Lecture Organic chemistry - Chapter 13: Alkynes: The C≡C Triple bond. This chapter presents the following content: Introduction to alkynes, natural occurrence and uses of alkynes, physical properties of alkynes, spectroscopy of alkynes, nomenclature of alkynes, acid-base properties of alkynes,. | Like alkenes, but ending -ene turns into –yne. HC CH Ethyne 5 4 3 2 1 1-Pentyne Br 1 2 3 4 5 6 5-Bromo-2-hexyne -ol -yne > OH 1 2 3 2-Propyn-1-ol Chapter 13: Alkynes: The C C Triple Bond Naming Priority: When the alkyne contains also double bonds, it is called an enyne. However, despite being an “yne”, numbering begins closest to either group: 1 2 3 4 5 6 3-Hexen-1-yne 1-Penten-4-yne When double and triple bond are equidistant from each terminus: ene first (alphabetical) 1 2 3 4 5 6 7 1-Hepten-4-yne 1 2 3 4 5 Substituents: Ethynyl 2-Propynyl (or propargyl) Smaller R is a substituent to larger R 3-Cyclobutyl-1-hexyne Ethynylcyclohexane but 1 2 2-Propenyl (or allyl) 1 2 Remember: Two perpendicular π bonds; sp hybrids R C C R Ethyne Structure The Triple Bond Is Energetic Heat of hydrogenation: more than two alkene bonds (which would be ~ -60 kcal mol-1) Acetylene torch: ~3,500 C Internal triple bond is more stable : : RC C H + B R C C + HB Hydrogens get more acidic (blue) - - pKa ~ . | Like alkenes, but ending -ene turns into –yne. HC CH Ethyne 5 4 3 2 1 1-Pentyne Br 1 2 3 4 5 6 5-Bromo-2-hexyne -ol -yne > OH 1 2 3 2-Propyn-1-ol Chapter 13: Alkynes: The C C Triple Bond Naming Priority: When the alkyne contains also double bonds, it is called an enyne. However, despite being an “yne”, numbering begins closest to either group: 1 2 3 4 5 6 3-Hexen-1-yne 1-Penten-4-yne When double and triple bond are equidistant from each terminus: ene first (alphabetical) 1 2 3 4 5 6 7 1-Hepten-4-yne 1 2 3 4 5 Substituents: Ethynyl 2-Propynyl (or propargyl) Smaller R is a substituent to larger R 3-Cyclobutyl-1-hexyne Ethynylcyclohexane but 1 2 2-Propenyl (or allyl) 1 2 Remember: Two perpendicular π bonds; sp hybrids R C C R Ethyne Structure The Triple Bond Is Energetic Heat of hydrogenation: more than two alkene bonds (which would be ~ -60 kcal mol-1) Acetylene torch: ~3,500 C Internal triple bond is more stable : : RC C H + B R C C + HB Hydrogens get more acidic (blue) - - pKa ~ 25! Why? 50% s-character K Alkynes Are Relatively Acidic - + Li Li Li + + + CH3MgBr MgBr + CH4 pKa 25 pKa 50 H H + Na NH2 + : : H Na C C : : - - NH3 + NaNH2 Na + + pKa 33 1 equiv : - Synthetic Use of Acidity - Preparation Of Alkynes Elimination E2 of dihaloalkanes C C H H X X C C C C Br Br Na B: - H X B: - NaNH2 excess NH3 liq. H+, H2O work up 75% Application in synthesis: RCH CHR R C C R Br Br Br Br Br2 NaNH2 NH3 liq 1,5-Hexadiyne 2. Alkylation of Alkynyl Metals SN2 rules apply Li THF Li I ∆ 90% Best: RI, THF, ∆. RBr or RCl need “coordinating” additives: . ; or HMPA solvent. Remember: Grignards don’t work for coupling with RX (but . for or carbonyls). H2N NH2 O + CH3MgBr MgBr CH2 O OH + LiNH2 (1 equiv) Li O OH + 2 CH3MgBr MgBr BrMg HO OH Li + Li O CH3 OH CH3CH O Reactions Of Alkynes 1. Reductions a. Complete hydrogenation H2, Pt 100% b. Partial hydrogenation with “poisoned” Lindlar’s catalyst: syn addition gives cis product: Pd-CaCO3, Pb(OCCH3)2, quinoline O N H2, Lindlar 100% H H
