CH_2-O^-$) of Ethanol.

  • In the Acetate ion, the negative charge is shared between two oxygen atoms.

  • The dispersal of electrical charge makes $CH_3COO^-$ significantly more stable than the $CH_3CH_2O^-$ ion.

  • Because the conjugate base ($CH_3COO^-$) is more stable than the parent acid ($CH_3COOH$), the equilibrium shifts to the right side, facilitating the release of the $H^+$ ion.

Inductive Effect in Substituted Propionic Acids

• $\beta$-Chloro Propionic Acid vs. Propionic Acid:

  • Structure of Propionic Acid: $CH_3-CH_2-COOH$

  • Structure of $\beta$-Chloro Propionic Acid: $Cl-CH_2-CH_2-COOH$

  • $\beta$-Chloro propionic acid is stronger than propionic acid due to the electron-withdrawing effect of the chlorine atom.

  • Inductive Effect ($-I$): Chlorine is highly electronegative and pulls electrons away from the $-COOH$ group through sigma (σ) bonds. This decreases electron density around the $-COOH$ group, making it easier to release $H^+$.

  • Conjugate Base Stability: When propionic acid loses $H^+$, it forms $CH_3-CH_2-COO^-$. In $\beta$-chloro propionic acid, the conjugate base ($Cl-CH_2-CH_2-COO^-$) benefits from the electron-withdrawing chlorine, stabilizing the negative charge.

• $\beta$-Chloro vs. $\alpha$-Chloro Propionic Acid:

  • Structure of $\alpha$-Chloro Propionic Acid: $CH_3-CH(Cl)-COOH$

  • Comparison: $\beta$-chloro propionic acid is a weaker acid than $\alpha$-chloro propionic acid.

  • Effect of Distance: In $\alpha$-chloro propionic acid, the chlorine atom is directly attached to the carbon adjacent to the carboxyl group. This allows for a more effective electron-withdrawing effect on the $-OH$ bond.

  • In $\beta$-chloro propionic acid, the chlorine is two carbons away from the $-COOH$ group. The increased distance weakens the inductive effect ($-I$). Therefore, as the distance between the chloro group and $-COOH$ increases, acidic strength decreases.

Hybridization and Unsaturated Acids

• Propenoic Acid vs. Propanoic Acid:

  • Propanoic Acid (A): $CH_3-CH_2-COOH$

  • Propenoic Acid (B): $CH_2=CH-COOH$

  • Hybridization Difference:

    • In (A), carbons are singly bonded ($sp^3$ hybridized) with $25\%$ s-character.

    • In (B), the doubly bonded carbon is $sp^2$ hybridized and possesses $33.3\%$ s-character.

  • Electronegativity: Higher s-character results in higher electronegativity. Doubly bonded carbon ($sp^2$) exerts a $-I$ effect, whereas singly bonded carbon ($sp^3$) exhibits a $+I$ effect.

  • Acidity Results: Because the $CH_2=CH-$ group in (B) exhibits a $-I$ effect, the conjugate base is more stable than that of (A). Thus, (B) releases the $H^+$ ion more readily.

Acidity of Dicarboxylic Acids

• Oxalic Acid vs. Malonic Acid:

  • Structures: Oxalic acid ($HOOC-COOH$); Malonic acid ($HOOC-CH_2-COOH$).

  • Dissociation Constants ($K_a$):

    • Oxalic acid $K_{a1} = 5400 \times 10^{-5}$

    • Malonic acid $K_{a1} = 140 \times 10^{-5}$

  • Higher $K_a$ (lower $pK_a$) indicates greater acidic strength.

  • Oxalic acid is stronger than malonic acid because its first dissociation constant is significantly higher.

• Succinic Acid vs. Propanoic Acid:

  • Succinic acid ($HOOC-CH_2-CH_2-COOH$) has a higher $K_{a1}$ ($6.4 \times 10^{-5}$) than Propanoic acid.

  • The presence of two carboxyl groups in succinic acid introduces additional electron-withdrawing effects through induction, increasing the acidity of each $-COOH$ compared to propanoic acid which has only one.

• Mono Glutarate Anion vs. Mono Succinate Anion:

  • Mono Glutarate Anion: Derived from Glutaric acid ($HOOC-(CH_2)_3-COOH$).

  • Mono Succinate Anion: Derived from Succinic acid ($HOOC-(CH_2)_2-COOH$).

  • The mono glutarate anion is more acidic than the mono succinate anion. The increased distance and effectively distributed electron-withdrawing effects in the glutarate structure lead to a more stable conjugate base.

• Second Dissociation Step Difficulty: In dicarboxylic acids, the second step of dissociation is usually difficult ($K_{a1} > K_{a2}$) because:

  • (i) The ion produced after the first dissociation is already negatively charged ($COO^-$). Removing a positive hydrogen ion ($H^+$) from a negative ion requires overcoming strong electrostatic attraction.

  • (ii) The resulting dianion (doubly charged negative ion) is less stable because like charges are separated, requiring more energy to maintain.

