Chapter 9: Alkynes 9.1: Sources of Alkynes (please read) 9.2: Nomenclature Systematic Nomenclature: Prefix-Parent-Suffix Naming Alkynes: Suffix: -yne Many of the same rules for alkenes apply to alkynes 1. Number the carbon chain from the end of the carbon nearest the triple bond 2. The alkyne position is indicated by the number of the alkyne carbon in the chain 3. Compounds with two triple bonds are referred to as diynes, three triple bonds as triynes, etc 9.3: Physical Properties of Alkynes (please read) 205 9.4: Structure and Bonding in Alkynes: sp Hybridization H C C H bond angles: H-C-C = 180° (linear geometry acetylene bond distances: (ethyne) C-H = 106 pm C≡C = 120 pm Each carbon is sp hybridized – linear geometry C≡C bond consists of one σ–bond (sp hybridized orbitals) and two π–bond (unhybridized p-orbitals) (see ch. 2 notes) Bond dissociation energies (ΔH°C-C) ΔH° C=C = 611 KJ/mol ΔH° C≡C = 820 KJ/mol ΔH° C–C = 368 KJ/mol ΔH° C–C = 368 KJ/mol π -bond = 243 KJ/mol π-bonds = 452 KJ/mol 226 KJ/mol per π -bond The π-bond of an alkene is ~17 KJ/mol more stable 206 than the π-bond of an alkyne. 1 H H H H ~121° 180° ~111° H 106 pm 111 pm 110 pm C C C C H C C H H 134 pm 120 pm H 153 pm H H H hybridization sp3 sp2 sp of C geometry tetrahedral trigonal planar linear ΔH°C-C 368 kJ/mol 611 kJ/mol 820 kJ/mol ΔH°C-H 410 kJ/mol 452 kJ/mol 536 kJ/mol % s character 25% 33% 50% pKa 62 45 26 207 9.5: Acidity of Acetylene and Terminal Alkynes In general, the C-H bond of hydrocarbons are very weak acids. The R-C≡C-H bond of a terminal acetylene is weakly acid, pKa ~26. H H _ + H C H H C + H pKa = 60 H H H H H _ + pK = 45 C C C C + H a H H H H _ + R C C H R C C + H pKa = 26 A strong base can deprotonate terminal acetylenes to generate an acetylide anion. _ _ R C C + B:H R C C H + B: Na + Na + pKa = pKa = 26 208 2 9.6: Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes - Acetylide anions are strong nucleophiles and will undergo nucleophilic substitution reactions with primary alkyl halides, resulting in the formation of a C-C bond. _ R THF R R C C + C Br R C C C + NaBr H H Na + H SN2 H new C-C bond formed Alkylation of acetylide anions is a general method of making higher alkynes from simpler alkynes. + H C C H + H2N: Na H C C H C C + H3CH2CH2C-Br H3CH2CH2C C C H pentyne + H3CH2CH2C C C H + H2N: Na H3CH2CH2C C C pentyne H CH CH C C C CH H3CH2CH2C C C + H3C-I 3 2 2 3 2-hexyne 209 Alkylation of acetylide anions is generally limited to primary alkyl halides. Acetylide anions act as a base with secondary and tertiary alkyl halides resulting in E2 elimination. 9.7: Preparation of Alkynes by Elimination Reactions Double dehydrohalogenation reaction H X R X2 R R R NaNH2 H X H vicinal NaNH dihalide R 2 R R R X H H R NaNH2 R X X geminal dihalide 3 equivalents of NaNH2 are required for preparing terminal alkynes from from 1,2- or 1,1-dihaloalkane. 