Alkenes and Alkynes

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Lesson 3

• Alkenes and Alkynes

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Alkenes (CnH2n)

• Organic compounds that contain double
  carbon-carbon bonds
• aka unsaturated compounds organic compounds
  that contain double and triple bonds
• General formula CnH2n

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Alkynes (CnH2n-2)

• Organic compounds that contain triple
  carbon-carbon bonds
• General formula CnH2n-2

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Naming Alkenes and Alkynes

• Step 1 Find the longest continuous chain that
  contains the double or triple bond(s) (parent
  chain).

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• Step 2 Number the chain, starting at the end of
  the chain closest to the double or triple bond.
  Use appropriate prefix to indicate the number of
  carbon atoms in the parent chain of the alkene.

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• Step 3 Use the ending -ene to indicate that it
  is an alkene (double bond) or -yne for an
  alkyne (triple bond).

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• Step 4 Branches on alkenes or alkynes are named
  and numbered in the same way as branched alkanes.

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Drawing Alkenes and Alkynes

• Step 1 Draw a straight chain containing the
  number of carbon atoms indicated by the name of
  the parent carbon chain.

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• Step 2 Number the carbon atoms in the parent
  chain from left to right.

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• Step 3 Double or triple bonds are inserted in the
  parent chain after the carbon as numbered in the
  name.

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• Step 4 Attach all branches to their numbered
  locations

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• Step 5 Add enough hydrogens so that all carbons
  will have four bonds.

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Alkene Properties

• 1. React quickly with small, diatomic molecules
  (ie hydrogen, halogens)

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• 2. Participates in
• addition reactions converts unsaturated
  hydrocarbons to saturated hydrocarbons
• hydrogenation addition reactions which convert
  unsaturated hydrocarbons to saturated ones by the
  addition of hydrogen only.

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• Used as starting materials in the petrochemical
  industry for the manufacture of gasoline,
  plastics, etc.
• 4. No rotation around a carbon-carbon double bond
• 5. Alkenes are non-polar

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• Alkenes are unsaturated (contain all single C-C
  bonds and have the maximum number of hydrogens)
• Structural isomers exist for all alkenes larger
  than propene

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• LDFs only
• As the number of carbons increase, boiling and
  melting points increase also

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Alkyne Properties

• No rotation around carbon-carbon multiple bonds.
• React in addition and hydrogenation
• reactions.
• 3. As the number of carbons increase, boiling and
  melting points increase also.

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• Alkynes are non-polar and are soluble in
  non-polar solvents only.
• LDFs only
• Alkynes are unsaturated
• Structural isomers exist for alkynes larger than
  propyne

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Geometric Isomerism

• Isomers of alkenes that differ only in the
  arrangement of their substituted groups
• Possible whenever each carbon of the double bond
  has at least one substituent
• Isomers will differ with respect to their
  physical and chemical properties

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• Trans substituted groups are on opposite sides
  of the double bond
• Cis substituted groups are on the same side of
  the double bond