H H H H H C C H C C H H - College of Arts and Science 10: Alkynes 10.1 Introduction to Alkynes...

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  • Chapter 10: Alkynes 10.1 Introduction to Alkynes

    HC-C HC-H

    % s character

    pKa

    368 KJ/mol

    410 KJ/mol

    sp3

    25%

    5060

    632 KJ/mol

    464 KJ/mol

    sp2

    33%

    44

    820 KJ/mol

    536 KJ/mol

    sp

    50%

    25

    hybridization of C

    geometry tetrahedral trigonal planar linear

    H

    H HH

    C C

    HH 1.54

    ~ 111

    1.1 C C HH1.20

    1801.06

    C CH

    H H

    H1.34

    ~ 122

    ~ 116 1.08

    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. Compounds with two triple bonds are referred to as diynes, three triple bonds as triynes, etc

    214

    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 R1 R2R H

    terminal internal

    107

  • 10.3 Acidity of Acetylene and Terminal Alkynes In general, the C-H bond of hydrocarbons are very weak acids. The R-CCH bond of a terminal acetylene is weakly acid, pKa ~25. A strong base can deprotonate terminal acetylenes to generate an acetylide anion.

    pKa = 50-60 pKa = 44 pKa = 25

    R C C H + B: R C C_

    Na +

    + B:H_ Na +

    pKa = 25 pKa =

    C CH

    H H

    HC C

    H

    H

    H

    _

    + H+CH

    HHH C

    H

    HH

    _

    + H+

    R C C H R C C_

    + H+

    215

    10.4 Preparation of Alkynes a. Elimination reactions of vic- and gem-dihalides

    recall from Chapter 8: H

    X+ H2O + KX

    KOH

    Double dehydrohalogenation reaction

    Note: 3 equivalents of NaNH2 are required for preparing terminal alkynes from from 1,2- or 1,1-dihaloalkane.

    216

    R1

    R2 Br2

    Br

    Br

    R1

    R2

    H

    H

    R2

    R1

    H

    Br

    H

    Br

    vic-dibromide

    gem-dibromide

    R1

    R2H

    Br

    vinyl bromideNaNH2, NH3

    NaNH2, NH3

    + NH3 + NaBr

    NaNH2R1 R2

    + NH3 + NaBr

    b. From terminal alkynes (Chapter 10.10)

    108

  • 217

    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 CC bond.

    Alkylation of acetylide anions is a general method of making higher alkynes from simpler alkynes.

    C CR HH2N Na+ C CR

    deprotonation acetylideanion

    C CR C BrR

    HHacetylide

    anion

    nucleophilicsubstitution

    C CR CR'

    HH

    pKa = 25

    + H2N-H

    pKa = 38

    Increasing oxidation state

    C C C C C C

    C OH C O CO

    ORCO2

    C NH2 C NH C N

    C Cl C ClCl

    C ClCl

    ClC Cl

    ClCl

    Cl

    218

    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

    109

  • 219

    10.5 Reduction of Alkynes Catalytic hydrogenation The hydrogenation of an alkyne cannot be stopped at the alkene stage under normal hydrogenation conditions (H2, metal catalyst)

    R1 C C R2H2, Pd/C

    C CR1

    H H

    R2

    H2, Pd/CC CR1H H

    R2H H

    cis alkene intermediate

    alkyne alkane

    HCC = 820 KJ/mol HC-C = 368 KJ/mol H1st -bond = 264 KJ/mol

    H2nd -bond = 188 KJ/mol

    HC=C = 632 KJ/mol HC-C = 368 KJ/mol

    H-bond = 264 KJ/mol

    HC-C = 368 KJ/mol

    alkane alkene alkyne

    220

    syn-addition of H2

    Hydrognation can be stopped at the cis-alkene stage with Lindlars catalyst

    The -bonds of alkynes are slightly more reactive toward hydrogenation than the -bond of an alkene.

