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Chapter 4 Alkynes

Chapter 4 Alkynes. 4.1 Nomenclature 4 .2 Structure of Alkynes 4.3 Reactions of alkynes 4.3.1 Acidity of Terminal Alkynes 4.3.2 Application of the Acidity of Terminal Alkynes in Organic Synthesis 4.3.3 Hydrogenation of Alkynes 1. Syn-addition of H 2 :

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Chapter 4 Alkynes

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  1. Chapter 4 Alkynes 4.1 Nomenclature 4.2 Structure of Alkynes 4.3 Reactions of alkynes 4.3.1 Acidity of Terminal Alkynes 4.3.2 Application of the Acidity of Terminal Alkynes in Organic Synthesis 4.3.3 Hydrogenation of Alkynes 1. Syn-addition of H2: Synthesis of cis-Alkyenes 2. Metal-Ammonia Reduction of Alkynes Synthesis of trans-Alkenes

  2. 4.3.4 Electrophilic Addition of Alkynes • a. Addition of HX • b. Hydration of Alkynes • c. Addition of X2 • d. Hydroboration and Oxidation of Alkynes • 4.4 Oxidation of Alkynes • 4.5 Preparation of Unsaturated • Hydrocarbons • 4.5.1 Preparation of alkenes • Dehydrohalogenation of alkyl halides • 2. Dehydration (脱水)of alcohols • 4.5.1 Preparation of Alkynes • Dehydrohalogenation of vicinal alkyl • dihalides • b. Alkylation of alkynide ions

  3. Suffix: ane yne Ethynyl (乙炔基) P134,4.13 Alkynes: Hydrocarbons containing a carbon-carbon triple bond. Terminal or mono-substituted alkynes Internal or disubstituted (内炔) 1. Numbering starts from the end near the first multiple bond. 4.1 Nomenclature 2. When there is a choice, double bonds receive lower numbers than triple bonds. 1-Butyne • Alkenyne (烯炔) 1-penten-4-yne (1-戊烯-4-炔) 3-Penten-1-yne (3-戊烯-1-炔)

  4. 2p 2p sp 2s 1s 2p 1s 2s 1s 4.2 Structure of Alkynes P18, 1.10 Acetylene (ethyne): spHybrid orbitals sp-hybridized state C: Exited state Ground state Hybri- dization Promotion of electron The shape of an sp hybrid obital: Each sp hybrid orbital: 50% s character 50% p character

  5. 180° An sp-hybridized carbon atom: The two sp hybride orbitals are oriented 180°away from each other, perpendicular to the two remaining p orbitals. Geometric structure of sp-hybridized C atoms is linear. In the mole. of acetylene, the formation of C-Cσbond : sp-sp overlap; The formation C-H σbonds: sp-1s overlap.

  6. The formation C-H σbonds: sp-1s overlap. The formation of two C-Cπbonds: 2py-2py overlap and 2pz-2pz overlap.

  7. H H πbond σbond Carbon-carbon triple bonds πbond

  8. pKa: 25 44 50 4.3 Reactions of alkynes 4.3.1 Acidity of Terminal Alkynes Acetylene: Carbanion Acidity: Conjugate base (共轭碱) pKa: 3.2 15.8 16-17 26 38

  9. Stronger base Weaker base Weaker acid Stronger acid 4.3.2 Application of the Acidity of Terminal Alkynes in Organic Synthesis 1. Preparation of Metal Alkynides P138,4.16 Acid-base reaction 2. Formation of C-C Bonds (Synthesis of Alkynes) Sodium alkynides are useful intermediates as nucleophile.

  10. : Identification of Primary halides Ch. P77 3. Formation of other metal alkynides Reagent: NH3 solution of AgNO3 or NH3 solution of CuCl Ch.P52(五)(1) 4.3.3 Hydrogenation of Alkynes

  11. Quinoline: • Syn-addition of H2: • Synthesis of cis-Alkyenes Lindlar catalyst (96%) Lindlar catalyst: Pd/ CaCO3, Pb(Ac)2-quinoline (喹啉) P-2 catalyst: Ch: P53

  12. Excess HX Alkenyl halide Alkyl dihalide 2. Metal-Ammonia Reduction of Alkynes Synthesis of trans-Alkenes 4.3.4 Electrophilic Addition of Alkynes • Reactivity: Alkynes < Alkenes • Regioselectivit: Follow Markov.’s Rule a. Addition of HX

  13. 1-Hexyne An-enol(烯醇) Transformation of Func- tional groups: b. Hydration of Alkynes 2-Hexanone Keto-enol tautomerism(酮式－烯醇式互变异构):

  14. c. Addition of X2 d. Hydroboration and Oxidation of Alkynes Ch: P67 6.4 Oxidation of Alkynes An internal alkynes: Identification of C-C triple bonds

  15. (84%) 85 ℃ 6.5 Preparation of Unsaturated Hydrocarbons 6.5.1 Preparation of alkenes 1. Dehydrohalogenation of alkyl halides: (脱卤化氢) Bromocyclohexane Cyclohexene (81%) 2. Dehydration (脱水)of alcohols Elimination

  16. 6.5.1 Preparation of Alkynes • Dehydrohalogenation of vicinal alkyl • dihalides Diphenylacetylene(85%) b. Alkylation of metal alkynide

  17. Problems to Chapter 4. Alkyne P146. 4.25(b) 4.26(b),(c) 4.29(c) 4.41 Show the reactions. 4.42 4.44 4.45(c) 4.48 4.52 4.56

  18. Additional problems: • 1.Predict the products from reaction of 2- • hexyne with the following reagents: • 2 equiv Br2 (b) 1equiv HBr (c) Excess HBr • (d) Li in NH3 (e) H2O, H2SO4, HgSO4 2. How would you carry out the following reaction?

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