phenols - SUMMARY Introduction to Phenols Reactions of...

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Unformatted text preview: SUMMARY Introduction to Phenols Reactions of phenol via electropbilic aromatic substitution - Halogenation 0 Nitration ' Sulfonation 0 FriedeI-Craft alkylation 0 Friedel-Craft acylation Claisen Rearrangement Miscellaneous Reactions Phenol 1 Phenol 2 Phenol 3 Phenol 4 Phenols GENERAL INFORMATION Phenols 1 Phenols (Ar—OH) IntroductiOn: Phenols are compounds with a hydroxyl group (—OH) attached directly to a benzene ring. This class of compounds is abundant in nature; they can be found in vanilla beans, vitamin E, and petroleum. In addition, phenols and their derivatives are widely synthesized industrially to produce resins, adhesives, and food preservatives. Phenol, or carbolic acid, was one of the first antiseptics. Physical Properties: ' Phenols have higher melting and boiling points than arenes and aryl halides of equivalent molecular weight. Reason: Phenols are able to form intermolecular hydrogen bonds. 0 Depending on the substituent group, ortho-substituted phenols may have much lower melting and boiling points than their meta or para isomers. Reason: Some substituents (e.g., nitro suhstituents) in the ortho position can form intramolecular hydrogen bonds with the hydroxyl group of the phenol, thereby disrupting the intermolecular hydrogen bond. 0 Phenols are also more soluble in water than the corresponding arenes and aryl halides. Reason: Phenols are able to form hydrogen bonds with water molecules. Chemical Properties: ‘ - The bond angle formed by C—O—H is about 109°, and the overall structure of phenol is planar. 0 Phenols are more acidic than alcohols of equivalent molecular weight. Compound: Alcohol < Phenol < Carboxylic Acid Acidity: Least =>=> Most Reason: After loss of the proton from the hydroxyl group, the negative charge remaining on the phenoxide oxygen is stabilized by the resonance. In the case of alcohols, the negative charge on the alkoxide ion is not stabilized by resonance. 0 Effect of electronrwithdrawing and -donating groups on the acidity of phenols: 1. In general, electron-donating groups (e.g., alkyl, alkoxy; see Benzenes 7) destabilize the phenoxide ion and thus decrease the acidity of the corresponding phenol. 2. Electron—withdrawing groups (e.g., halogen; see Benzenes 7) have the opposite effect. An electron- withdrawing group in the ortho or para (but not meta) positions can participate in resonance stabilization of the phenoxide anion and increase the acidity of the corresponding phenol. Groups such as nitro, acyl, cyano, and sulfonyl greatly increase the acidity of the corresponding phenol, Whereas other groups have much smaller effects. Nomenclature: Phenol refers to a benzene ring with an OH substituent. The carbon bearing the OH group is numbered 1, and other substituents receive the lowest numbers possible; the substituents are then named in alphabetical order: on 5 ’ 2 cnzcu3 5 3 Br 4 5-Brorno-2-ethylphenol Spectroscopy: H NMR: The hydroxyl proton in phenol has a characteristic absorption in the range of 3 = 12—15 ppm. IR: The O——H bond has a characteristic absorption at 3600 cm‘l. The C—O bond has an absorption in the range of 1200—1250cm‘1. REACTIONS OF PHENOLS Phenols 2 VIA ELECTROPHILIC AROMATIC S UBSTITUTION Keys: 1. Phenols readily undergo electrophilic aromatic substitution reactions in which the hydrogens at the ortho and para positions can be replaced by a halogen (—X), nitro group (—NOZ), sulfonyl group (—503), alkyl group (—R), or acyl group (—COR). 2. The powerful activating hydroxyl group in phenol promotes multiple electrophilic substitution reactions in some cases. Substitution at the ortho and para positions is particularly favored because the corresponding intermediates are more stable than those involved in meta substitution. 3. These substitution reactions have already been described in detail in Benzenes 2 through 6. Summary of Electrophilic Aromatic Substitution Reactions OH OH ' 0H Br Brg —> «9 Br 1. Halogenation (see Benzenes 2) Phenol ‘ m—Bromophenol p-Brornophenol 2. Nitration (see Benzenes 3) OH. OH OH mo2 _ HNO; ' —> + H1804 N02 Phenol m—Nitmphenol p-Nitmphenol 3. Sulfonation (sec Benzenes 4) OH OH OH scan so; —> + H2504 SOgH Phenol 2fiydroxylbcnzcnesulfonic 4-Hydroxylhcnzenesulfonic acid acid 4. Friedel-Craft alkylation (see Benzenes 5) 0H Cl 0H (EH3 OH \ CHCH; CH3CHCH3 —> + A103 Phenoi - Z-Isopmpylphenol /CHCH3 CH3 4ilsoprupylphen0] 5. Friedel-Craft acylation (see Benzenes 6) OH ‘ O OH (I? OH H ccna CH3CCI —> + AlClg Phenol n a-Hydmxyacelnphennne Hac /c\o p-Hydroxyacemphenone REACTIONS OF PHENOLS Phenols 3 CLAISEN REARRANGEMEN ll OH OH 1} NH omen: CH2 2} BICH2CH=CH2 —’ 3} heating Phenol o—Allylphenol Keys: 1. This reaction converts phenols to o—allylphenols. 2. The reaction can be broken down into two stages: 3. Conversion of phenols to ally] phenyl ethers (see Mechanism). b. Conversion of allyl phenyl ethers to o—allylphenols through a pericyclic mechanism involving the rearrangement of electrons in the transition state (see Mechanism). 3. Nearly all allyl phenol erhers can undergo the Claisen rearrangement reaction. Mechanism: Stage I 1. In a reaction with NaH, phenol is converted to a phenoxide ion. 2. In a fashion similar to the Williamson ether reaction (see Ether 2 for details), the phenoxide ion reacts with 3-brom0propene to form the allyl phenyl ether product. Stage II 3. Briefly, on heating, the allyl phenyi ether undergoes a pericyclic rearrangement where the C—0 bond is broken and a new C-C bond is formed simultaneously. The reaction proceeds through a six-membered ring transition state (not sh0wn in the figure below}. Tautomerization of hydrogen to oxygen completes the reaction. (IJHDC CH CH (-3 "FE H: 2 OH Na:0=’\/ 2 2 oj [CH ll“ l)NaH Q 2 Phenol Phenoxide Ally} phenol ether ion hear 0 CH2 H H 2H CH2/ rearrangement -\ H *dro en Gal ’ E w 0H CH2CH=CH2 rrAllylphenol REACTIONS OF PHENOLS Phenols 4 MISCELLANEOUS REACTIONS 1. Oxidation of phenol: Phenols can be readily oxidized to form quinones (2,5-cyclohexadiene-1,4-dione). A number of oxidizing agents can be used in this reaction, for instance, sodium dichromate or potassium nitrosodisulfonate [(KSO 3) 2N0]. OH 0 (KSO3),NO ___._——* H30 Phenol o Quinone 2. Formation of ethers: Phenols can behave like alcohols and undergo a Williamson ether reaction to generate ethers (see Ether: 2). OH 0 — CHchZCI-I3 K C0 + CHJCHacnzar e“;, Acetone Phenol l—Bromopropane Propyl phenyl ether ...
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This note was uploaded on 09/13/2009 for the course CHEM 12-636 taught by Professor Hubbard during the Spring '09 term at University of Georgia Athens.

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phenols - SUMMARY Introduction to Phenols Reactions of...

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