Phenols - Acidity of phenols (Pka value)

• Introduction to phenols
• Definition and structure of phenols
• Comparison of phenols with alcohols
• Acidic nature of phenols
• Factors affecting acidity of phenols

Introduction to phenols

• Phenols are a class of organic compounds that contain hydroxyl (-OH) functional group attached to an aromatic ring.
• They are derived from benzene by the replacement of one hydrogen atom by hydroxyl group.
• The general formula of phenols is C6H5OH, where C6H5 represents the aromatic ring.

Definition and structure of phenols

• Phenols are organic compounds in which a hydroxyl group (-OH) is directly attached to an aromatic ring.
• They have a planar structure due to the delocalization of pi electrons in the aromatic ring.
• The presence of the hydroxyl group imparts unique properties to phenols, such as acidity and solubility in water.

Comparison of phenols with alcohols

• Phenols and alcohols both contain hydroxyl (-OH) functional group, but the placement of the -OH group differs.
• In alcohols, the hydroxyl group is attached to a saturated carbon atom, while in phenols, the hydroxyl group is attached directly to an aromatic ring.
• Phenols are more acidic than alcohols due to the stabilization of the phenoxide ion formed after ionization.

Acidic nature of phenols

• Phenols are acidic in nature due to the presence of a strongly electron-withdrawing aromatic ring.
• The oxygen atom of the hydroxyl group withdraws electron density from the aromatic ring, leading to the formation of a stabilized phenoxide ion.
• The formation of a stabilized phenoxide ion increases the tendency of phenols to donate a proton, making them acidic.

Factors affecting acidity of phenols

• Substituent effect: Electron-withdrawing groups increase the acidity of phenols by stabilizing the phenoxide ion. Electron-donating groups decrease the acidity.
• Resonance effect: The delocalization of electrons in the aromatic ring contributes to the stability of the phenoxide ion, increasing acidity.
• Inductive effect: Electron-withdrawing groups attached to the aromatic ring destabilize the phenoxide ion, decreasing acidity.
• Degree of ionization: The extent to which phenols ionize in water determines their acidity. Higher ionization leads to higher acidity.

Examples of phenols

• Phenol (C6H5OH): A widely used disinfectant and antiseptic.
• Cresol (C7H8O): Found in coal tar and used in the production of antioxidants and pharmaceuticals.
• Eugenol (C10H12O2): Present in clove oil and used as a local antiseptic and anesthetic.
• Bisphenol A (C15H16O2): Used in the production of plastics, such as polycarbonate and epoxy resins.

11. Factors affecting acidity of phenols

• Substituent effect:
  • Electron-withdrawing groups increase acidity (e.g., -NO2, -CN)
  • Electron-donating groups decrease acidity (e.g., -CH3, -OH)
• Resonance effect:
  • Delocalization of electron density contributes to stability of the phenoxide ion, increasing acidity
  • Examples: Resonance stabilization in phenol and cresol derivatives
• Inductive effect:
  • Electron-withdrawing groups attached to the aromatic ring destabilize the phenoxide ion, decreasing acidity
  • Examples: Meta-substituted phenols versus ortho/para-substituted phenols
• Degree of ionization:
  • Determines the extent of phenol ionization in water and influences acidity
  • Higher ionization leads to higher acidity
  • Equilibrium equation: C6H5OH ⇌ C6H5O- + H+

12. Substituent effect on acidity - Electron-withdrawing groups

• Electron-withdrawing groups increase acidity of phenols
• Examples of electron-withdrawing groups:
  • Nitro group (-NO2)
  • Cyanide group (-CN)
• Electron density is withdrawn from the aromatic ring, stabilizing the phenoxide ion
• The more electron-withdrawing the substituent, the more acidic the phenol

13. Substituent effect on acidity - Electron-donating groups

• Electron-donating groups decrease acidity of phenols
• Examples of electron-donating groups:
  • Methyl group (-CH3)
  • Hydroxyl group (-OH)
• Electron density is donated to the aromatic ring, destabilizing the phenoxide ion
• The more electron-donating the substituent, the less acidic the phenol

14. Resonance effect on acidity - Delocalization of electron density

• Resonance stabilization increases acidity of phenols
• Delocalization of electron density occurs in the aromatic ring
• Stabilizes the phenoxide ion by distributing the negative charge
• Example: Resonance structure of phenol
  • O^- resonates between carbon atoms in the ring, not solely attached to one carbon atom

15. Inductive effect on acidity - Electron-withdrawing groups on the aromatic ring

• Inductive effect decreases acidity of phenols
• Electron-withdrawing groups destabilize the phenoxide ion
• Example: Meta-substituted phenols versus ortho/para-substituted phenols
  • Meta-substituted phenols are less acidic than ortho/para-substituted phenols
  • Electron-withdrawing groups at the meta position have a greater destabilizing effect

16. Degree of ionization and acidity of phenols

• The degree of ionization affects the acidity of phenols
• Stronger acids have a higher degree of ionization
• Equilibrium equation: C6H5OH ⇌ C6H5O- + H+
• Higher ionization leads to higher acidity
• Acidity is quantified using pKa values (negative logarithm of the acid dissociation constant Ka)

17. Comparison of pKa values for phenols

• pKa values determine the relative acidity of phenols
• Lower pKa values indicate stronger acids
• Example: Comparison of pKa values for various phenols
  • Phenol: pKa = 9.99
  • Cresol: pKa = 10.18
  • Methyl phenols: pKa = 10.19-10.44
• Generally, more electron-withdrawing groups lead to lower pKa values and higher acidity

18. Applications of phenols in daily life

• Phenols are widely used in various industrial and daily life applications
• Examples of applications:
  • Disinfectants and antiseptics: Phenol, cresol, and their derivatives are used for sterilization purposes.
  • Pharmaceuticals: Some phenols have antibacterial and antifungal properties and are used in the formulation of drugs.
  • Plastics: Phenols, such as Bisphenol A, are used in the production of polycarbonate and epoxy resins.
  • Cosmetics: Phenolic compounds are used in skincare products for their antioxidant properties.

