It takes Alkynes to make a world.

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Chapter 9

• It takes Alkynes to make a world.

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Homework

• Pp. 384-389
• Day 1 16-24
• Day 2 25-29, 31, 33, 34
• Work any and all problems that you can do.

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Alkynes

• Alkynes are hydrocarbons that contain a
  carbon-carbon triple bond
• Simple alkynes have the generic structural
  formula CnH2n-2.
• Acetylene is the simplest alkyne and can be
  produced from Calcium Carbide.

4
Structure and Bonding in Alkynes

• The carbons in the triple bond are sp hybridized.
• sp hybridization results in a 180o bond angle
  (linear geometry).

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Carbon Hybridization and Electronegativity
Electrons in an orbital with more s character are
closer to thenucleus and more strongly held.
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Preparation of Alkynes

• Synthetic methods involve two major reaction
  types
• 1. Functional Group Transformations
• 2. Carbon-carbon Bond Formations

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Formation of Sodium Acetylide

• Sodium acetylide is a good nucleophile and can be
  produced by reacting acetylene with sodium amide
• Note Since Mr. Singer does not know how to make
  a triple bond with Power Point, he will now
  sketch this reaction on the board.
• Once sodium acetylide is produced, it can be used
  to make terminal alkynes of most any length.
• Ex synthesize 1-heptyne from acetylene and any
  necessary organic and inorganic reagents.

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Preparation of Alkynes by Elimination Reactions

• Alkynes can be produced by reacting vicinal or
  geminal dihalides with sodium amide.
• Using the following substrates and any necessary
  organic or inorganic reagents Synthesize propyne
• a. 2-propanol c. isopropyl bromide

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Limitation to Use of Sodium Acetylide

• Sodium acetylide can only be used on primary
  alkyl halides. For secondary and tertiary alkyl
  halides, elimination reactions predominate.

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Preparation of Alkynes by Elimination Reactions

• Both geminal dihalides and vicinal dihalides will
  react with sodium amide to produce alkynes.
• Used most often for terminal alkynes. Since
  terminal alkynes are acidic, quenching with water
  is required to produce the alkyne.

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Reactions of Alkynes

• Alkynes can undergo two classes of reactions
• Reactions involving the acidity of terminal
  alkynes.
• Alkene-like electrophilic addition reactions.

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Hydrogenation of Alkynes

• Alkynes, like alkenes, react with hydrogen over a
  metal catalyst to produce alkanes.
• The addition is syn and if the stoichiometry
  is controlled, cis alkenes can be produced.
• The best way to control the reaction is by use of
  the Lindlar catalyst.

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Lindlar Catalyst

• The Lindlar Catalyst is palladium on calcium
  carbonate. It contains lead acetate and
  quinoline. Lead acetate and quinoline poison the
  catalyst for the hydrogenation of alkenes.

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Metal Ammonia Reductions of Alkynes

• Group I metals in liquid ammonia effectively
  reduce alkynes to alkenes. The addition is anti,
  therefore the resulting alkynes are trans.

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Addition of Hydrogen Halides to Alkynes

• Hydrogen halides add to alkynes much like
  alkenes. The additions follows Markovnikovs
  rule.
• However, hydrogen bromide adds to alkynes in the
  presence of peroxides via a free-radical
  mechanism. The rxn is antiMarkovnikov.

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Hydration of Alkynes

• Water adds to alkynes forming aldehydes. The
  standard method is to use aqueous sulfuric acid
  over a mercury II sulfate or mercury II oxide
  catalyst.

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Addition of Halogens.

• Halogens add to alkynes forming tetrahaloalkanes.

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Ozonolysis of Alkynes

• Ozonolyis of alkynes results in the formation of
  carboxylic acids.