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Alkynes

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Chapter 8 Introduction An alkyne is a hydrocarbon that contains a carbon-carbon triple bond I. Alkynes: An Overview A. Electronic Structure ... – PowerPoint PPT presentation

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Title: Alkynes


1
Chapter 8
Alkynes An Introduction to Organic Synthesis
2
Introduction
  • An alkyne is a hydrocarbon that contains a
    carbon-carbon triple bond

3
  • Acetylene is the simplest alkyne.
  • It is produced industrially from methane by high
    temperature decomposition (pyrolysis)
  • It is the starting material of many organic
    molecules

4
I. Alkynes An Overview
  • Electronic Structure
  • Naming Alkynes

5
A. Electronic Structure
  • Carbon-carbon triple bond results from
  • sp hybrid orbital on each C forming a s bond and
  • unhybridized py and pz orbitals forming a p bond

6
  • The remaining sp orbitals form bonds to other
    atoms at 180º to C-C triple bond.
  • The triple bond is shorter and stronger than
    single or double bond

7
  • The triple bond is shorter and stronger than
    single or double bond
  • Breaking a p bond in acetylene (HCCH) requires
    318 kJ/mole (in ethylene it is 268 kJ/mole)

8
B. Naming Alkynes
  • Like alkanes and alkenes, alkynes are named
    according to the system devised by the
    International Union of Pure and Applied Chemistry
    (IUPAC).

9
Steps to naming alkynes1
  • Find the longest continuous carbon chain
    containing the triple bond
  • Name using the suffix yne to indicate an alkyne.
  • The position of the triple bond is indicated by
    giving the number of the first alkyne carbon
  • Numbering of chain with triple bond is set so
    that the smallest number possible includes the
    triple bond

10
Steps to naming alkynes2
  • If more than one triple bond is present
  • Indicate the position of each and use the
    suffixes -diyne, -triyne,
  • A compound with two triple bonds is a diyne
  • A triyne has three triple bonds

11
Steps to naming alkynes3
  • If a triple and a double bond are present
  • Name the compound an enyne
  • An enyne has a double bond and triple bond
  • Number from chain that ends nearest a double or
    triple bond - double bond is preferred if both
    are present in the same relative position

12
  • Alkynes as substituents are called alkynyl

13
Practice Problem Name the following compounds
14
Practice Problem There are seven isomeric
alkynes with the formula C6H10. Draw
and name them
15
II. Synthesis of Alkynes
  • Elimination Reactions of Dihalides
  • Alkylation of Acetylide Anions

16
A. Elimination Reactions of Dihalides
  • Treatment of a 1,2 dihaloalkane (vicinal
    dihalide) with KOH or NaNH2 produces a two-fold
    elimination of HX and formation of an alkyne
  • Intermediate is a vinyl halide

17
  • Vicinal dihalides are available from addition of
    bromine or chlorine to an alkene

18
  • Vinyl halides give alkynes when treated with
    strong base.

19
Dehydrohalogenation of Vicinal dihalides
  • Dehydrohalogenation of vicinal dihalides results
    in twofold elimination (loss) of HX and formation
    of an alkyne
  • uses strong base (KOH or NaNH2)
  • Intermediate is a vinyl halide

20
B. Alkylation of Acetylide Anions
21
III. Reactions of Alkynes
  • Addition of HX and X2
  • Hydration of Alkynes
  • Reduction of Alkynes
  • Oxidative Cleavage of Alkynes

22
III. Reactions of Alkynes
  • Alkyne Acidity Formation of Acetylide Anions
  • Alkylation of Acetylide Anions
  • An Introduction to Organic Synthesis

23
Introduction
  • Addition reactions of alkynes are similar to
    those of alkenes
  • Alkynes react with many electrophiles to give
    useful products by addition

24
A. Addition of HX and X2
  • Addition of excess H-X to an alkyne gives a
    dihalide product
  • Regiochemistry is Markovnikov (X attaches to the
    more highly substituted sp carbon and H adds to
    the less highly substituted side)

25
Addition of X2 (where X Br or Cl)
  • Addition of excess X2 to an alkyne gives a
    tetrahalide product.
  • Initial addition of X2 (Br2 or Cl2) to an alkyne
    gives trans intermediate

26
Addition of HX to Alkynes Involves Vinylic
Carbocations
27
  • Addition of H-X to alkyne should produce a
    vinylic carbocation intermediate
  • Secondary vinyl carbocations form less readily
    than primary alkyl carbocations
  • Primary vinyl carbocations probably do not form
    at all
  • Nonethelss, H-Br can add to an alkyne to give a
    vinyl bromide if the Br is not on a primary carbon

28
  • Vinylic carbocations are less stable than
    similarly substituted alkyl carbocations.
  • Vinyl carbocations have sp-hybridized carbons and
    thus lack stabilizing hyperconjugative
    interactions
  • Stability of carbocations
  • 3º alkyl gt 2º alkyl gt 1º alkyl 2º vinyl gt
    CH3 1º vinyl

29
Addition of HX and X2 Summary
  • Addition of excess H-X to an alkyne gives a
    dihalide
  • Markovnikov Regiochemistry
  • Addition of excess X2 to an alkyne gives a
    tetrahalide product.

