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Home , Alkyl, Alkylation, Alkyne, Pentyne

Chapter 9: 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 apply to alkynes 1. Number the chain from the end of the carbon nearest the 2. The 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)

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9.4: Structure and Bonding in Alkynes: sp Hybridization

H C C H bond angles: H-C-C = 180° (linear geometry 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 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

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9.5: Acidity of Acetylene and Terminal Alkynes In general, the C-H bond of are very weak . The R-C≡C-H bond of a terminal acetylene is weakly , 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 to generate an anion. _ _ R C C + B:H R C C H + B: Na + Na + pKa = pKa = 26 208

2 9.6: Preparation of Alkynes by of Acetylene and Terminal Alkynes - Acetylide anions are strong and will undergo nucleophilic substitution reactions with primary 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

+ 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- 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 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

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: 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” catalyst

Pd on CaCO3 + Pb(OAc)4 + quinoline () “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- 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 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 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 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 ( 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 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

hydration Alkynes or 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 methyl

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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

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6 of Alkynes - anti-Markovnikov hydration of an alkyne (complementary to the Hg(II) catalyzed method)

1) BH , THF 3 O 2) H2O2, NaOH 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)

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9.14: 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

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 Prepare from 1-pentyne

H3CH2CH2C C CH CH3CH2CH2CH2CH2CH2CH2CH3

Prepare (Z)-2- from 1-pentyne

H3CH2CH2C C CH

Z-2-hexene

Prepare 4-octanone from acetylene

O HC CH

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