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Organic Chemistry. William H. Brown & Christopher S. Foote. Alkynes. Chapter 10. Chapter 10. 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”. Cycloalkynes.

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organic chemistry

Organic Chemistry

William H. Brown & Christopher S. Foote

alkynes
Alkynes

Chapter 10

Chapter 10

nomenclature
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”
cycloalkynes
Cycloalkynes
  • The smallest cycloalkyne isolated is cyclononyne
    • the C-C-C bond angle about the triple bond is approximately 156°
physical properties
Physical Properties
  • Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton
acidity
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
acidity7
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)
acidity8
Acidity
  • Water is a stronger acid than acetylene; hydroxide ion is not a strong enough base to convert acetylene to its anion
alkylation of acetylides
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.
alkylation of acetylides10
Alkylation of Acetylides
  • Alkylation of acetylide anions is the most convenient method for the synthesis of terminal alkynes
alkylation of acetylides11
Alkylation of Acetylides
  • Alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne
alkylation of acetylides12
Alkylation of Acetylides
  • With 2° and 3° alkyl halides, E2 is the major reaction
preparation
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
preparation14
Preparation
  • Alkynes are also prepared by double dehydrohalogenation of geminal dihalides
preparation15
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
preparation16
Preparation
  • A side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C
allenes
Allenes
  • Most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions
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
reduction19
Reduction
  • With the Lindlar catalyst, reduction stops at addition of one mole of H2
    • this reduction shows syn stereoselectivity
reduction20
Reduction
  • Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity
na nh 3 l reduction
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
na nh 3 l reduction22
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
hydroboration
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
hydroboration24
Hydroboration
  • To prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia)2BH
hydroboration25
Hydroboration
  • Treatment of a terminal alkyne with (sia)2BH results stereospecific and regioselective hydroboration
hydroboration26
Hydroboration
  • Treatment of an alkenylborane with H2O2 in aqueous NaOH gives an enol
hydroboration27
Hydroboration
  • Enol:a compound containing an OH group on one carbon of a C=C
  • The enol is in equilibrium with a compound formed by migration of a hydrogen atom from oxygen to carbon and the double bond from C=C to C=O
  • The enol and keto forms aretautomersand their interconversion is called tautomerism
    • the keto form generally predominates at equilibrium
    • we discuss keto-enol tautomerism in detail in Section 16.11
hydroboration28
Hydroboration
  • Hydroboration/oxidation of an internal alkyne gives a ketone
hydroboration29
Hydroboration
  • Hydroboration/oxidation of a terminal alkyne gives an aldehyde
addition of x 2
Addition of X2
  • Alkynes add one mole of bromine to give a dibromoalkene
    • addition shows anti stereoselectivity
addition of x 231
Addition of X2
  • The intermediate in bromination of an alkyne is a bridged bromonium ion
addition of hx
Addition of HX
  • Alkynes undergo regioselective addition of first one mole of HX and then a second mole to give a dibromoalkane
addition of hx33
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
addition of h 2 o hydration
Addition of H2O:hydration
  • In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water
addition of h 2 o hydration35
Addition of H2O:hydration
  • 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
addition of h 2 o hydration36
Addition of H2O:hydration
  • Step 3: proton transfer to solvent
  • Step 4: tautomerism of the enol gives the keto form
addition of h 2 o hydration37
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
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 synthesis39
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
organic synthesis40
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 synthesis41
Organic Synthesis
  • Target molecule: cis-3-hexene
organic synthesis42
Organic Synthesis
  • starting materials are acetylene and bromoethane
organic synthesis43
Organic Synthesis
  • Target molecule: 2-heptanone
organic synthesis44
Organic Synthesis
  • starting materials are acetylene and 1-bromopentane
prob 10 9
Prob 10.9

Predict bond angles about each highlighted atom.

prob 10 10
Prob 10.10

State the orbital hybridization of each highlighted atom.

prob 10 11
Prob 10.11

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

prob 10 12
Prob 10.12

How many stereoisomers are possible for enanthotoxin?

prob 10 13
Prob 10.13

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

prob 10 15
Prob 10.15

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

prob 10 17
Prob 10.17

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

prob 10 18
Prob 10.18

Propose a mechanism for this reaction.

prob 10 21
Prob 10.21

Show how to convert propene to each compound.

prob 10 22
Prob 10.22

Show how to bring about each conversion.

prob 10 24
Prob 10.24

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

prob 10 26
Prob 10.26

Show how to bring about each conversion.

prob 10 31
Prob 10.31

Show how to bring about this conversion.

alkynes59
Alkynes

End Chapter 10