Chapter 9
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Transcript of Chapter 9
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Chapter 9Phenols and Aryl Halides: Nucleophilic Aromatic Substitution
Lecture Notes in Chem. 206(Organic Chemistry II )Assoc. Prof. Joel R. Salazar
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Structure and Nomenclature of Phenols Phenols have hydroxyl groups bonded directly to a
benzene ring Naphthols and phenanthrols have a hydroxyl group bonded
to a polycyclic benzenoid ring
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Synthesis of Phenols Laboratory Synthesis
Phenols can be made by hydrolysis of arenediazonium salts
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Industrial Syntheses 1. Hydrolysis of Chlorobenzene (Dow Process)
Chlorobenzene is heated with sodium hydroxide under high pressure
The reaction probably proceeds through a benzyne intermediate
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2. Alkali Fusion of Sodium Benzenesulfonate Sodium benzenesulfonate is melted with sodium hydroxide
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3. From Cumene Hydroperoxide Benzene and propene are the starting materials for a three-step
sequence that produces phenol and acetone Most industrially synthesized phenol is made by this method
The first reaction is a Friedel-Crafts alkylation
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The second reaction is a radical chain reaction with a 3o benzylic radical as the chain carrier
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The third reaction is a hydrolytic rearrangement (similar to a carbocation rearrangement) that produces acetone and phenol A phenyl group migrates to a cationic oxygen group
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Reactions of Phenols as Acids Strength of Phenols as Acids
Phenols are much stronger acids than alcohols
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Phenol is much more acidic than cyclohexanol
Experimental results show that the oxygen of a phenol is more positive and this makes the attached hydrogen more acidic The oxygen of phenol is more positive because it is attached
to an electronegative sp2 carbon of the benzene ring Resonance contributors to the phenol molecule also make
the oxygen more positive
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Distinguishing and Separating Phenols from Alcohols and Carboxylic Acids Phenols are soluble in aqueous sodium hydroxide because of their relatively
high acidity Most alcohols are not soluble in aqueous sodium hydroxide A water-insoluble alcohol can be separated from a phenol by extracting the phenol into
aqueous sodium hydroxide
Phenols are not acidic enough to be soluble in aqueous sodium bicarbonate (NaHCO3) Carboxylic acids are soluble in aqueous sodium bicarbonate Carboxylic acids can be separated from phenols by extracting the carboxylic acid into
aqueous sodium bicarbonate
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Other Reactions of the O-H Group of Phenols
Phenols can be acylated with acid chlorides and anhydrides
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Phenols in the Williamson Ether Synthesis Phenoxides (phenol anions) react with primary alkyl
halides to form ethers by an SN2 mechanism
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Cleavage of Alkyl Aryl Ethers Reaction of alkyl aryl ethers with HI or HBr leads to
an alkyl halide and a phenol
Recall that when a dialkyl ether is reacted, two alkyl halides are produced
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Reaction of the Benzene Ring of Phenols (electrophilic aromatic substitution) Bromination
The hydroxyl group is a powerful activating group (ortho - para director)in electrophilic aromatic substitution and usually the tribromide is obtained Monobromination can be achieved in the presence of carbon
disulfide at low temperature
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Nitration Nitration produces o- and p-nitrophenol
Low yields occur because of competing oxidation of the ring
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Sulfonation Sulfonation gives mainly the the ortho (kinetic)
product at low temperature and the para (thermodynamic) product at high temperature
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The Kolbe Reaction Carbon dioxide is the electrophile for an electrophilic
aromatic substitution with phenoxide anion The phenoxide anion reacts as an enolate The initial keto intermediate undergoes tautomerization to
the phenol product Kolbe reaction of sodium phenoxide results in salicyclic
acid, a synthetic precursor to acetylsalicylic acid (aspirin)
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Aryl Halides and Nucleophilic Aromatic Substitution
Simple aryl and vinyl halides do not undergo nucleophilic substitution
Back-side attack required for SN2 reaction is blocked in aryl halides
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SN2 reaction also doesn’t occur in aryl (and vinyl halides) because the carbon-halide bond is shorter and stronger than in alkyl halides Bonds to sp2-hybridized carbons are shorter, and therefore
stronger, than to sp3-hybridized carbons
Resonance gives the carbon-halogen bond some double bond character
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Nucleophilic Aromatic Substitution by Addition-Elimination: The SNAr Mechanism Nucleophilic substitution can occur on benzene rings
when strong electron-withdrawing groups are ortho or para to the halogen atom The more electron-withdrawing groups on the ring, the
lower the temperature required for the reaction to proceed
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The reaction occurs through an addition-elimination mechanism A Meisenheimer complex, which is a delocalized carbanion,
is an intermediate The mechanism is called nucleophilic aromatic substitution
(SNAr)
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The carbanion is stabilized by electron-withdrawing groups in the ortho and para positions
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Nucleophilic Aromatic Substitution through an Elimination-Addition Mechanism: Benzyne Under forcing conditions, chlorobenzene can undergo
an apparent nucleophilic substitution with hydroxide Bromobenzene can react with the powerful base amide
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The reaction proceeds by an elimination-addition mechanism through the intermediacy of a benzyne (benzene containing a triple bond)
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When chlorobenzene labeled at the carbon bearing chlorine reacts with potassium amide, the label is divided equally between the C-1 and C-2 positions of the product This is strong evidence for an elimination-addition
mechanism and against a straightforward SN2 mechanism
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Sample problem Explain why the meta product is more stable
NaNH2/ NH3
========
CF3
Cl
CF3
NH2
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Benzyne can be generated from anthranilic acid by diazotization The resulting compound spontaneously loses CO2 and N2 to
yield benzyne The benzyne can then be trapped in situ using a Diels-Alder
reaction
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Reactions of the Side Chain of Alkylbenzenes Benzylic Radicals and Cations
When toluene undergoes hydrogen abstraction from its methyl group it produces a benzyl radical A benzylic radical is a radical in which the carbon bearing
the unpaired electron is directly bonded to an aromatic ring
Departure of a leaving group by an SN1 process from a benzylic position leads to formation of a benzylic cation
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Benzylic radicals and cations are stabilized by resonance delocalization of the radical and positive charge, respectively
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Halogenation of the Side Chain: Benzylic Radicals Benzylic halogenation takes place under conditions
which favor radical reactions Reaction of N-bromosuccinamide with toluene in the
presence of light leads to allylic bromination Recall N-bromosuccinamide produces a low concentration of
bromine which favors radical reaction
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Reaction of toluene with excess chlorine can produce multiple benzylic chlorinations
When ethylbenzene or propylbenzene react under radical conditions, halogenation occurs primarily at the benzylic position
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Alkenylbenzenes
Additions to the Double Bond of Alkenylbenzenes Additions proceed through the most stable
benzylic radical or benzylic cation intermediates
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Oxidation of the Side Chain Alkyl and unsaturated side chains of aromatic rings
can be oxidized to the carboxylic acid using hot KMnO4
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Reduction of Aromatic Compounds Aromatic rings are inert to catalytic hydrogenation under
conditions that reduce alkene double bonds Can selectively reduce an alkene double bond in the
presence of an aromatic ring
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Reduction of Aromatic Compounds
Reduction of an aromatic ring requires more powerful reducing conditions (high pressure or rhodium catalysts)
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Reduction of Aryl Alkyl Ketones
Aromatic ring activates neighboring carbonyl group toward reduction
Ketone is converted into an alkylbenzene by catalytic hydrogenation over Pd catalyst