Alkenes and Alkynes 5'15'3 and 7'17'4

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Alkenes and Alkynes5.1-5.3 and 7.1-7.4

• Lecture 18

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Unsaturated Hydrocarbons

• Unsaturated hydrocarbon Contains one or more
  multiple bonds.
• Alkene Contains a carbon-carbon double bond and
  has the general formula CnH2n.
• Alkyne Contains a carbon-carbon triple bond and
  has the general formula CnH2n-2.

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Unsaturated Hydrocarbons

• Arenes Benzene and its derivatives (Ch 21-22)
• The phenyl group is not reactive under any of
  the conditions we describe in Ch 6.

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Structure of Alkenes

• A double bond consists of
• one sigma bond formed by the overlap of sp2
  hybrid orbitals and one pi bond formed by the
  overlap of parallel 2p orbitals.
• the two carbon atoms of a double bond and the
  four atoms bonded to them lie in a plane, with
  bond angles of approximately 120.

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Structure of Alkenes

• it takes approximately 264 kJ (63 kcal)/mol to
  break the pi bond in ethylene that is, to rotate
  one carbon by 90 with respect to the other so
  that there is no overlap between 2p orbitals on
  adjacent carbons.

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Cis,Trans Isomerism in Alkenes

• Cis,trans isomers Isomers that have the same
  connectivity but a different arrangement of their
  atoms in space due to the presence of either a
  ring or a carbon-carbon double bond.

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IUPAC Nomenclature

• 1. Number the longest chain of carbon atoms that
  contains the double bond in the direction that
  gives the carbons of the double bond the lowest
  numbers.
• 2. Locate the double bond by the number of its
  first carbon.
• 3. Name substituents.
• 4. Number the carbons, locate and name
  substituents, locate the double bond, and name
  the main chain.

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Common Names

• Despite the precision and universal acceptance of
  IUPAC nomenclature, some alkenes, particularly
  low-molecular-weight alkenes, are known almost
  exclusively by their common names.

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Common Names

• the common names methylene, vinyl, and allyl are
  often used to show the presence of the following
  alkenyl groups

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The Cis,Trans System

• Configuration is determined by the orientation of
  atoms of the main chain.

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The E,Z System

• Uses priority rules (Chapter 3).
• If groups of higher priority are on the same
  side, the configuration is Z (German, zusammen).
• If groups of higher priority are on opposite
  sides, the configuration is E (German, entgegen).

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The E,Z System

• Example Name each alkene and specify its
  configuration by the E,Z system.

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Cis,Trans Isomerism

• Cycloalkenes
• In small-ring cycloalkenes, the configuration of
  the double bond is cis.
• These rings are not large enough to accommodate a
  trans double bond.

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Cis,Trans Isomerism

• Trans-cyclooctene is the smallest trans
  cycloalkene that has been prepared in pure form
  and is stable at room temperature.
• The cis isomer is 38 kJ (9.1 kcal)/mol more
  stable than the trans isomer.
• The trans isomer is chiral even though it has no
  chiral center.

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Dienes, Trienes, and Polyenes

• For alkenes containing two or more double bonds,
  change the infix -en- to -adien-, -atrien-, etc.
• Those containing several double bonds are often
  referred to more generally as polyenes.

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Dienes, Trienes, and Polyenes

• For alkenes with n double bonds, each of which
  can show cis,trans isomerism, 2n stereoisomers
  are possible.

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Physical Properties

• Alkenes are nonpolar compounds.
• The only attractive forces between their
  molecules are dispersion forces.
• The physical properties of alkenes are similar to
  those of alkanes.

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Nomenclature

• IUPAC use the infix -yn- to show the presence of
  a carbon-carbon triple bond.
• Common names prefix the substituents on the
  triple bond to the word acetylene.

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Cycloalkynes

• Cyclononyne is the smallest cycloalkyne isolated.
• It is quite unstable and polymerizes at room
  temp.
• The C-C-C bond angle about the triple bond is
  approximately 155, indicating high angle strain.

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Physical Properties

• Similar to alkanes/alkenes of comparable
  molecular weight and carbon skeleton.

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Acidity

• The pKa of acetylene and terminal alkynes is
  approximately 25, which makes them stronger acids
  than ammonia but weaker acids than alcohols
  (Section 4.1).
• Terminal alkynes react with sodium amide to form
  alkyne anions.

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Acidity

• Terminal alkynes can also react with sodium
  hydride or lithium diisopropylamide (LDA).
• Hydroxide ion is not a strong enough base to
  convert a terminal alkyne to an alkyne anion.