Organic Chemistry

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Organic Chemistry
William H. Brown Christopher S. Foote
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Alkenes I

• Chapter 5

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Unsaturated Hydrocarbons

• Unsaturated hydrocarbon contains one or more
  carbon-carbon double or triple 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 20-21)

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Benzene Phenyl Group

• We do not study benzene and its derivatives until
  Chapters 20 21
• but, we show structural formulas of compounds
  containing the phenyl group before that time
• the phenyl group is not reactive under any of
  the conditions we describe in Ch 6-19

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Structure of Alkenes

• The two carbon atoms of a double bond and the
  four atoms attached to them lie in a plane, with
  bond angles of approximately 120
• A double bond consists of
• one sigma bond formed by overlap of sp2 hybrid
  orbitals
• one pi bond formed by overlap of parallel 2p
  orbitals
• it takes approx. 264 kJ (63 kcal)/mol to break
  the pi bond in ethylene

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Restricted rotation about CC
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Cis,Trans Isomerism

• Cis,trans isomers isomers that have the same
  connectivity but a different arrangement of their
  atoms in space due to the presence of 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 carbon, locate and name
  substituents, locate the double bond, and name
  the main chain.

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Alkenyl Groups
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Common Names

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

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Configuration

• The cis,trans system configuration is determined
  by the orientation of atoms of the main chain

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Configuration - E,Z

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

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Configuration - E,Z

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

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Cis,Trans Isomerism

• Cycloalkenes
• configuration of the double bond in cyclopropene
  through cycloheptene must be cis
• cyclopentene is only slightly puckered

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Cis,Trans Isomerism

• the configuration of the double bond in
  cyclohexene must be cis
• cyclohexene is puckered

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Cis,Trans Isomerism

• Cycloalkenes
• trans-cyclooctene is the smallest
  trans-cycloalkene stable at 25C
• the cis isomer is 38 kJ (9.1 kcal)/mol more
  stable than the trans isomer.

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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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Terpenes

• Terpene a compound whose carbon skeleton can be
  divided into two or more units identical with the
  carbon skeleton of isoprene

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Terpenes

• Myrcene, C10H16, a component of bayberry wax and
  oils of bay and verbena
• Menthol, from peppermint

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Terpenes

• ?-Pinene, from turpentine
• camphor, from the camphor tree

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Terpenes

• Vitamin A (retinol)
• how many stereoisomers are possible for this
  terpene alcohol?

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Prob 5.7

• Using the VSEPR model, predict all bond
  angles about each circled atom.

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Prob 5.12

• Name these alkenes and cycloalkenes.

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Prob 5.13

• Arrange the groups in each set in order of
  increasing priority.

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Prob 5.14

• Assign an E or Z configuration to each
  compound. Also assign a cis or trans
  configuration to each.

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Prob 5.16

• For each molecule that shows cis,trans
  isomerism, draw the cis isomer.

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Prob 5.17

• Draw structural formulas for all compounds of
  molecular formula C5H10 that are
• (a) alkenes that do not show cis,trans isomerism.
• (b) alkenes that do show cis,trans isomerism.
• (c) cycloalkanes that do not show cis,trans
  isomerism.
• (d) cycloalkanes that do show cis,trans isomerism.

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Prob 5.19

• Draw a structural formula of a bromoalkene of
  molecular formula C5H9Br that shows
• (a) neither E,Z isomerism nor chirality.
• (b) E,Z isomerism but not chirality.
• (c) chirality but not E,Z isomerism.
• (d) both chirality and E,Z isomerism.

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Prob 5.23

• Which alkenes exist as pairs of cis,trans
  isomers? For each that does, draw the trans
  isomer.

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Prob 5.24

• Show that four stereoisomers are possible for
  this compound. Draw the stereoisomer with an E
  configuration at the double bond and an R
  configuration at the stereocenter.

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Prob 5.28

• Show that warburganal is a terpene. Label all
  stereocenters and state how many stereoisomers
  are possible for a molecule of this structure.

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Prob 5.29

• Show that santonin is a terpene. Label all
  stereocenters and tell how many stereoisomers are
  possible for the molecule of this structure.

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Prob 5.30

• (a) Show that zoapatanol is a terpene.
• (b)Specify the configuration at the double
  bond to the seven-membered ring.
• (c) How many stereoisomers are possible for
  this molecule?

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Prob 5.31 Pyrethrin II

• Label all stereocenters in pyrethrin II. How
  many stereoisomers are possible for this molecule?

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Prob 5.31 Pyrethrosin

• Show that pyrethrosin is a terpene. Label all
  stereocenters in pyrethrosin and state how many
  stereoisomers are possible for it.

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Alkenes I

• End Chapter 5