Organic Chemistry

1
Organic Chemistry

• William H. Brown Christopher S. Foote

2
Alkynes
Chapter 10

• Chapter 10

3
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 name acetylene

4
Cycloalkynes

• The smallest cycloalkyne isolated is cyclononyne
• the C-C-C bond angle about the triple bond is
  approximately 156

5
Physical Properties

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

6
Acidity

• A major difference between the chemistry of
  alkynes and that of alkenes and alkanes is the
  acidity of the hydrogen bonded to a triply bonded
  carbon (Section 4.4E)
• the pKa of acetylene is approximately 25, which
  makes it a stronger acid than ammonia but weaker
  than alcohols

7
Acidity

• Acetylene reacts with sodium amide to form sodium
  acetylide
• it can also be converted to its metal salt by
  reaction with sodium hydride or lithium
  diisopropylamide (LDA)

8
Acidity

• Water is a stronger acid than acetylene
  hydroxide ion is not a strong enough base to
  convert acetylene to its anion

9
Alkylation of Acetylides

• Acetylide anions are both strong bases and good
  nucleophiles
• They undergo SN2 reactions with alkyl halides,
  tosylates, and mesylates to form new C-C bonds to
  alkyl groups that is, they undergo alkylation
• because acetylide anions are also strong bases,
  alkylation is practical only with methyl and 1
  halides
• with 2 and 3 halides, E2 is the major reaction.

10
Alkylation of Acetylides

• Alkylation of acetylide anions is the most
  convenient method for the synthesis of terminal
  alkynes

11
Alkylation of Acetylides

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

12
Alkylation of Acetylides

• With 2 and 3 alkyl halides, E2 is the major
  reaction

13
Preparation

• Treatment of a vicinal dibromoalkane with two
  moles of base, most commonly sodium amide,
  results in two successive E2 reactions and
  formation of an alkyne

14
Preparation

• Alkynes are also prepared by double
  dehydrohalogenation of geminal dihalides

15
Preparation

• An alkene can be converted to an alkyne
• for a terminal alkene to a terminal alkyne, 3
  moles of NaNH2 are required
• for an internal alkene to an internal alkyne,
  only 2 moles of NaNH2 are required

16
Preparation

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

17
Allenes

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

18
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

19
Reduction

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

20
Reduction

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

21
Na/NH3(l) Reduction

• Step 1 a one-electron reduction of the alkyne
  gives a radical anion
• Step 2 an acid-base reaction gives an alkenyl
  radical and amide ion

22
Na/NH3(l) 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

23
Hydroboration

• Addition of borane to an internal alkynes gives a
  trialkenylborane
• Treatment of a trialkenylborane with acetic acid
  results in stereoselective replacement of B by H

24
Hydroboration

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

25
Hydroboration

• Treatment of a terminal alkyne with (sia)2BH
  results stereospecific and regioselective
  hydroboration

26
Hydroboration

• Treatment of an alkenylborane with H2O2 in
  aqueous NaOH gives an enol

27
Hydroboration

• Enol a compound containing an OH group on one
  carbon of a CC
• The enol is in equilibrium with a compound
  formed by migration of a hydrogen atom from
  oxygen to carbon and the double bond from CC to
  CO
• The enol and keto forms are tautomers and their
  interconversion is called tautomerism
• the keto form generally predominates at
  equilibrium
• we discuss keto-enol tautomerism in detail in
  Section 16.11

28
Hydroboration

• Hydroboration/oxidation of an internal alkyne
  gives a ketone

29
Hydroboration

• Hydroboration/oxidation of a terminal alkyne
  gives an aldehyde

30
Addition of X2

• Alkynes add one mole of bromine to give a
  dibromoalkene
• addition shows anti stereoselectivity

31
Addition of X2

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

32
Addition of HX

• Alkynes undergo regioselective addition of first
  one mole of HX and then a second mole to give a
  dibromoalkane

33
Addition of HX

• the intermediate in addition of HX is a 2
  vinylic carbocation
• reaction of the vinylic cation with halide ion
  gives the product

34
Addition of H2Ohydration

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

35
Addition of H2Ohydration

• Step 1 attack of Hg2 gives a bridged
  mercurinium ion intermediate, which contains a
  three-center, two-electron bond
• Step 2 water attacks the bridged mercurinium ion
  intermediate from the side opposite the bridge

36
Addition of H2Ohydration

• Step 3 proton transfer to solvent
• Step 4 tautomerism of the enol gives the keto
  form

37
Addition of H2Ohydration

• 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

38
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

39
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 acetylide anions
  (Section 10.5)
• 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

40
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

41
Organic Synthesis

• Target molecule cis-3-hexene

42
Organic Synthesis

• starting materials are acetylene and bromoethane

43
Organic Synthesis

• Target molecule 2-heptanone

44
Organic Synthesis

• starting materials are acetylene and
  1-bromopentane

45
Prob 10.9

• Predict bond angles about each highlighted
  atom.

46
Prob 10.10

• State the orbital hybridization of each
  highlighted atom.

47
Prob 10.11

• Describe each highlighted carbon-carbon bond
  in terms of the overlap of atomic orbitals.

48
Prob 10.12

• How many stereoisomers are possible for
  enanthotoxin?

49
Prob 10.13

• Show how to prepare each alkyne from the
  given starting material.

50
Prob 10.15

• Complete each acid-base reaction and predict
  whether the equilibrium lies toward the left or
  toward the right.

51
Prob 10.17

• Draw a structural formula for the enol
  intermediate and the carbonyl compound formed in
  each reaction.

52
Prob 10.18

• Propose a mechanism for this reaction.

53
Prob 10.21

• Show how to convert propene to each compound.

54
Prob 10.22

• Show how to bring about each conversion.

55
Prob 10.24

• Propose mechanisms for Steps (1) and (2) and
  reagents for (3) and (4).

56
Prob 10.25
57
Prob 10.26

• Show how to bring about each conversion.

58
Prob 10.31

• Show how to bring about this conversion.

59
Alkynes
End Chapter 10