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Organic Chemistry II - Phenols and Aryl Halides - Document preview page 1

Organic Chemistry II - Phenols and Aryl Halides - Page 1

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Organic Chemistry II - Phenols and Aryl Halides

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Organic Chemistry II - Phenols and Aryl Halides - Page 1 preview imageStudy GuideOrganic Chemistry II–Phenols and Aryl Halides1. Reactions of Phenolic HydrogenPhenols showacidic behaviorbecause the oxygen atom canrelease its hydrogen relativelyeasily. 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 theacidic nature of thephenolic–OH group.Why Phenols Are AcidicIn phenols, the hydrogen attached to oxygen can be removed more easily than in alcohols. When thishydrogen is lost:•Aphenoxide ionis formed•The negative charge is stabilized by thearomatic ringThis stability makes phenols acidic enough to react with bases.1.Reactions with BasesBecause phenols are acidic, they react readily withstrong basesto formsalts.What Happens•The base removes the acidic hydrogen from phenol•Aphenoxide saltis produced•Water is formed as a by-productExample•Phenol + sodium hydroxide →sodium phenoxide + waterThis reaction is often used toconfirm the acidic nature of phenols.
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Organic Chemistry II - Phenols and Aryl Halides - Page 2 preview imageStudy Guide2.Esterification of PhenolPhenols can react with certain acid derivatives to formesters. This process is known asesterification.Reagents UsedPhenols form esters when reacted with:•Acid anhydrides•Acid chlorides
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Organic Chemistry II - Phenols and Aryl Halides - Page 3 preview imageStudy GuideKey Point•Phenolsdo not react easily with carboxylic acids•However, they react readily withmore reactive acid derivativesExamples•Phenol + acetic anhydride →phenyl acetate•Phenol + acetyl chloride →phenyl acetate + HClThese reactions are useful for preparingaromatic esters.3.Williamson Ether Synthesis (from Phenol)Phenols can also be converted intoethersusing theWilliamson ether synthesis, which follows anSN mechanism.Step-by-Step Process1.Phenol reacts with a strong base (such as NaOH) to formsodium phenoxide2.Sodium phenoxide reacts with analkyl halide3.Anetheris formedExample•Phenol → sodium phenoxide →anisole (phenyl methyl ether)This method is widely used to preparearyl ethers.
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Organic Chemistry II - Phenols and Aryl Halides - Page 4 preview imageStudy GuideWhy These Reactions Matter•Acid–base reactions show theacidity of phenols•Esterification producesuseful aromatic esters•Williamson synthesis allows formation ofethers from phenols•Together, these reactions highlight thechemical versatility of phenolsKey Takeaway•Phenols are acidic due to the ease of hydrogen loss from the–OH group•Phenols react with bases to formphenoxide salts•Acid–base reactions confirm theacidic natureof phenols•Phenols form esters withacid anhydrides and acid chlorides•Phenols do not esterify easily with carboxylic acids•Phenols can form ethers via theWilliamson ether synthesis•Ether formation proceeds through anSN mechanism2.Reactions of Phenolic Benzene RingsThehydroxyl (–OH) groupin phenol has a powerful effect on the benzene ring. It donates electrondensity into the ring, making the ringhighly reactive toward electrophilic substitution reactions.This activating effect is so strong that many reactions of phenols occurwithout a catalyst, unlikereactions 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 theortho and para positions•As a result, electrophiles attack these positions more easilyBecause of this strong activation, phenols react faster and under milder conditions than benzene.
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Organic Chemistry II - Phenols and Aryl Halides - Page 5 preview imageStudy Guide1.Halogenation of PhenolsPhenols react readily with halogens such as bromine or chlorine. The extent of substitution dependson thereaction conditions.Typical Behavior•Substitution occurs mainly atortho and para positions•Products may bemono-, di-, or tri-substitutedExample: Bromination•When phenol reacts withaqueous bromine, all ortho and para positions are substituted•The product formed is2,4,6-tribromophenol
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Organic Chemistry II - Phenols and Aryl Halides - Page 6 preview imageStudy GuideControlled Monobromination•Monobromination can be achieved by:oCarrying out the reaction atvery low temperaturesoUsingcarbon disulfide (CS₂)as the solventThese conditions slow the reaction and limit substitution to one position.2.Nitration of PhenolPhenol undergoes nitration much more easily than benzene.
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Organic Chemistry II - Phenols and Aryl Halides - Page 7 preview imageStudy GuideReaction Conditions•Dilute nitric acid•Room temperatureProducts•A mixture of:oOrtho-nitrophenoloPara-nitrophenolThe strong activating effect of the–OH group allows nitration under mild conditions.3.Sulfonation of PhenolSulfonation of phenol withconcentrated sulfuric acidistemperature dependentand controlled bythermodynamics.
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Organic Chemistry II - Phenols and Aryl Halides - Page 8 preview imageStudy GuideTemperature Effects•At25 °C:oAn equilibrium mixture formsoTheortho product predominates•At100 °C:oThe equilibrium is disruptedoThepara-hydroxybenzenesulfonic acidforms almost exclusivelyHigher temperature favors themore stable para product.
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Organic Chemistry II - Phenols and Aryl Halides - Page 9 preview imageStudy Guide4.Kolbe Reaction (Kolbe–Schmitt Reaction)TheKolbe reactioninvolves the reaction of aphenoxide ionwithcarbon dioxideto form acarboxylate salt, which is then converted into a carboxylic acid.Reaction Steps1.Phenol is converted tosodium phenoxide2.Sodium phenoxide reacts withCO₂3.Acid treatment produces ahydroxybenzoic acidMechanism Highlights•The reaction proceeds through acarbanion intermediate•The electron-deficient carbon in CO₂is attracted to theelectron-rich aromatic ring•The intermediate undergoesketo–enol tautomerizationto give the final product
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Organic Chemistry II - Phenols and Aryl Halides - Page 10 preview imageStudy GuideThis reaction is an important method for introducing a–COOH groupinto a phenolic ring.Key Takeaway•The–OH group in phenolstrongly activatesthe benzene ring•Phenols undergo electrophilic substitutionmore easily than benzene•Substitution occurs mainly atortho and para positions•Halogenation can give mono-, di-, or tri-substituted products•Aqueous bromine produces2,4,6-tribromophenol•Low temperature and CS₂allowmonobromination•Dilute nitric acid givesortho-and para-nitrophenols•Sulfonation istemperature controlled•Higher temperatures favor thepara sulfonated product•TheKolbe reactionintroduces a carboxyl group using CO₂•Kolbe reaction proceeds via acarbanion intermediateand tautomerization
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Document Details

University
Texas A&M University
Course
Organic Chemistry II
Subject
Chemistry

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