tailieunhanh - Lecture Organic chemistry - Chapter 22: What does the benzene ring do to its substituents?

Chapter 22 - What does the benzene ring do to its substituents? After attending lectures, reading the text and working assigned problems related to this chapter, students will have acquired the following knowledge and abilities: Given the IUPAC name of a phenol derivative, be able to draw its structure; know how resonance influences the stability of benzylic cations, anions, and radicals as compared to similar non-benzylic species; be able to apply a benzyl ether as a protecting group in multi-step synthesis; given a series of phenols, be able to rank their relative acidity. | Chapter 22: What does the benzene ring do to its substituents? Benzylic resonance for +/./- All stabilized by the overlapping p orbitals Benzylic radicals are reactive intermediates in the α-halogenation of alkylbenzenes Therefore: Benzylic halogenation No EAS (Which needs FeBr3) Mechanism of benzylic halogenation Substitution occurs only at the benzylic position: Attack on the benzene destroys aromaticity! The two propagation steps Benzylic cation (SN1 reaction) Mechanism: Octet! Needs e-pushers; parent is too slow SN2 is accelerated Transition state is delocalized SN sp2 Benzylic Acidity: Resonance in Benzylic Anions Compare CH2=CH-CH3 pKa ~ 40 Benzylic Oxidation-Reduction or Na2Cr2O7, H+ C-C bond broken via: Can be stopped here (Recall electrophilic aromatic substitution: alkyl acyl) Neutral, mild Protecting group Discarded Deprotected Untouched Benzylic hydrogenolysis: Unique! Compare tert-Bu ethers as protecting groups, need acid for deprotection Compare allylic oxidation wth . | Chapter 22: What does the benzene ring do to its substituents? Benzylic resonance for +/./- All stabilized by the overlapping p orbitals Benzylic radicals are reactive intermediates in the α-halogenation of alkylbenzenes Therefore: Benzylic halogenation No EAS (Which needs FeBr3) Mechanism of benzylic halogenation Substitution occurs only at the benzylic position: Attack on the benzene destroys aromaticity! The two propagation steps Benzylic cation (SN1 reaction) Mechanism: Octet! Needs e-pushers; parent is too slow SN2 is accelerated Transition state is delocalized SN sp2 Benzylic Acidity: Resonance in Benzylic Anions Compare CH2=CH-CH3 pKa ~ 40 Benzylic Oxidation-Reduction or Na2Cr2O7, H+ C-C bond broken via: Can be stopped here (Recall electrophilic aromatic substitution: alkyl acyl) Neutral, mild Protecting group Discarded Deprotected Untouched Benzylic hydrogenolysis: Unique! Compare tert-Bu ethers as protecting groups, need acid for deprotection Compare allylic oxidation wth MnO2 Benzenol (Phenol) Keq = 1013 Nomenclature (Note: Phenol is an enol!) Phenol ethers: Alkoxybenzenes Phenoxy (from “phenyloxy”) . Methoxybenzene (Anisole) Natural products 11/6/2014 10 Acidity (Enolate!) Substituent effects: Inductive + resonance sp2 Multiple resonating e-withdrawing substituents increase the acidity to that of mineral acids Preparation of Phenols Problem with EAS: However, when a leaving group is on the ring: Nucleophilic aromatic substitution is possible! We learned earlier that SN2 is not possible on haloalkenes. How does aromatic nucleophilic substitution work? Electrophilic aromatic substitution needs “+OH” equivalent: e-sextet. Difficult to generate, problem is unsolved. : : When electron withdrawing groups are present, nucleophilic aromatic addition–elimination takes place. Without e-withdrawing groups: harsh conditions enforce an elimination-addition sequence through reactive benzyne intermediate. With super L = N2, phenyl cation intermediates. And then .

• Hoá học
• Organic chemistry
• Lecture Organic chemistry
• Hóa học hữu cơ
• Bài giảng Hóa học hữu cơ
• Benzene substituents
• Benzylic halogenation