310_18 - Organic Organic Lecture Series CH 310 N LECTURE 18...

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Unformatted text preview: Organic Organic Lecture Series CH 310 N LECTURE 18 Textbook Assignment: Chapter 22 Reactions of Benzene Homework (for credit): Problem Set – 6 Posted Today’s Topics: Acidity of phenols; e- donating & withdrawing groups Notice & Announcements: EXAM 2: Pick up after class 1 Organic Lecture Series Benzene & Aromaticity Chapter 21 2 Organic Organic Lecture Series Acidity of Phenols – the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion: 3 Organic Lecture Series Acidity of Phenols Alkyl and halogen substituents effect acidities by inductive effects: – alkyl groups are electron-releasing – halogens are electron-withdrawing 4 Acidity of Phenols Organic Organic Lecture Series – nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect 5 Organic Lecture Series Acidity of Phenols – part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect – in addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge 6 Organic Organic Lecture Series Acidity of Phenols • Phenols are weak acids and react with strong bases to form water-soluble salts – water-insoluble phenols dissolve in NaOH(aq) 7 Organic Lecture Series Acidity of Phenols – most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3 8 Organic Organic Lecture Series Alkyl-Aryl Ethers • Alkyl-aryl ethers can be prepared by the Williamson ether synthesis – but only using phenoxide salts and haloalkanes – haloarenes are unreactive to SN2 reactions X + RO - N a + no reaction 9 Alkyl-Aryl Ethers Organic Lecture Series The following two examples illustrate 1. the use of a phase-transfer catalyst 2. the use of dimethyl sulfate as a methylating agent 10 Benzylic Oxidation Organic Organic Lecture Series • Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4 – halogen and nitro substituents are also unaffected by these reagents – an alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group 11 Not responsible for mechanism Benzylic Oxidation Organic Lecture Series – if there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group H3 C CH 3 K2 Cr 2 O 7 1,4-Dimethylbenzene ( -xylene) p Not responsible for mechanism H2 SO4 O HOC O COH 1,4-Benzenedicarboxylic acid (terephthalic acid) 12 Organic Organic Lecture Series Benzylic Chlorination • Chlorination (and bromination) occurs by a radical mechanism 13 Organic Lecture Series Benzylic Bromination • Bromination can be conducted with NBS: 14 Organic Organic Lecture Series Benzoyl Peroxide as a Radical initiator 15 Organic Lecture Series Benzylic Reactions • Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates – the benzyl radical is a hybrid of five contributing structures 16 Benzylic Halogenation Organic Organic Lecture Series – benzylic bromination is highly regioselective – benzylic chlorination is less regioselective 17 Organic Lecture Series Reactions of Benzene & Its Derivatives Chapter 22 18 Organic Organic Lecture Series Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: 19 Organic Lecture Series Reactions of Benzene Sulfonation: H + SO3 H2 SO4 SO3 H Benzenesulfonic acid Alkylation: H + RX A lX 3 R + HX An alkylbenzene Acylation: O H + RC X O A lX 3 CR + H X An acylbenzene 20 Organic Organic Lecture Series Carbon-Carbon Bond Formations: 21 Organic Lecture Series Electrophilic Aromatic Substitution • Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile • In this section: – several common types of electrophiles – how each is generated – the mechanism by which each replaces hydrogen 22 ...
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