30
Practice Problem What products would you expect
from the following reactions?
31
B. Hydration of Alkynes
  • Hydration of alkynes is the addition H-OH to an
    alkyne
  • Mercury (II) catalyzes Markovnikov oriented
    addition
  • Hydroboration-oxidation gives the non-Markovnikov
    product

32
Mercury(II)-Catalyzed Hydration of Alkynes
  • Mercuric ion (as the sulfate) is a Lewis acid
    catalyst that promotes addition of water in
    Markovnikov orientation
  • The immediate product is a vinylic alcohol, or
    enol, which spontaneously rearranges to a ketone
  • Alkynes do not react with aqueous protic acids

33
Keto-enol Tautomerism
  • Isomeric compounds that can rapidily interconvert
    by the movement of a proton are called tautomers
    and the phenomenon is called tautomerism
  • Enols rearrange to the isomeric ketone by the
    rapid transfer of a proton from the hydroxyl to
    the alkene carbon
  • The keto form is usually so stable compared to
    the enol that only the keto form can be observed

34
Mechanism of Mercury (II) catalyzed Hydration
  • Addition of Hg(II) to alkyne gives a vinylic
    carbocation intermediate
  • Water adds and loses a proton
  • A proton from aqueous acid replaces Hg(II)

35
Hydration of Unsymmetrical Alkynes
  • Unsymmetrically substituted internal alkyne
  • If the alkyl groups at either end of the C-C
    triple bond are not the same, both products can
    form and this is not normally useful

36
Hydration of Unsymmetrical Alkynes
  • Terminal alkyne
  • If the triple bond is at the first carbon of the
    chain (then H is what is attached to one side)
    this is called a terminal alkyne
  • Hydration of a terminal alkyne always gives the
    methyl ketone, which is useful

37
Mercury(II)-Catalyzed Hydration Summary
  • H-OH adds to an alkyne to give an enol which
    converts to ketone
  • Markovnikov regiochemistry
  • Mercury-containing vinylic carbocation
    intermediate

38
Practice Problem What product would you obtain
by hydration of the following alkynes?
39
Practice Problem What alkynes would you start
with to prepare the following ketones?
40
Hydroboration/Oxidation of Alkynes
  • BH3 (borane) adds to alkynes to give a vinylic
    borane
  • Oxidation with H2O2 produces an enol that
    converts to the ketone or aldehyde
  • Process converts alkyne to ketone or aldehyde
    with Non-Markovnikov orientation
  • This is opposite to mercuric ion catalyzed
    hydration

41
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42
Comparison of Hydration of Terminal Alkynes
  • Hydroboration/oxidation converts terminal alkynes
    to aldehydes because addition of water is
    non-Markovnikov
  • Mercury(II) catalyzed hydration converts terminal
    alkynes to methyl ketones

43
Hydroboration-oxidation Summary
  • Borane adds to an alkyne to give a vinylic borane
    which is then oxidized by alkaline H2O2 to give
    an enol which converts to ketone or aldehyde
  • Non-Markovnikov regiochemistry

44
Practice Problem What alkyne would you start
with to prepare each of the following
compounds by a hydroboration/oxidation
reaction?
45
C. Reduction of Alkynes
  • Reduction of alkynes is the addition H-H to an
    alkyne
  • Complete Catalytic hydrogenation using a metal
    catalyst (Pd/C) or
  • Partial Catalytic hydrogenation using Lindlar
    catalyst
  • Reduction with Lithium/ammonia

46
Catalytic Hydrogenation
  • Addition of H2 over a metal catalyst (such as
    palladium on carbon, Pd/C) converts alkynes to
    alkanes (complete reduction)
  • The addition of the first equivalent of H2
    produces an alkene intermediate, which is more
    reactive than the alkyne so the alkene is not
    observed

47
Conversion of Alkynes to cis-Alkenes
  • Addition of H2 using the Lindlar catalyst
    (chemically deactivated palladium on calcium
    carbonate) produces a cis alkene
  • The two hydrogens add syn (from the same side of
    the triple bond)

48
Conversion of Alkynes to trans-Alkenes
  • Alkynes are reduced to trans alkenes with sodium
    or lithium in liquid ammonia
  • Anhydrous ammonia (NH3) is liquid below -33 ºC
  • Alkali metals dissolve in liquid ammonia and
    function as reducing agents
  • The reaction involves a radical anion
    intermediate

49
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50
Reduction of Alkynes Summary
  • Addition of H2 over a metal catalyst (Pd/C) to an
    alkyne produces an alkane

51
  • Addition of H2 using the Lindlar catalyst
    produces a cis alkene
  • syn stereochemistry

52
  • Alkynes are reduced to trans alkenes with sodium
    or lithium in liquid ammonia
  • a radical anion intermediate
  • anti stereochemistry

