hapter 10: Alkynes 10.1 Introduction to Alkynes ~ 111 ~ 122 1.06 Å 180 1.1 Å ~ 116 1.08 Å 1.54 Å 1.34 Å 1.20 Å hybridization of sp 3 sp 2 sp geometry tetrahedral trigonal planar linear 368 KJ/mol 632 KJ/mol 820 KJ/mol Δ - Δ - % s character pk a 410 KJ/mol 25% 50 60 464 KJ/mol 33% 44 536 KJ/mol 50% 25 213 10.2: 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. ompounds with two triple bonds are referred to as diynes, three triple bonds as triynes, etc Terminal alkyne: Triple bond is at the end of the carbon chain Internal alkyne: Triple bond is not at the end of the carbon chain terminal internal 214 107
10.3 Acidity of Acetylene and Terminal Alkynes In general, the - bond of hydrocarbons are very weak acids. The - bond of a terminal acetylene is weakly acid, pk a ~25. _ + + pk a = 50-60 _ + + pk a = 44 _ + + pk a = 25 A strong base can deprotonate terminal acetylenes to generate an acetylide anion. _ + B: Na + _ + B: Na + pk a = 25 pk a = 215 10.4 Preparation of Alkynes a. Elimination reactions of vic- and gem-dihalides recall from hapter 8: K Double dehydrohalogenation reaction + 2 + K 2 NaN 2, N 3 vic-dibromide NaN 2 NaN 2, N 3 vinyl bromide + N 3 + Na + N 3 + Na gem-dibromide Note: 3 equivalents of NaN 2 are required for preparing terminal alkynes from from 1,2- or 1,1-dihaloalkane. b. From terminal alkynes (hapter 10.10) 216 108
10.10 Alkylation of Terminal Alkynes - Acetylide anions are strong nucleophiles and will undergo nucleophilic substitution reactions with methyl or primary alkyl halides, resulting in the formation of a bond. pk a = 25 2 N Na + deprotonation acetylide anion + 2 N- pk a = 38 acetylide anion nucleophilic substitution Alkylation of acetylide anions is a general method of making higher alkynes from simpler alkynes. ' 217 Alkynes undergo many of the same reaction as alkenes Alkynes are in a higher oxidation state as alkenes; therefore the products from the reaction of alkynes are often in a higher oxidation than the alkene counterparts Increasing oxidation state l l l l l l l l l l 2 N 2 N N 218 109
10.5 eduction of Alkynes atalytic hydrogenation The hydrogenation of an alkyne cannot be stopped at the alkene stage under normal hydrogenation conditions ( 2, metal catalyst) 2, Pd/ 2, Pd/ alkyne cis alkene intermediate alkane alkane alkene alkyne Δ - = 368 KJ/mol Δ = = 632 KJ/mol Δ - = 368 KJ/mol Δ π-bond = 264 KJ/mol Δ Ξ = 820 KJ/mol Δ - = 368 KJ/mol Δ 1st π-bond = 264 KJ/mol Δ 2nd π-bond = 188 KJ/mol 219 The π-bonds of alkynes are slightly more reactive toward hydrogenation than the π-bond of an alkene. Lindlar s catalyst: poisoned palladium catalyst Pd on a 3 + Pb(Ac) 4 + quinoline (amine) poisons reduce the catalysts activity so only the most reactive functional groups are hydrogenated 2, Lindlar's catalyst 2 syn-addition of 2 ydrognation can be stopped at the cis-alkene stage with Lindlar s catalyst 3 2, Lindlar's atalyst 2 (87%) 3 Prostaglandin E 2 220 110
Dissolving metal reduction Na (0) metal in liquid ammonia (N 3 ) Na (0) in N 3 Na + + e (solvated electron) mechanism Na (0), N 2 3 anti-addition of 2 221 10.6 ydrohalogenation of Alkynes addition of to an alkyne follows Markovnikov s rule vinyl halide trans addition of only useful for terminal or symmetrical acetylenes ( = ) Mechanism? + rate = k [alkyne][] 2 vinyl carbocation 222 111
gem-dihaloalkanes are obtained with excess - addition follows Markovnikov s rule = or = excess gem-dihaloalkane Anti-Markovnikov addition of to terminal alkynes in the presence of peroxides (radical mechanism) + not observed Polar&mechanism&(Markovnikov&addition), peroxides not observed adical&mechanism&(anti7markovnikov&addition) peroxides 223 10.8 alogenation of Alkynes 2 10.7 ydration of Alkynes Alkenes - anti addition of 2 Alcohols 2 excess Alkynes - Ketones or Aldehydes g(ii) catalyzed hydration of alkynes: similar to oxymercuration - Markovnikov Addition 2 S 4, 2 g(ii)s 4, enol tautomerization methyl ketone 224 112
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 (=) = Δ = 632 KJ/mol - 381-435 1448 KJ/mol enol keto = Δ = 735 KJ/mol - 368-410 1513 KJ/mol Tautomerization is acid- and base-catayzed 225 ydroboration of Alkynes - anti-markovnikov hydration of an alkyne (complementary to the g(ii) catalyzed method) 1) B 2 6, TF 2) 2 2, Na 2 enol tautomerization 1 aldehyde g(ii)-catalyzed hydration and hydroboration are equivalent for internal alkynes. ydration of an internal alkyne is only useful for symmetrical acetylenes ( = ). 1 1) B 3, TF 2) 2 2, Na + Ketone products gs 4, 2 S 4, 2 226 113
10.9 zonolysis of Alkynes zonolysis of alkenes (hap. 9.11): 3 4 1) 3 2) Zn + 3 4 ketones and aldehydes zonolysis of Alkynes terminal alkyne 1) 3 2) 2 + 1) 3 2) 2 internal alkyne + Alkynes are less reactive toward ozonolysis than alkenes. An alkenes can be oxidatively cleaved by ozone in the presence of an alkyne. 227 10.11 Synthesis Strategies 228 114