7-1 Alkynes – Chapter 7 nomenclature - (chapter 5), structure, classification acidity of terminal...

7-7-11

Alkynes – Chapter 7 Alkynes – Chapter 7

nomenclature - (chapter 5), structure, classification

acidity of terminal acetylenes - (chapter 4)

alkylation

prep - dehydrohalogenation (-2HX) or double -elimination

reduction - H2/M (chapter 6) and chemical

hydroboration (chapter 6) - protonation

- oxidation (hydration)

enol-keto tautomerism

addition of HX, X2, H2O (chapter 6)

synthesis(chapter 6)

(blue - chemistry from previous chapters)

H2

HH

Lindlarcatalyst

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NomenclatureNomenclature IUPAC: use the infix -ynyn- to show the presence of

a carbon-carbon triple bond

Common names: prefix the substituents on the triple bond to the word “acetylene”

Common name:IUPAC name:

DimethylacetyleneVinylacetylene2-Butyne 1-Buten-3-yne

1

1,6-Heptadiyne3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne

23

4

1

2 3 4

5

6 12

34

56

7 7

7-7-33

CycloalkynesCycloalkynes Cyclononyne is the smallest cycloalkyne isolated

• it is quite unstable and polymerizes at room temp• the C-C-C bond angle about the triple bond is

approximately 155°, indicating high angle strain

Cyclononyne

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Physical PropertiesPhysical Properties Similar to alkanes and alkenes of comparable

molecular weight and carbon skeleton

0.766174-36

0.746125-79

0.71671-132

0.69040-900.69127-32

(a gas)8-126(a gas)-23-102(a gas)-84-81

Densityat 20°C(g/mL)

Boiling Point(°C)

MeltingPoint(°C)FormulaName

1-Decyne

1-Octyne

1-Hexyne

1-Pentyne2-Butyne

1-ButynePropyne

Ethyne HC CHCH3C CH

CH3C CCH3CH3(CH2 )2C CH

CH3CH2C CH

CH3(CH2 )3C CH

CH3(CH2 )5C CH

CH3(CH2 )7C CH

7-7-55

Acetylene Acetylene – – linear geometrylinear geometry – sp hybridization - 180o bond angles

R-C-C-R’

-bond 180o

bond angle

CC C R’R

90o angle bond to orbitals)

7-7-66

AcetyleneAcetylene

7-7-77

AcidityAcidity

The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1)• terminal alkynes react with sodium amide to form

alkyne anions NH2H-C C-H H-C C:- NH3+

pKa 38(Weaker acid)

pKa 25(Stronger acid)

+

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Acidity of organic compoundsAcidity of organic compounds

HC

CCH3

pKa = 25

H-Csp

H CC CH3

H

H

>

pKa = 44

H-Csp2

H CC CH3

H

H

H

H>

pKa = 50

H-Csp3

Why ? recall chapter 4The more s-character, the more acidic the C-H bond

50% 33% 25%

7-7-99

Relative basicity (acidity)Relative basicity (acidity)

also - NH2 H-NH3

C

CH3C

H

+ O

C

Li

CH3H3C

H

C

CH3C

+

Li

OHC

CH3H3C

H

C

CH3C

H

+ N

H3C

Li

H

CC

H3C

+ N

H3C

Li

HH

pKa = 16

pKa = 38

7-7-1010

AcidityAcidity

• terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA)

• because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion

Na+H– [(CH3)2CH]2N– Li+

Sodium hydride Lithium diisopropylamide (LDA)

Keq = 10-9.3

pKa 15.7pKa 25

-- ++

(Stronger acid)W(eaker acid)

HC CH HC COH H2O

7-7-1111

Alkylation of Alkyne Alkylation of Alkyne AnionsAnions Alkyne anions are both strong bases and good

nucleophiles They participate in nucleophilic substitutionnucleophilic substitution

reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylationalkylation• because alkyne anions are also strong bases,

alkylation is practical only with methyl and 1° halides• with 2° and 3° halides, elimination is the major reaction

HC CHHC C- Na+

H

Br

Sodium acetylide

Bromo-cyclohexane

Acetylene Cyclohexene

Na+Br -++ +

elimin-ation(Ch 9)

