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Organic Chemistry

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Title: Organic Chemistry


1
Alkynes
Chapter 7
2
7.2 Nomenclature
  • A. IUPAC use the infix -yn- to show the presence
    of a carbon-carbon triple bond
  • B. Common names prefix the substituents on the
    triple bond to the word acetylene

IUPAC name
2-Butyne
1-Buten-3-yne
Common name
3
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

4
7.3 Physical Properties, Table 7-1
  • Similar to alkanes and alkenes of comparable
    molecular weight and carbon skeleton

5
7.4 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

6
pKa values, Table 4-1
7
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

8
7.5 A. 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 whereas with 2 and 3 halides,
    elimination is the major reaction

9
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

10
B. 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

11
Preparation from Alkenes
  • for a terminal alkene to a terminal alkyne, 3
    moles of base are required

12
Preparation from Alkenes
  • a side product may be an allene, a compound
    containing adjacent carbon-carbon double bonds,
    CCC

R
H
C
C
C
X
H
H
R
R
An allene
H
R
R
An alkyne
13
Allene
  • Allene a compound containing a CCC group
  • the simplest allene is 1,2-propadiene, commonly
    named allene

14
Allenes
  • most allenes are less stable than their isomeric
    alkynes, and are generally only minor products in
    alkyne-forming dehydrohalogenation reactions

15
7.6 A. Addition of X2
  • Alkynes add one mole of bromine to give a
    dibromoalkene
  • addition shows anti stereoselectivity

16
Addition of X2
  • the intermediate in bromination of an alkyne is a
    bridged bromonium ion

17
B. 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

2-Bromopropene
Propyne
2,2-Dibromopropane
18
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

19
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

20
7.7 A. Hydroboration
  • Addition of borane to an internal alkyne gives a
    trialkenylborane
  • addition is syn stereoselective

21
Hydroboration
  • Treating an alkenylborane with H2O2 in aqueous
    NaOH gives an enol

22
Enols
  • 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 CC to CO
  • keto forms generally predominate at equilibrium
  • keto and enol forms are tautomers and their
    interconversion is called tautomerism

23
Hydroboration
  • to prevent dihydroboration with terminal alkynes,
    it is necessary to use a sterically hindered
    dialkylborane, such as (sia)2BH
  • a terminal alkyne treated with (sia)2BH results
    in stereoselective and regioselective
    hydroboration

24
Hydroboration
  • hydroboration/oxidation of an internal alkyne
    gives a ketone
  • hydroboration/oxidation of a terminal alkyne
    gives an aldehyde

O
3-Hexanone
3-Hexyne
25
B. Addition of H2O hydration
  • In the presence of sulfuric acid and Hg(II)
    salts, alkynes undergo addition of water

O

Propyne
26
Addition of H2O hydration
  • Step 1 attack of Hg2 (an electrophile) on the
    triple bond (a nucleophile) gives a bridged
    mercurinium ion

27
Addition of H2O hydration
  • Step 2 attack of water (a nucleophile) on the
    bridged mercurinium ion intermediate (an
    electrophile) opens the three-membered ring

28
Addition of H2O hydration
  • Step 3 proton transfer to solvent gives an
    organomercury enol
  • Step 4 tautomerism of the enol gives the keto
    form

29
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

30
7.8 A. 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

31
Reduction
  • With the Lindlar catalyst, reduction stops at
    addition of one mole of H2
  • this reduction shows syn stereoselectivity

32
B. 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

33
C. Dissolving Metal Reduction
  • Reduction of an alkyne with Na or Li in liquid
    ammonia converts an alkyne to an alkene with anti
    stereoselectivity

34
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)

35
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
36
7.9 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

37
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

38
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

39
Organic Synthesis
  • Target molecule cis-3-hexene

40
Organic Synthesis
  • starting materials are acetylene and bromoethane

41
Organic Synthesis
  • Target molecule 2-heptanone

42
Organic Synthesis
  • starting materials are acetylene and
    1-bromopentane

43
Alkynes
End Chapter 7
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