Chapter 10: Alkynes 10.1 Introduction to Alkynes

ΔH°C-C

ΔH°C-H

% s character

pKa

368 KJ/mol

410 KJ/mol

sp3

25%

50–60

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

HH1.54 Å

~ 111°

1.1 Å C C HH1.20 Å

180°1.06 Å

C CH

H H

H1.34 Å

~ 122°

~ 116° 1.08 Å

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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

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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

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10.3 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 ~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+

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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.

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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)

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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 C–C bond.

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

C CR HH2N– Na+

C CRdeprotonation acetylide

anion

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

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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

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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 CR1

H HR2

H H

cis alkene intermediate

alkyne alkane

ΔH°CΞC = 820 KJ/mol ΔH°C-C = 368 KJ/mol ΔH°1st π-bond = 264 KJ/mol

ΔH°2nd π-bond = 188 KJ/mol

ΔH°C=C = 632 KJ/mol ΔH°C-C = 368 KJ/mol

ΔH°π-bond = 264 KJ/mol

ΔH°C-C = 368 KJ/mol

alkane alkene alkyne

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syn-addition of H2

Hydrognation can be stopped at the cis-alkene stage with Lindlar’s catalyst

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 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 R2C C

H

R1 R2

HH2,Lindlar's catalyst

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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), NH3

anti-addition of H2

mechanism

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10.6 Hydrohalogenation of Alkynes – addition of HX to an alkyne follows Markovnikov’s 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

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gem-dihaloalkanes are obtained with excess HX - addition follows Markovnikov’s rule

R1 C C R2HX

R1 CCX

R2

Hexcess

R1 CC R2

H

XX

H

gem-dihaloalkane

HX

R C C H HBrR1 C

C 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

H–Br not observed Polar&mechanism&(Markovnikov&addition)

H–Br, peroxides not observed Radical&mechanism&(Anti7Markovnikov&addition)

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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

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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)

C=C ΔH° = 632 KJ/mol C-O 381 O-H 435

1448 KJ/mol

C=O ΔH° = 735 KJ/mol C-C 368 C-H 410

1513 KJ/mol

CCO

H

CC HO

enol keto

Tautomerization is acid- and base-catayzed

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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,

H2OR1 C C R2

Ketone products

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10.9 Ozonolysis of Alkynes

R2

R1 R3

R4

1) O32) Zn

R1

R2

OR4

R3O+ ketones and

aldehydes

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 R2

O

HO

terminal alkyne

internal alkyne

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10.11 Synthesis Strategies

114