7-7-1212

Alkylation of Alkyne AnionsAlkylation of Alkyne Anions

• alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes

• alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne

HC C - Na+

Br

Na+Br-

Sodiumacetylide

+ +

1-Bromobutane 1-Hexyne

CH3CH2C C- Na+

CH3CH2C CCH2CH3

CH3CH2-Br

Na+Br-+

Bromoethane

3-Hexyne

+

Sodium butynide

7-7-1313

CC

CH2

CH3

H3C CC

CH2

H3C

+ H-NH2

NH2

I CH

HHC

CCH2

CH3

H3C

Alkylation of AcetylidesAlkylation of Acetylides

HC C: CCl

CH3

+

Na

HH

CC

H

C

H3C

+ :Cl:HH

Na

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Preparation from AlkenesPreparation from Alkenes Treatment of a vicinal dibromoalkane with two

moles of base, most commonly sodium amide, results in two successive dehydrohalogenationdehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyneCH3CH=CHCH3

CH3CH-CHCH3

Br Br

CH2Cl2

Br2

2NaNH2NH3(l)

CH3C CCH3 2NaBr 2NH3

2-Butene+

+-33oC

2-ButyneSodiumamide

+ +

Note! alkenes are precursors of diBrs

dehydrohalogenationdehydrohalogenation

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Preparation from AlkenesPreparation from Alkenes

• for a terminal alkene to a terminal alkyne, 3 moles of base are required

CH3(CH2)3CH=CH2

Br2CH3(CH2)3CH-CH2

3NaNH2

H2OCH3(CH2)3C CH

1-Hexene 1,2-Dibromohexane

1-Hexyne

CH3(CH2)3C C-Na+

Sodium salt of 1-hexyne

-2HBr

Br Br

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Preparation from AlkenesPreparation from Alkenes

• a side product may be an alleneallene, a compound containing adjacent carbon-carbon double bonds, C=C=C

A haloalkene(a vinylic halide)

An allene

An alkyne

RCC CR

R

HR

H X HNaNH2RC–C=CR-HBr

C C CR

R R

H

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AlleneAllene Allene:Allene: a compound containing a C=C=C group

• the simplest allene is 1,2-propadiene, commonly named allene

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AllenesAllenes

• most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions

CH2=C=CH2

CH2=C=CHCH3

CH3C CH

CH3C CCH3 H0 = -16.7 kJ (-4.0 kcal)/mol

H0 = -6.7 kJ (-1.6 kcal)/mol

7-7-1919

Electrophilic Addition of XElectrophilic Addition of X22

C

R

C

R'

CBr

CRBr

R' Br21 eq.

Br2

1 eq.or XS

C

Br

CRBr

R'Br

Br

Alkynes add first mole of bromine to give a dibromoalkene (2nd mole give tetrabromoalkane)

•addition shows anti stereoselectivity

CH3C CCH3 Br2CH3COOH, LiBr

C C

BrH3C

Br CH3

anti addition

(E)-2,3-Dibromo-2-butene2-Butyne

+

7-7-2020

Addition of XAddition of X22

• the intermediate in bromination of an alkyne is a bridged bromonium ion

C CH3C CH3

BrBr

C C

Br

H3C CH3

C C

Br

H3C CH3

Br

H3C

C C

Br

Br CH3

7-7-2121

Addition of HXAddition of HX Alkynes undergo regioselective addition of either

1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed

2,2-Dibromopropane2-BromopropenePropyne

CH3C CH

Br

Br

BrHBr CH3C=CH2 CH3CCH3

HBr

7-7-2222

Addition of HXAddition of HX

• the intermediate in addition of HX is a 2° vinylic carbocation

• reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product

CH3C CH H-Br CH3C=CH2 BrA 2° vinyliccarbocation

+++

CH3C=CH2 Br

Br

CH3C=CH2

++

2-BromopropeneCH3C=CH2 Br

Br

CH3C=CH2

++

7-7-2323

Addition of HXAddition of HX

• in the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation

• of the two possible carbocations, the favored one is the resonance-stabilized 2° carbocation

+

H

Br

BrCH2CH3C

H

CH2CH3C

Br

Br

H

CH3C CH2

CH2CH3C

Br

H

Br

Br

BrCH3CCH3

Resonance-stabilized 2° carbocation

slower

faster

1° Carbocation

+

+

7-7-2424

Acetylene chemistry - recall olefin chemistry

HH

H

B HRR +

HHH

B HR

R

HHH

B HR

R

R = H, alkyl

H

B HRR +

H

B HR

R

H

B HR

R

HydroborationHydroboration

7-7-2525

HydroborationHydroboration Addition of borane to an internal alkyne gives a

trialkenylborane• addition is syn stereoselective

BH3THF

BR R3-Hexyne

+

A trialkenylborane(R = cis-3-hexenyl group)