19. Safety precautions while handling phenols

• Phenols are toxic and can cause harm if not handled properly
• Safety precautions while handling phenols:
  • Wear appropriate protective equipment, such as gloves, goggles, and lab coats, to prevent direct contact with the skin and eyes.
  • Handle phenols in a well-ventilated area to avoid inhalation of fumes.
  • Store phenols in a cool, dry, and secure place away from sources of ignition.
  • Dispose of phenols according to proper hazardous waste disposal regulations.

20. Summary

• Phenols are organic compounds containing a hydroxyl group (-OH) directly attached to an aromatic ring.
• Phenols are more acidic than alcohols due to the stabilization of the phenoxide ion formed after ionization.
• Factors affecting the acidity of phenols include substituent effect, resonance effect, inductive effect, and degree of ionization.
• Substituents can either increase or decrease the acidity of phenols.
• Resonance and inductive effects contribute to the stability or instability of the phenoxide ion.
• The degree of ionization determines the acidity of phenols, with higher ionization indicating higher acidity.
• Phenols have various applications in disinfectants, antiseptics, pharmaceuticals, plastics, and cosmetics.
• Safety precautions should be followed when handling phenols to prevent harm.

21. Substituent effect on acidity - Examples:

• Electron-withdrawing groups increase acidity:
  • Nitro group (-NO2) in 2-nitrophenol (pKa = 7.15)
  • Cyanide group (-CN) in 2-cyanophenol (pKa = 8.95)
• Electron-donating groups decrease acidity:
  • Methyl group (-CH3) in 2-methylphenol (pKa = 10.11)
  • Hydroxyl group (-OH) in 2-hydroxyphenol (pKa = 10.20)

22. Resonance effect on acidity - Examples:

• Resonance stabilization increases acidity:
  • Phenol (C6H5OH) has a pKa value of 9.99 due to resonance delocalization in the aromatic ring.
  • Resonance structure of phenol shows the delocalization of negative charge.

23. Inductive effect on acidity - Examples:

• Inductive effect decreases acidity:
  • Meta-substituted phenols are less acidic than ortho/para-substituted phenols.
  • Electron-withdrawing groups at the meta position have a greater destabilizing effect.
  • Example: 2,4,6-trinitrophenol (picric acid) is less acidic than 2,4,6-trimethylphenol (mesitol).

24. Degree of ionization and acidity - Examples:

• Degree of ionization affects acidity:
  • Phenol has a pKa value of 9.99, indicating it is a weak acid.
  • Higher ionization leads to higher acidity.
  • Example: 2-naphthol (pKa = 9.48) has higher acidity than phenol.

25. Comparison of pKa values for different phenols:

• Phenol (C6H5OH): pKa = 9.99
• Cresol (C7H8O): pKa = 10.18
• 2-nitrophenol (C6H5NO3): pKa = 7.15
• 2-hydroxyphenol (C6H6O2): pKa = 10.20
• 2-methylphenol (C7H8O): pKa = 10.11

26. Effect of substituents on pKa values:

• Electron-withdrawing groups decrease pKa values (increase acidity).
• Electron-donating groups increase pKa values (decrease acidity).
• Example: 2-nitrophenol (pKa = 7.15) is more acidic than 2-methylphenol (pKa = 10.11).

27. Resonance and acidity - Phenolate ion:

• Resonance delocalization increases the stability of the phenolate ion.
• Phenol resonates between carbon atoms in the aromatic ring.
• Stabilization of the phenolate ion increases the acidity of phenols.

28. Inductive effect and acidity - Meta-substituted phenols:

• Electron-withdrawing groups at the meta position have a destabilizing effect on the phenolate ion.
• Example: 3-nitrophenol is less acidic than 4-nitrophenol due to the inductive effect of the nitro group.

29. Comparison of phenols and alcohols:

• Phenols are more acidic than alcohols due to the presence of the aromatic ring.
• Alcohols have protonation sites on the carbon atom, while phenols have it on the oxygen atom.
• Phenoxide ions are stabilized by resonance, increasing the acidity of phenols.

30. Summary:

• Substituents can affect the acidity of phenols. Electron-withdrawing groups increase acidity, while electron-donating groups decrease acidity.
• Resonance stabilization contributes to the acidity of phenols by stabilizing the phenoxide ion.
• Inductive effects can either increase or decrease acidity, depending on the position of the substituent.
• The degree of ionization determines the acidity of phenols, with higher ionization leading to higher acidity.
• Comparing pKa values helps determine the relative acidity of different phenols.
• Understanding these factors is essential for comprehending the acidity trends in phenols and their practical applications.