Acidity of phenols, effect of substituents on acidity
Phenols are organic compounds featuring one or more hydroxyl (-OH) groups directly bonded to an aromatic ring. Unlike alcohols, phenols are more acidic and possess higher boiling points due to stronger hydrogen bonding. They often exist as colorless liquids or white solids. Phenols are versatile substances with applications spanning disinfectants, antiseptics, industrial chemical production for resins and plastics, and even pharmaceutical components. However, it’s crucial to handle phenols with caution as they can be highly toxic and caustic. In this article we will see acidity of phenols, effect of substituents on acidity, qualitative tests, structure and uses of phenol, cresols, resorcinol, naphthols.
Acidity of phenols
Phenols are considered weak acids due to their ability to donate a proton (H+) in aqueous solutions. To understand their acidity, let’s consider the concept of conjugate acid-base pairs:
Brønsted-Lowry Acid-Base Theory
- A Brønsted-Lowry acid is a substance that can donate a proton (H+).
- The corresponding conjugate base is the species formed after the acid donates its proton.
- For example, when a phenol (ArOH) reacts with water (H2O), it donates a proton to form the phenoxide ion (ArO-) and hydronium ion (H3O+).
Phenol Equilibrium
Phenol (ArOH) + H2O <=> Phenoxide ion (ArO-) + Hydronium ion (H3O+) |
Resonance Stabilization in Phenoxide Ion
- The key factor contributing to the acidity of phenols is the stability of the phenoxide ion formed after proton donation.
- The negative charge on the phenoxide ion is delocalized (spread out) across the aromatic ring due to resonance.
- Resonance structures can be drawn to depict the delocalization of the negative charge, showing that the negative charge isn’t confined to a single oxygen atom.
- This delocalization stabilizes the phenoxide ion, making it energetically favorable for the phenol to donate a proton and become acidic.
Comparison to Alcohols
Compared to alcohols (R-OH), phenols are more acidic due to the resonance stabilization of the phenoxide ion.
- In alcohols, the negative charge on the conjugate base (alkoxide ion) is not delocalized and is localized on the oxygen atom.
- The lack of resonance stabilization makes alkoxide ions less stable compared to phenoxide ions.
- As a result, alcohols are less willing to donate a proton and are weaker acids than phenols.
Other Factors Affecting Acidity
While resonance stabilization is the primary factor, other factors can slightly influence the acidity of phenols:
- Electron-withdrawing groups (like Cl, Br, NO2) attached to the aromatic ring can increase the acidity of phenols by stabilizing the phenoxide ion further. These groups withdraw electron density away from the oxygen atom, making the negative charge on the phenoxide ion more spread out and stable.
- Electron-donating groups (like CH3, NH2) can decrease the acidity by making it less favorable to donate a proton. These groups donate electron density towards the oxygen atom, making the negative charge on the phenoxide ion more localized and less stable.
Effect of Substituents on Acidity of Phenols
The acidity of phenols is significantly influenced by the nature of substituents attached to the benzene ring.
Electron-Withdrawing Groups (EWGs)
Increase acidity: These groups pull electron density away from the oxygen atom in the phenoxide ion, stabilizing it. This makes it easier for the phenol to lose a proton, increasing its acidity.
Examples: -NO₂, -CN, -COOH, -SO₃H, -F, -Cl, -Br, -I
Positional effect: The effect is more pronounced when the EWG is at the ortho or para position due to resonance stabilization.
Electron-Donating Groups (EDGs)
Decrease acidity: These groups donate electron density to the oxygen atom in the phenoxide ion, destabilizing it. This makes it harder for the phenol to lose a proton, decreasing its acidity.
Examples: -CH₃, -OCH₃, -NH₂
Positional effect: The effect is more pronounced when the EDG is at the ortho or para position due to resonance stabilization.
Inductive Effect
- In addition to resonance, the inductive effect also plays a role.
- Electron-withdrawing groups can withdraw electron density through the sigma bonds, increasing acidity.
- Electron-donating groups can donate electron density through the sigma bonds, decreasing acidity.
Summary table
| Substituent | Effect on acidity |
| Electron-withdrawing (NO₂, CN, COOH, etc.) | Increases acidity |
| Electron-donating (CH₃, OCH₃, NH₂) | Decreases acidity |
Example
p-nitrophenol is more acidic than phenol due to the electron-withdrawing nature of the nitro group.
p-cresol is less acidic than phenol due to the electron-donating nature of the methyl group.
