Chapter 8: Alkynes sp Hybridized Orbitals. hybridize one s-orbital and one p-orbitals. Two sp hybrid orbitals and two unhybridized p-orbital

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1 hapter 8: Alkynes sp ybridized rbitals hybridize one s-orbital and one p-orbitals leave two p-orbital unhybridized Two sp hybrid orbitals and two unhybridized p-orbital triple bonds- acetylene ( 2 2 ) 1

2 Alkyne omenclature Systematic omenclature (IUPA System) Prefix-Parent-Suffix aming Alkynes Suffix: -yne Many of the same rules for alkanes apply to alkenes 1. umber 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 Preparation of Alkynes a. Elimination reactions of 1,2-dihalides ecall: 3 4 K 3 4 K Br 2 Br a 2 2 abr Br Br cis or trans 1,2-dibromide vinyl bromide a 2 2 abr alkynes b. From other alkynes. Section 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 2

3 eactions of Alkynes: Addition of - Markovnikov addition 3 2 vinyl halide 3 2 gem- dihalide 3 2 trans addition of only useful if = or if = 3 2 gem- dihalide Addition of l l 2 trans addition of 2 vinyl carbocation Vinyl carbocations are generally less stable than the corresponding alkyl carbocation arbocation Stability > > ~ > ~ 3 alkyl 2 alkyl 1 alkyl 2 vinyl methyl carbocation 1 vinyl 3

4 ydration of Alkenes Alkenes Alkynes hydration hydration Alcohols Ketones or Aldehydes Mercury (II) atalyzed ydration: similar to oxymercuration - Markovnikov Addition gs 4, 3 3 methyl ketone gs 4, 3 1 Keto-enol tauromerization tautomer: constitutional isomers that interconvert enol keto = D = 611 KJ/mol = D = 735 KJ/mol D = -104 KJ/mol 4

5 Keto-Enol Tautomerization and the Structure of DA Proposed structure: Guanosine Guanosine Thymidine Thymidine ytidine Adenosine Adenosine ytidine Proposal inconsistent with hargaff s ule Adenosine Thymidine Guanosine ytidine Watson-rick (-Donohue) Base Pairing 5

6 ydroboration of Alkynes 1) B 3, TF 2) 2 2, a aldehyde 1) B 3, TF 2) 2 2, a ketones ydrogenation of Alkynes 2, catalyst D (hydrogenation) = -176 KJ/mol 2, catalyst D (hydrogenation) = -137 KJ/mol The second p-bond of an alkyne is slightly more reactive toward hydrogention Under normal hydrogenation conditions, hydrogenation of an alkyne can not be stopped at the alkene stage 2, Pd 2, Pd 6

7 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 atalyst cis -addition of 2 eaction can be stopped at the cis-alkene stage Dissolving Metal eduction: Li(0) metal in liquid ammonia ( 3 ) Li(0) in 3 e (solvated electron) Li, 3, ( 3 ) 3 trans-alkene 7

8 xidative cleavage of alkenes (sect. 7.8): 3 4 1) 3, 2 l, -78 2) Zn 3 4 Zn ketones and aldehydes 3 KMn 4 3 ketones KMn 4 3 KMn 4 3 carboxylic acid xidative leavage of Alkynes (sect.8.7) 1) 3 2) Zn -or- terminal alkyne KM 4 arboxylic acid 2 1) 3 2) Zn -or- internal alkyne KM 4 arboxylic acids Alkynes are less reactive toward oxidative cleavage than alkenes. Alkenes can be oxidatively cleaved in the presence of alkynes Acidity of Terminal Alkynes _ pka = 25 acetylide anion a 2 _ a 3 (pka = 35) _ pka = 45 _ pka = 60 8

9 Alkylation of the acetylide anion: formation of - bonds a substitution reaction between a nucleophile and alkyl halide Acetylide anions are strong nucleophiles and will undergo substitution reactions with alkyl halides _ Br a TF abr new - bond formed The acetylide anion displaces bromide from the alkyl bromide forming a new - bond. Alkylation is a general method of making alkynes from simpler alkynes. Alkylation of acetylide anions is generally limited to primary alkyl bromides and iodide rganic Synthesis Synthesis: making larger, more complex molecules out of smaller ones using known and reliable reactions Prepare octane from 1-pentyne a) a 2 b) I , Pd Prepare Z-2-butene from 1-pentyne a) a 2 b) 3 I 2, Lindlars atalyst Z-2-hexene work the problem backwards 9