Organic Lecture 3/5

Physical Properties and Reactivity of Phenols

• Influence of Substituents:

  • The nitro group (NO2) has a significant effect on acidity and reactivity compared to other groups.

  • Groups like carboxylic acids may require removal of water for reactions to proceed efficiently.

Electrophilic Aromatic Substitution (EAS) Reactions

• Mechanism Overview:

  • Reactions are acid-catalyzed (e.g., sulfuric acid, H2SO4).

  • Water removal might be necessary to shift equilibrium to favor product formation.

• Halogenation with Bromine Water:

  • Phenols easily react with bromine in water, demonstrating high reactivity as an activated benzene ring.

  • Reaction must be done in a polar solvent (like water) to stabilize the bromide ion (Br-).

  • Multiple additions can occur, confirming the presence of excess phenol.

• Nitration:

  • Using concentrated nitric acid will lead to multiple substitutions; dilute nitric acid can limit this to one substitution.

  • The hydroxyl (-OH) group acts as a strong electron-donor, enhancing reactivity.

Sulfonation

• Position Dependence on Temperature:

  • At low temperatures, sulfonyl group (-SO3H) prefers ortho substitution.

  • At high temperatures, para substitution is favored.

  • Sulfonyl group acts as a meta director and deactivator due to positive charge on sulfur.

• Regeneration and Removal:

  • Sulfonyl group can be removed using diluted sulfuric acid, demonstrating phenol's high reactivity.

Functional Group Tests

• Testing for Presence of Phenols:

  • Bromine water is a specific test for phenols, indicating their reactive nature in electrophilic substitution.

  • The reaction proceeds only in water, confirming phenol presence based on consistent reaction outcomes.

Acylation Reaction of Phenols

• Mechanism of O-Acylation:

  • Acid halides or anhydrides react with phenols to form esters via O-acylation (similar to Fischer esterification).

  • Lewis acids can facilitate acylation, changing reaction mechanism to involve EAS type reaction for electrophilic substitutions.

Diazotization and Azo Dye Formation

• Formation of Azo Compounds:

  • A diazonium ion reacts with phenol to form azo compounds, useful as dyes.

  • Modification of the phenolic structure allows tailoring of dye colors.

  • Azodyes are prevalent in commercial applications, though not as commonly used in textiles due to fading issues.

Hydroxycarbonyl Formation via Carboxylation

• Phenoxide Ion Formation and CO2 Reaction:

  • Phenol is converted to phenoxide ion using NaOH.

  • Phenoxide ion then reacts with CO2 gas under pressure to form carboxylic acids.

Reimer-Tiemann Reaction

• Mechanism Overview:

  • Reagent: chloroform and NaOH; forms ortho-substituted phenols upon heating.

  • The EAS type mechanism yields carbonyl products that restore aromaticity.

Oxidation and Reduction of Phenols

• Oxidation Resistance:

  • Phenols resist oxidation, but di-phenols can be oxidized to diketones under strong conditions.

• Mild Reduction Techniques:

  • Sodium thiosulfate (Na2S2O4) serves as a mild reducing agent for phenols, utilized in applications such as photography.

Spectroscopic Analysis of Phenols

• Infrared (IR) Spectroscopy Peaks:

  • Key peaks around 3200 cm-1 indicate -OH stretch; distinguish from carboxylic acids (which show broader peaks).

• Nuclear Magnetic Resonance (NMR) Peaks:

  • -OH protons show variable shifts due to hydrogen bonding; deuterium exchange can help confirm presence.

  • C-13 NMR indicates downfield shifts for carbons attached to oxygen due to deshielding effects.