Unsaturated Hydrocarbons II: Alkynes

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Unsaturated Hydrocarbons II Alkynes

• By
• Dr. Siham Lahsasni

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Alkynes Molecular and Structural Formulas

• The alkynes comprise a series of carbon and
  hydrogen based compounds that contain one triple
  bond. This group of compounds is a homologous
  series with the general molecular formula of
  CnH2n2
• The alkyne triple bond is composed of one s and
  two p covalent bonds.
• The simplest alkyne, ethyne (also known as
  acetylene), has two carbon atoms and the
  molecular formula of C2H2. The structural formula
  for ethyne is

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sp hybridization
This involves the mixing of one s- and one
p-orbital forming two sp-hybrid orbitals. The two
sp-hybrid orbitals are oriented in a linear
arrangement and bond angle is 180.
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Acetylene

• Both C are sp hybridized
• 2 C-H s bonds are made by the interaction of C
  sp with H1s orbitals (see red arrows)
• 1 C-C s bond is made by the interaction of C sp
  with another C sp orbital (see green arrow)
• 2 C-C p bonds are made by the interaction of the
  2 pairs of C p orbitals (see black arrows)

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Nomenclature

1. Identify the longest continuous chain of carbon
   atoms that contains the carbon-carbon triple
   bond. the - ane ending is changed to yne
2. Number the carbon atoms of the longest
   continuous chain, starting at the end closest to
   the triple bond.
3. The position of the triple bond is indicated by
   placing the lower of the pair of numbers assigned
   to the triple-bonded carbon atoms in front of the
   name of the alkyne.
4. The location and name of any substituent atom or
   group is indicated.

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IUPAC Propyne
Common Methyl acetylene
5-bromo-2-pentyne
IUPAC 2,6-Dimethyl-3-heptyne
Common Isobutylisopropylacetylene
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• Give the IUPAC and common names of the following
  compounds

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Physical Properties

• Nonpolar, insoluble in water.
• Soluble in most organic solvents.
• Boiling points similar to alkane of same size.
• Less dense than water.
• Up to 4 carbons, gas at room temperature.
• Terminal alkynes, R-C?C-H, are more acidic than
  other hydrocarbons.

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Preparation of alkynes

1. Dehydrohalogenation of alkyl dihalides

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• 2. Reaction of sodium Acetylide with Primary
  Alkyl Halides

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Reactions of alkynes

1. Addition of hydrogen

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2. Addition of halogen
3. Addition of hydrogen halide
4- Addition of water Hydration
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5. Addition of Born Hydried
6- Oxidation of alkynes
7- Ozonolysis
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Visual test for alkynes