210 3 9.8: Reactions of Alkynes - alkynes undergo addition reactions similar to alkenes. Increasing oxidation state C C C C C C Cl Cl C Cl C Cl C Cl Cl O C OH C O C OR 9.9: Hydrogenation of Alkynes - The hydrogenation of an alkyne cannot be stopped at the alkene stage under normal hydrogenation conditions (H2, metal catalyst) H H H2, Pd/C H H H2, Pd/C R C C R 1 2 C C R1 C C R2 R1 R2 H H cis alkene intermediate 211 The π-bonds of alkenes are slightly more reactive toward hydrogenation than the π-bond of an alkene. Lindlar’s catalyst: “poisoned” palladium catalyst Pd on CaCO3 + Pb(OAc)4 + quinoline (amine) “poisons” reduce the catalysts activity so only the most reactive functional groups are hydrogenated H2, Lindlar's Catalyst H H cis -addition R1 C C R2 C C of H2 R1 R2 Hydrognation can be stopped at the cis-alkene stage with Lindlar’s catalyst 9.10: Metal-Ammonia Reduction of Alkynes - Dissolving Metal Reduction: Li(0) metal in liquid ammonia (NH3) • Li(0) in NH3 e (solvated electron) Li, NH3, (CH3)3COH H R2 R1 C C R2 C C trans-alkene 212 R1 H 4 9.11: Addition of Hydrogen Halides to Alkynes Addition of HX to acetylenes: Markovnikov addition X HX R C C H C H R C only useful for H terminal or vinyl halide symmetrical acetylenes X HX (R = R ) C R 1 2 R C C R 2 1 2 R1 C H trans addition of HX Mechanism ? R + HX C H R C C H C H vinyl carbocation rate = k [alkyne][HX]2 213 Carbocation Stability R R H H R + H + ~ H C C R C + > R C + > H C + ~ R C C > H C + H R H H H H 3° 2° 1° 2° methyl 1° alkyl alkyl alkyl vinyl carbocation vinyl gem-dihaloalkanes are obtained with excess HX - addition follows Markovnikov’s rule X X X HX HX C R C R 2 R1 C C R2 2 R1 C R1 C excess H H H R2=H or R2=R1 gem-dihaloalkane Anti-Markovnikov addition of HBr to terminal alkynes in the Presence of peroxides (radical mechanism) H HX HBr C Br C Br R C C H R C peroxides 1 R1 C H H 214 5 9.13: Addition of Halogens to Alkynes X X X X2 X2 C R C R2 R C C R 2 1 2 R1 C R1 C excess X X X anti addition of X2 9.12: Hydration of Alkynes hydration Alkenes Alcohols hydration Alkynes Ketones or Aldehydes Hg (II) catalyzed hydration of alkynes: similar to oxymercuration - Markovnikov Addition HgSO4, H2SO4, OH O H O tautomerization R C C H 2 C H R1 C R CH3 H enol methyl ketone 215 Tautomerization - equilibrium between two isomers (tautomer), which differ by the migration of a proton and a π-bond. Keto-enol tautomerization - normally favors the keto form (C=O) H O O C=C ΔH° = 611 KJ/mol C=O ΔH° = 735 KJ/mol C C H C-O 380 C C C-C 368 O-H 426 C-H 420 enol keto Keto-enol tautomerization is acid-catalyzed 216 6 Hydroboration of Alkynes - anti-Markovnikov hydration of an alkyne (complementary to the Hg(II) catalyzed method) 1) BH , THF 3 O 2) H2O2, NaOH Aldehyde R C C H R product H Hg(II)-catalyzed hydration and hydroboration are equivalent for internal alkynes 1) BH3, THF HgSO4, H2SO4, 2) H2O2, NaOH O O H2O R1 C C R2 + R2 R1 R1 C C R2 R1 R2 Ketone products Hydration of an internal alkyne is only useful for symmetrical acetylenes (R1 = R2) 217 9.14: Ozonolysis of Alkynes Ozonolysis of alkenes (sect. XX): 1) O3 R1 R3 R1 R3 2) Zn ketones and O + O aldehydes R2 R4 R2 R4 Ozonolysis of Alkynes 1) O3 2) Zn O R C C H R C + H2CO3 terminal alkyne OH Carboxylic acid 1) O3 O O 2) Zn R1 C C R2 R1 C + C R2 OH HO internal alkyne Carboxylic acids Alkynes are less reactive toward ozonolysis than alkenes. An alkenes can be oxidatively cleaved by ozone in the presence of an alkyne. 218 7 C-C bond forming reactions are important for organic synthesis Prepare octane from 1-pentyne H3CH2CH2C C CH CH3CH2CH2CH2CH2CH2CH2CH3 Prepare (Z)-2-hexene from 1-pentyne H3CH2CH2C C CH Z-2-hexene Prepare 4-octanone from acetylene O HC CH 219 8.