Nucleophilic Aromatic Substitution of Benzyne
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The document discusses the elimination-addition mechanism of nucleophilic aromatic substitution involving benzyne. Aryl halides undergo substitution when treated with strong bases like KNH2 or NH3 at -33°C. The new substituent attaches to either the carbon that bore the leaving group or the adjacent carbon. The mechanism proceeds in three steps: 1) formation of the highly reactive intermediate benzyne, 2) relief of angle strain in benzyne, and 3) addition of the nucleophile to form the substitution product. Benzyne is a reactive dienophile that gives Diels-Alder adducts when generated in the presence of conjugated dienes. Other methods like treating 1-
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Nucleophilic Aromatic Substitution of Benzyne
- 1. Presentation By Aadil Ali Wani The Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution: Benzyne
- 2. Cl NH2 Aryl Halides Undergo Substitution When Treated With Very Strong Bases KNH2, NH3 –33°C (52%)
- 3. 2 NH3 –33°C Regiochemistry new substituent becomes attached to either the carbon that bore the leaving group or the carbon adjacent to it CH3 CH3 CH3 Br NaNH2, NH + NH2
- 4. + 2 Regiochemistry new substituent becomes attached to either the carbon that bore the leaving group or the carbon adjacent to it CH3 CH3 CH3 NaNH2, NH3 –33°C NH Br NH2
- 5. Regiochemistry CH3 NH2 NaNH2, NH3 –33°C CH3 CH3 CH3 NH2 + + NH2 NH2
- 6. Same result using 14C label * Cl KNH2, NH3 –33°C NH2 * + * NH2 (52%) (48%)
- 7. Cl • •• • NH2•H H Mechanism Step 1 H H •• • – •• H
- 8. – ••• Cl•H Cl • • •H • NH2•H H H 2 •• Mechanism Step 1 H H • •• • •• •• – •• H NH H H compound formed in this step is called benzyne
- 9. Benzyne H H H H Benzyne has a strained triple bond. It cannot be isolated in this reaction, but is formed as a reactive intermediate.
- 10. • NH2• Mechanism Step 2 H H – •• H H
- 11. •• • NH2• NH2H H Mechanism Step 2 H H H – H – •• •• H H Angle strain is relieved. The two sp-hybridized ring carbons in benzyne become sp2 hybridized in the resulting anion.
- 12. •• •• •• Mechanism Step 3 H H NH2 H – H NH2 H
- 13. NH2• HH NH2 H• •• •••• Mechanism Step 3 – •• •• H H H – H NH2 H NH2 H H
- 14. reaction of with NaOH Hydrolysis of Chlorobenzene 14C labeling * Cl indicates that the high- temperature NaOH, H2O 395°C chlorobenzene OH goes via benzyne. * + * OH (43%) (54%)
- 15. Diels-Alder Reactions of Benzyne
- 16. Other Routes to Benzyne Benzyne can be prepared as a reactive intermediate by methods other than treatment of chlorobenzene with strong bases. Another method involves loss of fluoride ion from the Grignard reagent of 1-bromo-2- fluorobenzene.
- 17. Mg, THF • •F •• F •• • • Other Routes to Benzyne Br MgBr •• heat •• + FMgBr
- 18. Benzyne as a Dienophile Benzyne is a fairly reactive dienophile, and gives Diels-Alder adducts when generated in the presence of conjugated dienes.
- 19. + F Benzyne as a Dienophile Br Mg, THF heat (46%)