Alkynes Rynes

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Chapter 11 Alkynes (R-ynes)
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General Characteristics of Alkynes
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Alkynes
R-ynes are either terminal or internal
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Alkynes (Structure and Bonding)
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Alkynes (Structure and Bonding)
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Alkynes (Structure and Bonding)
R-ynes undergo many of the same addition rxns as
R-enes
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Alkynes (Structure and Bonding)
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Naming Alkynes
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When naming R-ynes..
use the same rules as was used in naming
alkenes.
For example, how would you name these?
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Note In some cases the alkyne group can be
named as a substituent group.
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Practice Examples
3,4-dimethyl-1,5-octadiyne
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An Interesting Alkyne
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Preparation of R-ynes
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Preparation of Alkynes (E2)
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Reactions of R-ynes
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Reactions of Alkyne
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TERMINAL ALKYNES can act as Bronsted acids in
the presence of certain strong bases.
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Addition of HX
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Addition of X2
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Hydration
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An enol is simply an alkene with an OH on one
of the sp2 C atoms.
Is this an enol?
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The enol and the keto forms are special type of
constitutional isomers called tautomers.
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OK back to hydration again
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Lets take a look at the hydration mechanism
again
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Hydroboration/Oxidation
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HydroborationOxidation of Alkynes
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HydroborationOxidation
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Reactions of Acetylide Anions
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Acetylide anions are important synthetic
intermediates...... for preparing new C-C
bonds .
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Write a synthetic route for the preparation of
propanal from methanol.
1. PBr3 2. NaC?CH (must show its preparation )
3. BH3 4. H2O2, -OH
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Acetylide anions behave as other nucleophiles.
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For example, what is the product of this
reaction?
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What about the product of this epoxide reaction
?
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Synthesis
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How would you prepare 2-pentyne from ethyne?
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Synthesis

• Consider these questions
• Do functional groups change?
• Are new C-C bonds formed?
• Is stereochemistry or regiochemistry introduced?

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Synthesis
The process of planning a synthesis with two or
more steps is called retrosynthetic analysis
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How would you prepare 2-pentyne from ethyne?
Do this problem as a homework problem
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Alkynes
Synthesis
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Alkynes
Physical Properties

• The physical properties of alkynes resemble those
  of hydrocarbons having similar shape and
  molecular weight.
• Alkynes have low melting points and boiling
  points.
• Melting point and boiling point increase as the
  number of carbons increases.
• Alkynes are soluble in organic solvents and
  insoluble in water.

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Alkynes
Interesting Alkynes

• Acetylene (H-C?C-H) is a colorless gas that burns
  in oxygen to form CO2 and H2O. The combustion of
  acetylene releases more energy per mole of
  product formed than any other hydrocarbons. It
  burns with a very hot flame and is an excellent
  fuel.

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Alkynes
HydrationElectrophilic Addition of Water
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Alkynes
HydrationElectrophilic Addition of Water

• Consider the conversion of a general enol A to
  the carbonyl compound B. A and B are tautomers A
  is the enol form and B is the keto form of the
  tautomer.

• Equilibrium favors the keto form largely because
  the CO is much stronger than a CC.
  Tautomerization, the process of converting one
  tautomer into another, is catalyzed by both acid
  and base.