  • (iii) Formation of Hydrogen Bonds: If the distance is small, internal hydrogen bonding can occur after the first dissociation, stabilizing the monoanion and making it harder to remove the second $H^+$ (forming a stable chelate structure).

• Comparison of Succinic and Glutaric Acids:

  • $K_{a1}$ Succinic acid = $6.4 \times 10^{-5}$

  • $K_{a1}$ Glutaric acid = $4.5 \times 10^{-5}$

  • Succinic acid is stronger in its first dissociation step than glutaric acid.

Geometrical Isomerism: Maleic vs. Fumaric Acid

• Relative Acidities: Maleic acid (cis) is a stronger acid than Fumaric acid (trans) in the first dissociation step ($K_{a1}$).

• Intramolecular Hydrogen Bonding: In the maleate mono anion (conjugate base of maleic acid), the $-COOH$ and $-COO^-$ groups are on the same side of the double bond. This proximity allows for intramolecular hydrogen bonding, which stabilizes the anion.

• Fumaric Acid: The groups are on opposite sides, preventing hydrogen bonding. The double bond prevents rotation. Thus, Maleic acid loses its first $H^+$ faster.

• Second Dissociation Step: Interestingly, mono sodio maleate is a weaker acid than mono sodio fumarate.

  • Maleate mono anion $pK_a = 6.26$

  • Fumarate mono anion $pK_a = 4.38$

  • In the second step, the release of $H^+$ from mono sodio maleate is harder because the hydrogen is involved in the stable intramolecular hydrogen bond.

Acidity of Phenols

• Phenol vs. Ethanol:

  • Phenol Structure: $-OH$ attached to an $sp^2$ carbon of a phenyl ring.

  • Electronegativity: The phenyl ring exerts a $-I$ effect, making the $-OH$ bond polar and easier to break.

  • Resonance Stability: Phenoxide ion (conjugate base) is stabilized by resonance with the aromatic ring (5 resonating structures).

  • Ethanol: The alkyl group releases electrons ($+I$ effect), destabilizing the ethoxide ion. Ethanol is practically neutral.

  • Charge Separation: In Phenol, resonance structures involve charge separation ($+$ and $-$), making them less stable than the parent. However, in the Phenoxide ion, there is charge dispersal (spreading the negative charge), making the ion highly stable.

• $p$-Nitro Phenol vs. Phenol:

  • $p$-Nitro phenol is a stronger acid than phenol.

  • The nitro ($-NO_2$) group at the para position is a strong electron-withdrawing group ($-I$ and $-R$ effects), which stabilizes the negative charge on the phenoxide ion.

• Picric Acid (2,4,6-Trinitro Phenol):

  • Picric acid does not have a $-COOH$ group but is highly acidic.

  • The three $-NO_2$ groups strongly withdraw electrons, stabilizing the phenoxide negative charge so effectively that it can react with weak bases like sodium bicarbonate ($NaHCO_3$) to form a Picrate Salt, $H_2O$, and $CO_2$ gas.

• $p$-Cresol vs. $p$-Nitro Phenol:

  • $p$-Cresol is a weaker acid than $p$-nitro phenol.

  • $p$-Cresol has a methyl group ($-CH_3$), which is electron-donating, making the $-OH$ less acidic. $p$-Nitro phenol has a strong electron-withdrawing $-NO_2$ group.

• Benzoic Acid vs. Phenol:

  • Benzoic acid is more acidic than phenol. The conjugate base (benzoate ion) is more effectively stabilized by resonance.

• Nitro Isomer Comparison:

  • $o$-Nitro Phenol vs. Phenol: $o$-Nitro Phenol is more acidic due to the $-I/-R$ effects of the ortho nitro group.

  • $m$-Nitro vs. $p$-Nitro Phenol: $p$-Nitro phenol is more acidic because the $-NO_2$ group at the meta position cannot stabilize the phenoxide ion through resonance, only through inductive effects. The para position allows for resonance stabilization.

• Solubility logic: Phenols are soluble in $NaOH$ (forming soluble phenoxide salts) but not in $NaHCO_3$ because $NaHCO_3$ is a base too weak to deprotonate them (except for highly substituted ones like picric acid).

Basicity of Amines and Related Compounds

• Aniline vs. $p$-Nitro Aniline:

  • Aniline is a stronger base than $p$-nitro aniline.

  • In $p$-nitro aniline, the $-NO_2$ group (at para position) pulls electron density away from the $-NH_2$ group via inductive and resonance effects, making the nitrogen's lone pair less available for protonation.

  • Ortho and Para nitro anilines are weaker bases than aniline due to the "superimposable effect" of electron withdrawal.

• Effect of Nitro Group on Acid/Base Strength:

  • Introducing a Nitro group to Phenol enhances acidic strength (stabilizes the negative charge on the conjugate base).

  • Introducing a Nitro group to Aniline reduces basic strength (decreases electron density on the lone pair).

• Acetamide Stability: An aqueous solution of Acetamide ($CH_3CONH_2$) is neutral.

  • The $-NH_2$ is bonded to a carbonyl $C=O$ group, which is $sp^2$ hybridized and exerts a $-I$ effect.