H

7-7-2626

HydroborationHydroboration of an internal alkyne

H3CC

CCH3

BH

HH

+ H3C CC

H3C

B

H

HH

of a terminal alkyne – get dihydroboration

HC

CR

BH

HH

+H C

C

R

B

H

HH

H CC

R

B

H

HH

H

B

H

H

H2B-H

7-7-2727


• to prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia)2BH

• treatment of a terminal alkyne with (sia)2BH results in stereoselective and regioselective hydroboration

B-H

Di-sec-isoamylborane [(sia)2BH]

(sia)2BH+

1-Octyne An alkenylborane

B(sia)2H

H

7-7-2828

HydroborationHydroboration Terminal acetylene

hindered borane adds oncedi-sec-isoamylborane or HB(sia)2

H2O2NaOH

HR

H O

Henolunstable

HR

H OH

"keto"aldehyde

tautomerism H+ transfer (O to C)

HR

+

H Bsia

HR

H Bsia

sia

sia

7-7-2929

enol-keto tautomerism H+ transfer C O

R C

C O

H R C

C O

H

R' R'

H H[from hydroboration, oxymercuration(next) and other rxns]

mechanism e(-) arrows

R C

C O

H R C

C O

H

R' R'

H H

R C

C O

H R C

C O

H

R' R'

H Hor

Enol Ketone

7-7-3030


Treating an alkenylborane with H2O2 in aqueous NaOH gives an enol

• enol:enol: a compound containing an OH group on one carbon of a carbon-carbon double bond

• an enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=O

• keto forms generally predominate at equilibrium• keto and enol forms are tautomerstautomers and their

interconversion is called tautomerismtautomerism

1. BH3

2. H2O2, NaOH2-Butyne 2-Buten-2-ol

(an enol)2-Butanone(a ketone)

Keq = 6.7 x 106

(for keto-enoltautomerism)

H OH O

7-7-3131


• hydroboration/oxidation of an internal alkyne gives a ketone

• hydroboration/oxidation of a terminal alkyne gives an aldehyde

3-Hexanone3-Hexyne

1. BH32. H2O2, NaOH

O

1-Octyne

An enol

HH

OH

2. H2O2, NaOH

1. (sia)2BH

H

O

Octanal

7-7-3232

Addition of HAddition of H22O: hydrationO: hydration

In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water

+

Propanone(Acetone)

1-Propen-2-ol(an enol)

PropyneCH3C CH

OH O

HgSO4

H2SO4H2O CH3C=CH2 CH3CCH3

7-7-3333


• Step 1: attack of Hg2+ (an electrophile) on the triple bond (a nucleophile) gives a bridged mercurinium ion

• Step 2: attack of water (a nucleophile) on the bridged mercurinium ion intermediate (an electrophile) opens the three-membered ring

7-7-3434


• Step 3: proton transfer to solvent gives an organomercury enol

• Step 4: tautomerism of the enol gives the keto form

C C

Hg+

H

H3C

OH

H+

OH

H

C C

Hg+

H

H3C

OH

H3O+++

CH3-C-CH2-Hg+O

Enol formKeto form

C CHO

H3C Hg+

H

7-7-3535


• Step 5: proton transfer to the carbonyl oxygen gives an oxonium ion

• Steps 6 and 7: loss of Hg2+ gives an enol; tautomerism of the enol gives the ketone

H2O:CH3-C-CH2-Hg+O

O

H

H H+

+ CH3-C-CH2-Hg+O

H +

+

CH3-C-CH2-Hg+

OH

Hg2+CH3-C=CH2

OH

CH3-C-CH3

O

+

+

(enol form) (Keto form)

7-7-3636

Hydration of acetylenesHydration of acetylenes

internal yne

terminal yne - HBR2/[O] aldehyde

- Hg++/H2O ketone

R C C R' R CCH2R'

OR'C

RH2C

O

+

R C C H

R CH2 C

H

O

R CCH3

O

- “either” ketone(s)

