Acidity of Phenols Handwritten Notes (Organic Chemistry 2)
Download comprehensive handwritten notes on Acidity of Phenols for Organic Chemistry 2 (B.Pharm, 3rd Semester). These notes define phenols and acidity, explain the resonance structure of phenols, and detail the significant effect of electron-acceptor and electron-donor substituents on their acidic nature. Access these essential chemistry notes as a PDF or view them online for free.
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Acidity of Phenols: A Detailed Exploration (Organic Chemistry 2, B.Pharm 3rd Sem)
This comprehensive set of handwritten notes delves into the critical topic of "Acidity of Phenols," a fundamental concept in Organic Chemistry 2 for B.Pharm students in their 3rd semester. Phenols are organic compounds characterized by a hydroxyl (-OH) group directly attached to an aromatic hydrocarbon group, typically a benzene ring. Unlike simple aliphatic alcohols, phenols exhibit a distinct acidic character, which is significantly influenced by their unique molecular structure and the nature of any substituents present on the aromatic ring.
Defining Acidity and Phenols
The notes begin by clearly defining "What is phenols?" and establishing their basic structure. Following this, the concept of "acidity" is defined in the context of organic chemistry, specifically focusing on the ability of a compound to donate a proton (H⁺). The "acidic nature" of a compound is determined by the stability of its conjugate base once the proton is released. Phenols are notably more acidic than aliphatic alcohols, although less acidic than carboxylic acids. This difference in acidity is central to understanding their chemical behavior.
Resonance Structure of Phenols
The heightened acidity of phenols is primarily attributed to the resonance stabilization of their conjugate base, the phenoxide ion. When phenol loses its proton, it forms a phenoxide ion (C₆H₅O⁻). The negative charge on the oxygen atom in the phenoxide ion is not localized but can be delocalized into the aromatic ring through resonance. The notes meticulously illustrate the "resonance structure of phenols" and, more importantly, the phenoxide ion. This delocalization distributes the negative charge over multiple atoms (oxygen and carbons at ortho and para positions of the benzene ring), thereby stabilizing the phenoxide ion. A more stable conjugate base implies a stronger acid, explaining why phenols are acidic.
Effect of Substitution on Acidity of Phenols
The presence of substituents on the benzene ring significantly impacts the acidity of phenols. The "effect of substitution on acidity of phenols" is a crucial aspect discussed in detail. Substituents can either increase or decrease the stability of the phenoxide ion, thereby altering the acidity of the parent phenol.
- Define Electron Acceptor Group (Electron-Withdrawing Groups - EWG): These groups pull electron density away from the aromatic ring, typically through inductive effect (-I) or resonance effect (-M). When an electron-acceptor group is present on the phenol ring, particularly at ortho and para positions, it helps to further delocalize and stabilize the negative charge on the phenoxide ion. This increased stabilization makes the conjugate base more stable and, consequently, enhances the acidity of the phenol. Examples include nitro (-NO₂), cyano (-CN), carboxyl (-COOH), and halogen (-Cl, -Br). For instance, nitrophenols are considerably more acidic than phenol itself, with p-nitrophenol being a stronger acid than o-nitrophenol due to more effective resonance stabilization.
- Define Electron Donor Group (Electron-Releasing Groups - ERG): These groups push electron density into the aromatic ring, typically through inductive effect (+I) or resonance effect (+M). When an electron-donor group is present on the phenol ring, it increases the electron density within the ring and on the oxygen of the phenoxide ion. This destabilizes the phenoxide ion by intensifying the negative charge, making the conjugate base less stable and, consequently, decreasing the acidity of the phenol. Examples include alkyl groups (-CH₃, -C₂H₅) and alkoxy groups (-OCH₃). For instance, cresols (methylphenols) are less acidic than phenol.
The notes provide clear explanations and illustrative examples to demonstrate how the electronic nature and position of substituents profoundly influence the acidic strength of phenols. This comprehensive understanding of phenol acidity and substituent effects is vital for students to predict and explain the reactivity of these important organic compounds.
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