Phenols - Organic Chemistry - Lecture Notes, Study notes of Organic Chemistry

Download Phenols - Organic Chemistry - Lecture Notes and more Study notes Organic Chemistry in PDF only on Docsity! 1     Phenols Nomenclature The –OH group takes precedence as the parent phenol. OH phenol OH CH3 meta-cresol OH CH3 Cl 1 2 3 45 6 5-chloro-2-methylphenol Carboxyl and acyl groups take precedence over the –OH group. C O OH HO p-hydroxybenzoic acid The –OH group is a strong electron-donating group through the lone pair. O H O H O H O H Phenol can hydrogen bond with H2O and other phenol groups. O H O H H O Hb.p. 132 °C H O H docsity.com 2 Ortho-nitrophenol can engage in intramolecular hydrogen bonding and has a lower boiling point and lower water solubility than either the meta or para-isomers because there is less intermolecular hydrogen bonding and less hydrogen bonding with water. O H N O O ortho-nitrophenol intramolecular hydrogen bond O H N OO O H N O OH O H H O H para-nitrophenol Phenols are much more acidic than regular alcohols. It is completely deprotonated by sodium hydroxide. O HCH3CH2 pKa 16 + Na OH OCH3CH2 + H2O Keq ~ 1 O H + Na OH O + H2O Keq 105.7 pKa ~10 pKa 15.7 This is due to the fact that the resulting anion is stabilized by resonance donation of the lone pair onto the benzene ring. O O O O But it is not a strong enough acid to be significantly ionized by sodium bicarbonate. O H + Na+ -O C O OH O + HO C O OH pKa ~6 pKa 10 This allows separation of a mixture of phenol, benzoic acid and toluene in diethyl ether using simple acid-base chemistry. The mixture is first treated with a solution of aqueous sodium bicarbonate. This is will deprotonate the benzoic acid, making it soluble in the docsity.com 5 Sulfonation gives mainly the para-isomer. OH H2SO4 100 °C OH SO3H major OH + SO3H minor Friedel-Crafts alkylation gives mainly the para-isomer. OH (CH3)3CCl AlCl3 OH C CH3CH3 CH3 major + OH C minor CH3 CH3 CH3 Friedel-Crafts acylation also gives mainly the para-isomer. OH CH3CH2CH2 C O Cl AlCl3 OH CO CH2CH2CH3 major (74%) + OH C O CH2CH3CH3 minor (16%) So, in the presence of aluminum trichloride, we see acylation at carbon. In the absence of aluminum trichloride, we see acylation at oxygen. OH CH3CH2CH2 C O Cl O C O CH2CH2CH3 With anhydrides, acid activation is used and acylation also occurs on oxygen. docsity.com 6 OH CH3CH2 C O O O C O CH2CH3 C O CH2CH3 H2SO4 + HO C O CH2CH3 Acylation can also be done in basic conditions with anhydrides. Treatment of the phenol with sodium hydroxide deprotonates the phenol and makes the oxygen a better nucleophile. OH NaOH H2O O CH3CH2 C O O C O CH2CH3 O C CH3CH2 O O O CH2CH3 O C O CH2CH3 O C O CH2CH3 + Aryl Ethers For synthesis of aryl ethers, the phenol must be the nucleophile since there is no nucleophilic substitution at sp2 carbons. OH NaOH H2O O Br O If there is a strong electron withdrawing group, nucleophilic aromatic substitution can occur through an addition-elimination mechanism. F NO2 NaOCH3 HOCH3 NO2 F OCH3 OCH3 NO2 Cleavage of aryl ethers occurs only on the alkyl side of the ether since there is no nucleophilic substitution at sp2 carbons. O CH2CH3 H Br O CH2CH3 H Br O H + CH3CH2Br docsity.com