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Organic Chemistry II - Phenols and Aryl Halides

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    Study Guide Organic Chemistry II – Phenols and Aryl Halides 1. Reactions of Phenolic Hydrogen Phenols show acidic behavior because the oxygen atom can release its hydrogen relatively easily . This acidity is responsible for many of the characteristic reactions of phenols. In this section, we’ll look at the most common reactions that occur due to the acidic nature of the phenolic – OH group . Why Phenols Are Acidic In phenols, the hydrogen attached to oxygen can be removed more easily than in alcohols. When this hydrogen is lost: • A phenoxide ion is formed • The negative charge is stabilized by the aromatic ring This stability makes phenols acidic enough to react with bases. 1. Reactions with Bases Because phenols are acidic, they react readily with strong bases to form salts . What Happens • The base removes the acidic hydrogen from phenol • A phenoxide salt is produced • Water is formed as a by - product Example • Phenol + sodium hydroxide → sodium phenoxide + water This reaction is often used to confirm the acidic nature of phenols .

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    Study Guide 2. Esterification of Phenol Phenols can react with certain acid derivatives to form esters . This process is known as esterification . Reagents Used Phenols form esters when reacted with: • Acid anhydrides • Acid chlorides

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    Study Guide Key Point • Phenols do not react easily with carboxylic acids • However, they react readily with more reactive acid derivatives Examples • Phenol + acetic anhydride → phenyl acetate • Phenol + acetyl chloride → phenyl acetate + HCl These reactions are useful for preparing aromatic esters . 3. Williamson Ether Synthesis (from Phenol) Phenols can also be converted into ethers using the Williamson ether synthesis , which follows an SN mechanism . Step - by - Step Process 1. Phenol reacts with a strong base (such as NaOH) to form sodium phenoxide 2. Sodium phenoxide reacts with an alkyl halide 3. An ether is formed Example • Phenol → sodium phenoxide → anisole (phenyl methyl ether) This method is widely used to prepare aryl ethers .

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    Study Guide Why These Reactions Matter • Acid – base reactions show the acidity of phenols • Esterification produces useful aromatic esters • Williamson synthesis allows formation of ethers from phenols • Together, these reactions highlight the chemical versatility of phenols Key Takeaway • Phenols are acidic due to the ease of hydrogen loss from the – OH group • Phenols react with bases to form phenoxide salts • Acid – base reactions confirm the acidic nature of phenols • Phenols form esters with acid anhydrides and acid chlorides • Phenols do not esterify easily with carboxylic acids • Phenols can form ethers via the Williamson ether synthesis • Ether formation proceeds through an SN mechanism 2. Reactions of Phenolic Benzene Rings The hydroxyl ( – OH) group in phenol has a powerful effect on the benzene ring. It donates electron density into the ring, making the ring highly reactive toward electrophilic substitution reactions . This activating effect is so strong that many reactions of phenols occur without a catalyst , unlike reactions of benzene. Why Phenol Is So Reactive • The oxygen atom in the – OH group donates electrons to the ring • This increases electron density, especially at the ortho and para positions • As a result, electrophiles attack these positions more easily Because of this strong activation, phenols react faster and under milder conditions than benzene.

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    Study Guide 1. Halogenation of Phenols Phenols react readily with halogens such as bromine or chlorine. The extent of substitution depends on the reaction conditions . Typical Behavior • Substitution occurs mainly at ortho and para positions • Products may be mono - , di - , or tri - substituted Example: Bromination • When phenol reacts with aqueous bromine , all ortho and para positions are substituted • The product formed is 2,4,6 - tribromophenol

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    Study Guide Controlled Monobromination • Monobromination can be achieved by: o Carrying out the reaction at very low temperatures o Using carbon disulfide (CS ₂ ) as the solvent These conditions slow the reaction and limit substitution to one position. 2. Nitration of Phenol Phenol undergoes nitration much more easily than benzene.

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    Study Guide Reaction Conditions • Dilute nitric acid • Room temperature Products • A mixture of: o Ortho - nitrophenol o Para - nitrophenol The strong activating effect of the – OH group allows nitration under mild conditions. 3. Sulfonation of Phenol Sulfonation of phenol with concentrated sulfuric acid is temperature dependent and controlled by thermodynamics.

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    Study Guide Temperature Effects • At 25 °C : o An equilibrium mixture forms o The ortho product predominates • At 100 °C : o The equilibrium is disrupted o The para - hydroxybenzenesulfonic acid forms almost exclusively Higher temperature favors the more stable para product .

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    Study Guide 4. Kolbe Reaction (Kolbe – Schmitt Reaction) The Kolbe reaction involves the reaction of a phenoxide ion with carbon dioxide to form a carboxylate salt , which is then converted into a carboxylic acid. Reaction Steps 1. Phenol is converted to sodium phenoxide 2. Sodium phenoxide reacts with CO ₂ 3. Acid treatment produces a hydroxybenzoic acid Mechanism Highlights • The reaction proceeds through a carbanion intermediate • The electron - deficient carbon in CO ₂ is attracted to the electron - rich aromatic ring • The intermediate undergoes keto – enol tautomerization to give the final product

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    Study Guide This reaction is an important method for introducing a – COOH group into a phenolic ring. Key Takeaway • The – OH group in phenol strongly activates the benzene ring • Phenols undergo electrophilic substitution more easily than benzene • Substitution occurs mainly at ortho and para positions • Halogenation can give mono - , di - , or tri - substituted products • Aqueous bromine produces 2,4,6 - tribromophenol • Low temperature and CS ₂ allow monobromination • Dilute nitric acid gives ortho - and para - nitrophenols • Sulfonation is temperature controlled • Higher temperatures favor the para sulfonated product • The Kolbe reaction introduces a carboxyl group using CO ₂ • Kolbe reaction proceeds via a carbanion intermediate and tautomerization

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