Chapter 3: Alkenes, Alkynes,

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Chapter 3 Alkenes, Alkynes, Aromatic Compounds
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Introduction

• Alkenes contain carbon-carbon double bonds.
• Alkynes contain carbon-carbon triple bonds.
• Because alkenes and alkynes can add more
  hydrogens, there are often called unsaturated
  hydrocarbons.

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Nomenclature of Alkenes

• Alkenes are named similarly to alkanes with some
  additional rules
• I. The suffix (ending) -ene is used when
  double bonds are present. Do an example.

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Rules for Naming Alkenes (Contd)

• II. When a chain has four or more carbons, we
  number the position of the double bond, using
  only the lower of possible numbers.
• Do an example.
• III. Branches and halogen substituents are named
  as in the rules for alkanes. Do an example.

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Rules (Contd)

• IV. The longest chain must include the double
  bond. Do an example.
• V. Cyclic alkanes containing a double bond are
  called cyclic alkenes. Do some examples.
• VI. Compound with two or more double bonds are
  given the suffix (ending) -diene, -triene,
  etc.
• Show example of a vinyl group.

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Sp2 Hybridization and p Orbitals

• The hybridization of carbon involved in a double
  bond is sp2 (3 single bonds, 1 double bond). The
  orbital is 1/3 s character and 2/3 p character.
• The orbitals all lie in the same plane and the
  bond angle between them is 120. The unchanged p
  orbital is forms the double bond and is 90 to
  the sp2 orbitals.

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Geometry and Stereoisomerism

• A double bond consists of a single s bond and a
  single p bond.
• In order for the p bond to exist, the two p
  orbitals must be parallel to each other.
• Show example ethylene.

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Stereoisomerism (Contd)

• There is no rotation around a double bond.
• Due to the lack of rotation, there exists the
  same type of stereoisomerism as in alkanes.
• Show cis and trans 2-butene.

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Stereoisomerism (Contd)

• In order for there to be cis-trans stereoisomers,
  both double-bonded carbons must be connected to
  two different groups.

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Physical Properties of Alkenes

• Similar to alkanes (discuss trends ).

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Chemical Properties of Alkenes Addition Reactions

• Unlike alkanes, alkenes undergo a variety of
  reactions. Most reactions involve the double
  bond.
• Addition Reaction show example
• Addition of H2/catalyst
• Addition of X2

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Addition of HX

• Addition of a different group to each carbon the
  result is two possible products.
• e.g. propene
• Governed by Markovnikovs rule
• In the addition of HX, the hydrogen goes to the
  carbon that already has more hydrogens.

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Addition of Water

• Addition of H2O requires a catalyst, usally
  sulfuric acid. This reaction is called a
  hydration reaction. Follows Markovnikovs rule.

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Addition Polymers

• Polymerization reaction may be the most important
  reaction involving alkenes.
• Show polymerization of ethene.
• A chain made up of repeating units is called a
  polymer. The repeat unit is called the monomer.

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Alkynes

• Same rules as alkenes, except ending yne is used
  for the triple bond.

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Aromatic Hydrocarbons