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Organic Chemistry. William H. Brown Christopher S. Foote Brent L. Iverson. Alkynes. Chapter 7. D. imethylacetylene. V. inylacetylene. Nomenclature. IUPAC: use the infix - yn - to show the presence of a carbon-carbon triple bond

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organic chemistry

Organic Chemistry

William H. Brown

Christopher S. Foote

Brent L. Iverson

slide2

Alkynes

Chapter 7

nomenclature

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
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
Physical Properties
  • Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton
acidity
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
acidity7
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
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
alkylation of alkyne anions9
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
preparation from alkenes
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
preparation from alkenes11
Preparation from Alkenes
  • for a terminal alkene to a terminal alkyne, 3 moles of base are required
preparation from alkenes12

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

allene
Allene
  • Allene: a compound containing a C=C=C group
    • the simplest allene is 1,2-propadiene, commonly named allene
allenes
Allenes
  • most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions
addition of x 2
Addition of X2
  • Alkynes add one mole of bromine to give a dibromoalkene
    • addition shows anti stereoselectivity
addition of x 216
Addition of X2
  • the intermediate in bromination of an alkyne is a bridged bromonium ion
addition of hx

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

addition of hx18
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
addition of hx19
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
hydroboration
Hydroboration
  • Addition of borane to an internal alkyne gives a trialkenylborane
    • addition is syn stereoselective
hydroboration21
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
hydroboration22
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
hydroboration23

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

addition of h 2 o hydration

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

addition of h 2 o hydration25
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
addition of h 2 o hydration26
Addition of H2O: hydration
  • Step 3: proton transfer to solvent gives an organomercury enol
  • Step 4: tautomerism of the enol gives the keto form
addition of h 2 o hydration27
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
reduction
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
reduction29
Reduction
  • With the Lindlar catalyst, reduction stops at addition of one mole of H2
    • this reduction shows syn stereoselectivity
hydroboration protonolysis
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
dissolving metal reduction
Dissolving Metal Reduction
  • Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity
dissolving metal reduction32
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 reduction33

+

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
Organic Synthesis
  • 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
organic synthesis35
Organic Synthesis
  • 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 synthesis36

target

starting

molecule

materials

Organic Synthesis
  • 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
organic synthesis37
Organic Synthesis
  • Target molecule: cis-3-hexene
organic synthesis38
Organic Synthesis
  • starting materials are acetylene and bromoethane
organic synthesis39
Organic Synthesis
  • Target molecule: 2-heptanone
organic synthesis40
Organic Synthesis
  • starting materials are acetylene and 1-bromopentane
alkynes
Alkynes

End Chapter 7