53
Practice Problem Using any alkyne needed, how
would you prepare the following
alkenes?
  • trans-2-Octene
  • cis-3-Heptene
  • 3-Methyl-1-pentene

54
D. Oxidative Cleavage of Alkynes
  • Strong oxidizing reagents (O3 or KMnO4) cleave
  • internal alkynes, producing two carboxylic acids
  • terminal alkynes, producing a carboxylic acid and
    carbon dioxide

55
  • Neither process is useful in modern synthesis.
  • Historically, these were used to elucidate
    structures because the products indicate the
    structure of the alkyne precursor

56
Oxidative Cleavage of Alkynes Summary
57
Practice Problem Propose structures for alkynes
that give the following products on
oxidative cleavage by KMnO4
58
E. Alkyne Acidity Formation of Acetylide
Anions
  • Reaction of strong anhydrous bases (Na-NH2) with
    a terminal alkyne produces an acetylide ion
  • Terminal alkynes are relatively acidic

59
  • Terminal alkynes are weak Brønsted acids (pKa
    25)
  • Alkenes and alkanes are much less acidic.

60
  • Acetylide anions are more stable than either
    alkyl anions or vinylic anions because
  • They have sp-hybridized carbon and their negative
    charge is in a hybrid orbital with 50 s
    character, allowing the charge to be closer to
    the nucleus.

61
Formation of Acetylide Anions Summary
  • Reaction of strong anhydrous bases (Na-NH2) with
    a terminal alkyne produces an acetylide ion

62
Practice Problem The pKa of acetone, CH3COCH3,
is 19.3. Which of the following bases
is strong enough to deprotonate
acetone?
  • KOH (pKa of H2O 15.7)
  • Na -C?CH (pKa of C2H2 25)
  • NaHCO3 (pKa of H2CO3 6.4)
  • NaOCH3 (pKa of CH3OH 15.6)

63
F. Alkylation of Acetylide Anions
  • Reaction of an acetylide anion with a primary
    alkyl halide produces a larger alkyne
  • Acetylide anions can react as nucleophiles as
    well as bases
  • Acetylide anions can displace a halide ion from a
    1o alkyl halide

64
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65
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66
Limitations of Alkylation of Acetylide Ions
  • Reactions only are efficient with 1º alkyl
    bromides and alkyl iodides
  • Acetylide anions can behave as bases as well as
    nucelophiles
  • Reactions with 2º and 3º alkyl halides gives
    dehydrohalogenation, converting alkyl halide to
    alkene

67
Alkylation of Acetylide Anions Summary
  • Reaction of an acetylide anion with a primary
    alkyl halide produces a larger alkyne

68
Practice Problem Show the terminal alkyne and
alkyl halide from which the following
products can be obtained. If two
routes look feasible, list both
69
Practice Problem How would you prepare
cis-2-butene starting from propyne, an
alkyl halide, and any other reagents
needed? This problem cant be worked
in a single step. Youll have to carry
out more than one reaction.
70
G. An Introduction to Organic Synthesis
  • Organic synthesis may be used to
  • produce new molecules that are needed as drugs or
    materials
  • design, test and improve efficiency and safety
    for making known molecules
  • test ideas and methods, answering challenges

71
Synthesis as a Tool for Learning Organic Chemistry
  • In order to propose a synthesis, one must be
    familiar with reactions
  • What they begin with
  • What they lead to
  • How they are accomplished
  • What the limitations are
  • A synthesis combines a series of proposed steps
    to go from a defined set of reactants to a
    specified product

72
Strategies for Synthesis
  • Compare the target and the starting material
  • Consider reactions that efficiently produce the
    outcome.
  • Look at the product and think of what can lead to
    it

73
Example
  • Problem prepare octane from 1-pentyne
  • Strategy use acetylide coupling

74
Practice Problem Beginning with 4-octyne as
your only source of carbon and using
any inorganic reagents necessary, how
would you synthesize the following
compounds?
  • Butanoic acid
  • cis-4-Octene
  • 4-Bromooctane
  • 4-Octanol (4-hydroxyoctane)
  • 4,5-Dichlorooctane

75
Practice Problem Beginning with acetylene and
any alkyl halides needed, how would
you synthesize the following compounds?
  • Decane
  • 2,2-Dimethylhexane
  • Hexanal
  • 2-Heptanone

76
Chapter 8
The End
77
Addition of HX and X2 Summary
78
Hydration of Alkynes
  • Mercury (II) catalyzes Markovnikov oriented
    addition
  • Hydroboration-oxidation gives the non-Markovnikov
    product

79
Reduction of Alkynes Summary
80
Oxidative Cleavage of Alkynes Summary
81
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82
Formation of Acetylide Anions Summary
83
Alkylation of Acetylide Anions Summary
  • Reaction of an acetylide anion with a primary
    alkyl halide produces a larger alkyne

84
Alkylation of Acetylide Anions Summary
  • Reaction of an acetylide anion with a primary
    alkyl halide produces a larger alkyne
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