Key Points
- The strength of the substituent’s effect depends on its electron-withdrawing or donating ability.
- The position of the substituent on the benzene ring also influences acidity.
- Understanding the effect of substituents on phenol acidity is crucial for predicting the reactivity and properties of different phenolic compounds.
Qualitative Tests for Phenols
Phenols can be identified through several characteristic reactions:
Ferric Chloride Test
- Principle: Phenols react with ferric chloride to form colored complexes.
- Procedure: Add a few drops of ferric chloride solution to an aqueous solution of the phenol.
- Observation: Formation of a violet or blue coloration indicates the presence of phenol.
Bromine Water Test
- Principle: Phenols undergo electrophilic substitution with bromine water to form white precipitates of tribromophenol.
- Procedure: Add bromine water dropwise to an aqueous solution of the phenol.
- Observation: Decolorization of bromine water and formation of a white precipitate confirms the presence of phenol.
Libermann’s Nitroso Reaction
- Principle: Phenols react with nitrous acid to form nitroso compounds, which then undergo further reactions to produce colored products.
- Procedure: Treat phenol with nitrous acid followed by addition of sodium hydroxide.
- Observation: A deep blue color indicates the presence of phenol.
Litmus Test
- Principle: Phenols are weakly acidic and turn blue litmus paper red.
- Procedure: Dip blue litmus paper into the phenol solution.
- Observation: A color change from blue to red indicates the presence of phenol.
While carboxylic acids also show a positive litmus test, phenols differ in their reaction with sodium carbonate. Phenols do not produce effervescence with sodium carbonate, unlike carboxylic acids. These tests provide reliable qualitative evidence for the presence of phenolic groups in organic compounds.
Structure and uses of important phenol derivatives
Phenol
Structure
Phenol is an aromatic compound with a hydroxyl (-OH) group directly attached to a benzene ring.
Uses
- Disinfectant and antiseptic
- Production of plastics, resins, and nylon
- Intermediate in the synthesis of various organic compounds
- Used in the pharmaceutical industry for the production of aspirin, salicylic acid, and other drugs
- Employed as a preservative in some vaccines
Cresols
Structure
Cresols are methyl derivatives of phenol, with the methyl group (-CH3) attached to the benzene ring at the ortho, meta, or para positions.
Uses
- Disinfectants and antiseptics (more effective than phenol)
- Production of pesticides and herbicides
- Intermediate in the synthesis of dyes, pharmaceuticals, and antioxidants
- Used as a solvent and preservative
Resorcinol
Structure
Resorcinol is a benzene ring with two hydroxyl groups (-OH) attached at the meta positions.
Uses
- Antiseptic and antifungal agent
- Intermediate in the production of dyes, resins, and pharmaceuticals
- Used in the treatment of skin conditions like psoriasis and eczema
- Employed as a sunscreen and antioxidant
Naphthols
Structure
Naphthols are derived from naphthalene, a fused aromatic hydrocarbon, with a hydroxyl group attached to one of the rings.
There are two isomers: 1-naphthol and 2-naphthol.
Uses
- Dyes and pigments
- Intermediate in the synthesis of various organic compounds
- Used as antioxidants and stabilizers
- Employed in the production of pharmaceuticals and rubber chemicals
Note: All these compounds exhibit varying degrees of toxicity and require careful handling.
Summary
Phenols are compounds with a hydroxyl group attached to a benzene ring. They exhibit weak acidity due to resonance stabilization of the phenoxide ion. Electron-withdrawing groups enhance acidity while electron-donating groups reduce it. Phenols are detected by ferric chloride test (violet color), bromine water test (white precipitate), and Libermann’s nitroso reaction (blue color). Phenol itself is a disinfectant, used in plastics, and as a precursor for aspirin. Cresols are more potent disinfectants. Resorcinol finds use in medicine and as an intermediate. Naphthols are crucial for dye production and other organic syntheses. These compounds, while useful, should be handled carefully due to their toxicity.
If you have any questions, suggestions or want to contribute, feel free to reach us at pharmacareers123@gmail.com
For more regular updates you can visit our social media accounts,
Instagram: Follow us
Facebook: Follow us
WhatsApp: Join us
Telegram: Join us