7-7-3737

ReductionReduction Treatment of an alkyne with hydrogen in the

presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane

2-Butyne Butane

+CH3C CCH3Pd, Pt, or Ni

2H2 CH3CH2CH2CH33 atm

7-7-3838

ReductionReduction With the Lindlar catalyst, reduction stops at

addition of one mole of H2 • this reduction shows syn stereoselectivity

CH3C CCH3 H2 C C

CH3H3C

H H

Lindlarcatalyst

cis-2-Butene

+2-Butyne

7-7-3939

Hydroboration - ProtonolysisHydroboration - Protonolysis Addition of borane to an internal alkyne gives a

trialkenylborane• addition is syn stereoselective

• treatment of a trialkenylborane with acetic acid results in stereoselective replacement of B by H

BH3THF

BR R3-Hexyne

+

A trialkenylborane(R = cis-3-hexenyl group)

H

BR R

H3CH3COH

O

H H(CH3COO)3B

cis-3-Hexene

+

A trialkenylborane

+

7-7-4040

Dissolving Metal ReductionDissolving Metal Reduction Reduction of an alkyne with Na or Li in liquid

ammonia converts an alkyne to an alkene with anti stereoselectivity

R'R 2Na

2Na

NH3(l)

NH3(l)

R

R'H

H

H

H

2NaNH2

trans-4-Octene4-Octyne

+ +

7-7-4141

Dissolving Metal ReductionDissolving Metal Reduction

• Step 1: a one-electron reduction of the alkyne gives a radical anion

• Step 2: the alkenyl radical anion (a very strong base) abstracts a proton from ammonia (a very weak acid)

.

R-C C-R + Na + Na+

An alkenyl radical anion

R-C C-R. :

R-C C-R H N H

H R

C C

R

H

N H

H

+

An alkenylradical

+

Amideion

••

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Dissolving Metal ReductionDissolving Metal Reduction

• Step 3: a second one-electron reduction gives an alkenyl anion

• this step establishes the configuration of the alkene• a trans alkenyl anion is more stable than its cis isomer

• Step 4: a second acid-base reaction gives the trans alkene

+Na

RC C

R

H RC C

R

H

+Na+

An alkenyl anion

. :

R

C C

R

H

+ +

R

C C

R

H

H

A trans alkene

H N H

H

H N

HAmide ion

7-7-4343

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts A successful synthesis must

• provide the desired product in maximum yield• have the maximum control of stereochemistry and

regiochemistry• do minimum damage to the environment (it must be a

“green” synthesis)

Our strategy will be to work backwards from the target molecule

7-7-4444

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts We analyze a target molecule in the following

ways• the carbon skeleton: how can we put it together. Our

only method to date for forming new a C-C bond is the alkylation of alkyne anions (Section 7.5)

• the functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and how can they be changed to give the functional groups of the target molecule

7-7-4545

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts We use a method called a retrosynthesis and use

an open arrow to symbolize a step in a retrosynthesis

Retrosynthesis:Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials

targetmolecule

startingmaterials

7-7-4646

H3C

C CH

H CH2Br

HH

H3CC C

H H C H

O

H H

How does onemake butanalfrom 1-bromobutane?

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts

7-7-4747

H3CC C

H H CH2Br

HH

H3CC C

H H C H

O

H H

H3CC C

H H CH2OH

HH

NaOH DMF

PCC


7-7-4848

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts Target molecule: cis-3-hexene

-:C C:- 2CH3CH2Br

cis-3-Hexene

+

3-Hexyne

Acetylide dianion

Bromoethane

disconnect here

7-7-4949


• starting materials are acetylene and bromoethane

cis-3-Hexene3-Hexyne

HC CH1. NaNH2 3. NaNH2

2. CH3CH2Br 4. CH3CH2Br

5. H2

Lindlarcatalyst

Acetylene 1-Butyne

7-7-5050

Organic Synthesis : Beginning ConceptsOrganic Synthesis : Beginning Concepts Target molecule: 2-heptanone

2-Heptanone

1-Heptyne

2-Heptyne

HC C- +Acetylide anion

1-Bromopentane

An acid-catalyzed hydrationof this alkyne gives a mixture of 2-heptanone and 3-heptanone

O

Br

An acid-catalyzed hydration of thisalkyne gives 2-heptanone

7-7-5151


• starting materials are acetylene and 1-bromopentane

2-Heptanone

1-Heptyne

O

BrHC CH

1. NaNH2

2.

3. H2OH2SO4, HgSO4

7-1 Alkynes – Chapter 7 nomenclature - (chapter 5), structure, classification acidity of terminal...

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