CHEM 2222 Chapter 22 Notes
CHEM 2222 Chapter 22 Notes Chem 212 - Organic Chemistry II
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This 21 page Class Notes was uploaded by annafen on Sunday May 29, 2016. The Class Notes belongs to Chem 212 - Organic Chemistry II at Vanderbilt University taught by Dr. Alissa Hare in Spring 2016. Since its upload, it has received 7 views. For similar materials see Organic Chemistry II in Chemistry at Vanderbilt University.
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Date Created: 05/29/16
22.1 Nomenclature Nomenclature • The C-OH is the 1 carbon for numbering and is implied • Substituents are cited in alphabetical order and are numbered to give the lowest numbers possible • However the hydroxy substituent has lower priority than acyl and carboxyl groups 1 22.2 Structure and Bonding Structure of Phenol • Phenol is planar with resonance delocalization of the phenol nonbonding electron pair into the ring 2 22.3 Physical Properties Physical Properties • The hydroxyl group has a large effect on the physical properties of phenol • The OH hydrogen bonds to other phenol molecules, leading to a higher mp/bp than benzene • Can also H-bond with water and therefore is soluble in water • Ortho-substituents that interfere with the H-bonding ability of phenol will lower the mp/bp and solubility in water 3 22.4 Acidity of Phenols Acidity • Phenols are more acidic than aliphatic alcohols • The pK foa most phenols is ~10 • The benzene ring is slightly electron withdrawing and stabilizes the conjugate base by the inductive effect • Most importantly the anion is stabilized by resonance delocalization which stabilizes the conjugate base 4 22.5 Substituent Effects on the Acidity of Phenols Substituent Effects • Electron-donating substituents make a phenol less acidic by destabilizing the phenoxide ion by resonance or the inductive effect • The lone pair electrons of the substituent donate e- density into the benzene ring • Electron-withdrawing substituents make a phenol more acidic by stabilizing the phenoxide ion through delocalization of the negative charge and through inductive effects 5 Substituent Effects • The influence of a substituent on phenol acidity is also dependent on its position relative to the –OH • The effect can be additive with increasing substituents increasing the effect on acidity 6 22.6 Sources of Phenols Synthesis of Phenols 7 Baeyer-Villiger oxidation • Oxidative cleavage of a C-C bond adjacent to a carbonyl group • Use peroxy acids like MCPBA 8 22.8 Reactions of Phenols: Electrophilic Aromatic Substitution Electrophilic Aromatic Substitution • A hydroxyl group is a very powerful activating substituent • It is a strong ortho/para director • Halogenation - Phenols are so highly activated that they often react with Br and Cl without a catalyst – like anilines 2 2 • Nitration 9 Electrophilic Aromatic Substitution • Sulfonation • Friedel-Crafts alkylation – less basic than anilines so you don’t get the AlCl complex 3 • Friedel-Crafts acylation – can get O-acylation 10 22.9 Acylation of Phenols Acylation • Acyl chlorides and acid anhydrides react with phenols at the aromatic ring or the hydroxyl oxygen – depending on whether AlCl is present 3 • If aluminum chloride is present, Friedel-Crafts occurs • If absent, then O-acylation occurs 11 22.10 Carboxylation of Phenols Preparation of Aspirin • Synthesis of salicyclic acid - The Kolbe–Schmitt reaction • Synthesis of aspirin 12 22.11 Preparation of Aryl Ethers Williamson Ether Synthesis • Phenols react with 1º and 2º alkyl halides to yield ethers via the Williamson synthesis 13 22.12 Cleavage of Aryl Ethers by Hydrogen Halides Cleavage of Aryl Ethers • Cleavage of alkyl aryl ethers by hydrogen halides yields an alkyl halide and a phenol as the final products 14 22.13 Claisen Rearrangement of Allyl Aryl Ethers Claisen Rearrangement • Thermal rearrangement of an aryl allyl ether to an ortho-allyl phenol • The Claisen rearrangement involves a concerted, pericyclic mechanism, which is related to the Diels-Alder reaction 15 Claisen Rearrangement • A sigmatropic rearrangement is characterized by a transition state in which a 16 22.14 Oxidation of Phenols: Quinones Quinones • Phenols are more easily oxidized than alcohols • Occurs by two sequential 1 e transfers 17 22.15 Spectroscopic Analysis of Phenols Infra Red Spectroscopy • The O ⎯ H stretching is in the region and the peak due to C ⎯ O stretching appears around 18 UV-vis Spectroscopy and Mass Spectrometry • The pH of the solution affects the chromophore as it does for aniline • The molecular ion is often the major peak in the mass spectra of phenols 19 NMR Spectroscopy • 1H NMR - The range for the –OH proton is δ 4–12 with the exact chemical shift depending on concentration, solvent, and temperature • 13C NMR - The ⎯ OH group deshields the carbon to which it is attached by about 25 ppm, while shielding the ortho carbon by about 14 ppm • This is similar for aryl ethers 20
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