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Alkynes – Chapter 7 PowerPoint PPT Presentation


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Alkynes – Chapter 7 . nomenclature - (chapter 5) , structure, classification acidity of terminal acetylenes - (chapter 4) alkylation prep - dehydrohalogenation (-2HX) or double  -elimination reduction - H 2 /M (chapter 6) and chemical hydroboration (chapter 6) - protonation

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Alkynes – Chapter 7

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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)


Nomenclature l.jpg

D

imethylacetylene

V

inylacetylene

Nomenclature

  • IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond

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

IUPAC name:

2-Butyne

1-Buten-3-yne

Common name:


Cycloalkynes l.jpg

Cycloalkynes

  • 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


Physical properties l.jpg

Physical Properties

  • Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton


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90o angle

 bond to orbitals)

C

R

C

R’

Acetylene

– linear geometry

– sp hybridization

- 180o bond angles

R-C-C-R’

 -bond 180o

bond angle


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Acetylene


Acidity l.jpg

Acidity

  • 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


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H

H

H

H

H

C

C

C

H

>

CH

C

>

C

C

3

CH

3

CH

H

3

H

H

pKa = 50

pKa = 44

H-Csp3

H-Csp2

The more s-character,the moreacidic the C-H bond

50%

33%

25%

Acidity of organic compounds

pKa = 25

H-Csp

Why ? recall chapter 4


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pKa = 16

also -NH2

H-NH3

pKa = 38

Relative basicity (acidity)


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Acidity

  • 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


Alkylation of alkyne anions l.jpg

Alkylation of Alkyne Anions

  • Alkyne anions are both strong bases and good nucleophiles

  • They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation

    • 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


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Alkylation 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


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Alkylation of Acetylides


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Preparation from Alkenes

  • Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne

Note! alkenes are precursors of diBrs

dehydrohalogenation


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Preparation from Alkenes

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


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N

a

N

H

2

R

C

C

=

C

R

-

H

B

r

R

C

C

C

R

A haloalkene

(a vinylic halide)

Preparation from Alkenes

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

R

H

C

C

C

X

H

H

R

R

An allene

H

R

R

An alkyne


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Allene

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

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


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Allenes

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


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R'

Br

R'

Br

R'

Br

C

Br

C

2

C

Br

2

1 eq.

C

1 eq.

C

C

Br

or XS

Br

R

R

R

Br

Electrophilic Addition of X2

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

    • addition shows anti stereoselectivity


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Addition of X2

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


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B

r

B

r

H

B

r

H

B

r

C

H

C

C

H

C

H

C

=

C

H

C

H

C

C

H

3

3

2

3

3

B

r

Addition 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

Propyne

2-Bromopropene

2,2-Dibromopropane


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Addition 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


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Addition 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


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Hydroboration

Acetylene chemistry - recall olefin chemistry

R = H, alkyl


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Hydroboration

  • Addition of borane to an internal alkyne gives a trialkenylborane

    • addition is syn stereoselective


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Hydroborationof an internal alkyne

of a terminal alkyne – get dihydroboration


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Hydroboration

  • 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


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Hydroboration Terminal acetylene

hindered borane adds once

di-sec-isoamylborane or HB(sia)2

tautomerism H+ transfer (O to C)


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enol-keto tautomerismH+ transfer C  O

[from hydroboration,

oxymercuration

(next) and other rxns]

Enol

Ketone

mechanism e(-) arrows

or


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Hydroboration

  • Treating an alkenylborane with H2O2 in aqueous NaOH gives an 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 aretautomersand their interconversion is called tautomerism


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1

.

B

H

3

2

.

H

O

,

N

a

O

H

2

2

Hydroboration

  • hydroboration/oxidation of an internal alkyne gives a ketone

  • hydroboration/oxidation of a terminal alkyne gives an aldehyde

O

3-Hexyne

3-Hexanone


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O

H

H

S

O

2

4

C

H

C

C

H

C

H

C

C

H

H

O

C

H

C

=

C

H

3

3

3

2

3

2

H

g

S

O

4

1-Propen-2-ol

Propanone

(an enol)

(Acetone)

Addition of H2O: hydration

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

O

+

Propyne


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Addition of H2O: hydration

  • 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


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Addition of H2O: hydration

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

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


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Addition of H2O: hydration

  • 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


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Hydration of acetylenes

terminal yne - HBR2/[O] aldehyde

- Hg++/H2O  ketone

internal yne

- “either”  ketone(s)


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Reduction

  • 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


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Reduction

  • With the Lindlar catalyst, reduction stops at addition of one mole of H2

    • this reduction shows syn stereoselectivity


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Hydroboration - 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


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Dissolving Metal Reduction

  • Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity


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Dissolving 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)


Dissolving metal reduction42 l.jpg

+

N

a

N

a

Dissolving 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

R

R

.

:

+

+

C

C

C

C

R

R

H

H

An alkenyl anion


Organic synthesis beginning concepts l.jpg

Organic 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


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Organic 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


Organic synthesis beginning concepts45 l.jpg

target

starting

molecule

materials

Organic Synthesis : Beginning Concepts

  • We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis

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


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Organic Synthesis : Beginning Concepts

H

CH

Br

H

2

C

C

H

H

C

H

3

How does one

make butanal

from 1-bromobutane?

O

H

H

C

H

C

C

H

C

H

H

3


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H

H

CH

Br

2

C

C

H

H

C

H

3

H

H

CH

OH

2

C

C

H

H

C

H

3

O

H

H

C

H

C

C

H

C

H

H

3

Organic Synthesis : Beginning Concepts

NaOH

DMF

PCC


Organic synthesis beginning concepts48 l.jpg

Organic Synthesis : Beginning Concepts

  • Target molecule: cis-3-hexene


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Organic Synthesis : Beginning Concepts

  • starting materials are acetylene and bromoethane


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Organic Synthesis : Beginning Concepts

  • Target molecule: 2-heptanone


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Organic Synthesis : Beginning Concepts

  • starting materials are acetylene and 1-bromopentane


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