  • More importantly, resonance occurs where the nitrogen lone pair is delocalized into the carbonyl oxygen ($R-C(O^-)=NH_2^+$), making the lone pair unavailable and the substance almost neutral.

• Succinimide and Phthalimide:

  • These are imides where the $NH$ group is bonded between two electron-withdrawing $C=O$ groups.

  • This effect is so strong that the substance behaves as an acid rather than a base.

  • They react with $NaOH$ or $KOH$ to form salts (e.g., Potassium phthalimide).

  • Aqueous Succinimide can turn blue litmus red; Acetamide cannot.

Organic Structures and Chemical Formulas Catalog

1. Formic Acid: $H-COOH$

2. Acetic Acid: $CH_3-COOH$

3. Propionic / Propanoic Acid: $CH_3-CH_2-COOH$

4. Butyric Acid: $CH_3-CH_2-CH_2-COOH$

5. Isobutyric Acid: $(CH_3)_2CH-COOH$

6. Trimethyl Acetic Acid: $(CH_3)_3C-COOH$

7. Propenoic Acid: $CH_2=CH-COOH$

8. Malonic Acid: $HOOC-CH_2-COOH$

9. Succinic Acid: $HOOC-(CH_2)_2-COOH$

10. Glutaric Acid: $HOOC-(CH_2)_3-COOH$

11. Valeric Acid: $CH_3-(CH_2)_3-COOH$

12. Adipic Acid: $HOOC-(CH_2)_4-COOH$

13. 2-Butenedioic Acid: $HOOC-CH=CH-COOH$

14. Maleic Acid: Cis-isomer of butenedioic acid.

15. Fumaric Acid: Trans-isomer of butenedioic acid.

16. Maleate Mono Anion: Conjugate base of maleic acid involving internal H-bonding.

17. Fumarate Mono Anion: Conjugate base of fumaric acid.

18. Mono Sodio Maleate ($pK_a = 6.26$)

19. Mono Sodio Fumarate ($pK_a = 4.38$)

20. Benzoic Acid: $C_6H_5COOH$

21. Salicylic Acid: $o$-Hydroxy benzoic acid.

22. p-Hydroxy Benzoic Acid (PHBA)

23. Phenol: $C_6H_5OH$

24. Phenoxide Ion: $C_6H_5O^-$

25. o, p, m Nitro Phenol: (Ortho, Para, Meta isomers)

26. 2,4-Dinitro Phenol

27. 2,4,6-Trinitro Phenol: (Picric Acid)

28. p-Cresol: $p$-Methyl phenol.

29. 1°-Butyl Amine: $CH_3-CH_2-CH_2-CH_2-NH_2$

30. Ethanol Amine: $HO-CH_2-CH_2-NH_2$

31. Ethylene Diamine: $NH_2-CH_2-CH_2-NH_2$

32. Hydrazine: $H_2N-NH_2$

33. Hydroxyl Amine: $HO-NH_2$

34. Pyridine: $C_5H_5N$

35. Piperidine: $C_5H_{10}NH$

36. Aniline: $C_6H_5NH_2$

37. N-Me-Aniline: $C_6H_5NHCH_3$

38. N,N-Dimethyl Aniline (DMA): $C_6H_5N(CH_3)_2$

39. Toluidines: (o, m, p isomers of methyl aniline)

40. α-Naphthyl Amine (1°): Amino group at 1st position of naphthalene.

41. 2-Amino Naphthalene: (β-Naphthyl amine)

42. Diphenylamine (2°): $(C_6H_5)_2NH$

43. o, m, p Nitro Aniline

44. Acetamide: $CH_3CONH_2$

45. Succinimide: Cyclic imide of succinic acid.

46. Phthalimide: Cyclic imide of phthalic acid.

47. Sodium Succinimide

48. Acetyl Acetone (2,4-Pentanedione): $CH_3-C(=O)-CH_2-C(=O)-CH_3$

49. Ethyl Aceto Acetate (EAA): $CH_3-C(=O)-CH_2-C(=O)OC_2H_5$

50. Diethyl Malonate (DEM): $C_2H_5O-C(=O)-CH_2-C(=O)OC_2H_5$

51. Nitromethane: $CH_3-NO_2$

52. Methyl Cyanide / Acetonitrile: $CH_3-CN$

53. Triphenyl Methane (TPM): $(C_6H_5)_3CH$

54. Guanidine: $H_2N-C(=NH)-NH_2$

55. Urea: $H_2N-C(=O)-NH_2$

56. Thiourea: $H_2N-C(=S)-NH_2$

57. Toluene: $C_6H_5CH_3$

58. Cyclohexanone: $C_6H_{10}O$

59. 2,6-Dimethyl-4-Pyrone

60. Picolinic Acid: Pyridine-2-carboxylic acid.

61. Acetonitrile: $CH_3-C\equiv N$

62. Trimethyl Ammonium Chloride: $[(CH_3)_3NH]^+Cl^-$

63. 3,5-Dimethyl-4-Nitro Phenol (Weak acid)

64. 2,6-Dimethyl-4-Nitro Phenol (Strong acid)

65. N,N-Dimethyl-o-Toluidine (N,N-DMOT) (Very good base)