    Lindlars 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

    O

    RO OR

    OCH3

    O

    O

    HO OH

    CO2HO

    RO OR

    O

    OCH3

    H2, Lindlar's Catalyst

    Prostaglandin E2

    (87%)

    CCR1 R2 C CH

    R1 R2

    HH2,Lindlar's catalyst

    110

  • 221

    Dissolving metal reduction Na(0) metal in liquid ammonia (NH3)

    Na(0) in NH3 Na+ + e (solvated electron)

    CCR1 R2 C CH

    R1 H

    R2Na(0), NH3anti-addition of H2

    mechanism

    222

    10.6 Hydrohalogenation of Alkynes addition of HX to an alkyne follows Markovnikovs rule

    R C C HHX

    R CCX

    H

    Hvinyl halide

    R1 C C R2HX

    R1 CCX

    R2

    Htrans addition

    of HX

    only useful for terminal or symmetrical acetylenes (R1 = R2)

    R C C HHX R

    CC H

    H

    +

    vinylcarbocation

    Mechanism ?

    rate = k [alkyne][HX]2

    111

  • 223

    gem-dihaloalkanes are obtained with excess HX - addition follows Markovnikovs rule

    R1 C C R2HX

    R1 CCX

    R2

    Hexcess

    R1 CC R2

    H

    XX

    H

    gem-dihaloalkane

    HX

    R C C H HBr R1 CC Br

    Hperoxides

    HX

    R1 CC Br

    H

    H

    Anti-Markovnikov addition of HBr to terminal alkynes in the presence of peroxides (radical mechanism)

    R2=H or R2=R1

    C CR

    H H

    HC CR

    Br

    H

    HH

    H+ C CR

    H

    H

    HH

    Br

    HBr not observed Polar&mechanism&(Markovnikov&addition)

    HBr, peroxides not observed Radical&mechanism&(Anti7Markovnikov&addition)

    224

    10.8 Halogenation of Alkynes

    R1 C C R2X2

    R1 CCX

    R2

    X

    X2R1 C

    C R2

    X

    XX

    X

    anti addition of X2

    excess

    10.7 Hydration of Alkynes

    Alkenes Alcohols Alkynes Ketones or Aldehydes

    H-OH

    H-OH

    Hg(II) catalyzed hydration of alkynes: similar to oxymercuration - Markovnikov Addition

    CCR1 H C CHO

    R1 H

    HH2SO4, H2O

    Hg(II)SO4,

    enol

    tautomerization CC

    O

    R1H

    H H

    methyl ketone

    112

  • 225

    Tautomerization - equilibrium between two isomers (tautomer), which differ by the migration of a proton and a -bond.

    Keto-enol tautomerization - normally favors the keto form (C=O)

    C=C H = 632 KJ/mol C-O 381 O-H 435

    1448 KJ/mol

    C=O H = 735 KJ/mol C-C 368 C-H 410

    1513 KJ/mol

    CCO

    H

    CC HO

    enol keto

    Tautomerization is acid- and base-catayzed

    226

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

    CCR1 H C CH

    R1 H

    OH1) B2H6, THF

    2) H2O2, NaOH H2O

    enol

    tautomerization CCR1

    HHH

    O

    aldehyde

    Hg(II)-catalyzed hydration and hydroboration are equivalent for internal alkynes. Hydration of an internal alkyne is only useful for symmetrical acetylenes (R1 = R2).

    R1 C C R2R1

    OR2 + R2

    OR1

    1) BH3, THF2) H2O2, NaOH

    HgSO4, H2SO4,H2O R1 C C R2

    Ketone products

    113

  • 227

    10.9 Ozonolysis of Alkynes

    R2

    R1 R3

    R4

    1) O32) Zn

    R1

    R2O

    R4

    R3O+ ketones andaldehydes

    Ozonolysis of alkenes (Chap. 9.11):

    Ozonolysis of Alkynes

    Alkynes are less reactive toward ozonolysis than alkenes. An alkenes can be oxidatively cleaved by ozone in the presence of an alkyne.

    CCR1 H

    1) O32) H2O CR1

    O

    OH

    +

    O

    C

    O

    CCR1 R2

    1) O32) H2O CR1

    O

    OH

    + C R2O

    HO

    terminal alkyne

    internal alkyne

    228

    10.11 Synthesis Strategies

    114