Olefins and acetylenes

1
Olefins and acetylenes

• Chapter 3

2
Alkenes ethene

• H2CCH2 sp2-hybridized

3
Formation of ethene

• Ethene can be compared with the recombination of
  two methyl radicals

4
Additional overlap exists

• The additional overlap between the two p-orbitals
  gives rise to the second double bond

5
Formation of the s-bond

• Combination of the two sp2-orbitals generates a
  bonding s-orbital

6
Formation of the p-bond

• Combining the p-orbitals gives rise to a
  so-called p-bond

7
The double bond is stable

• There is no rotational freedom around the double
  bond !

8
Nomenclature of alkenes
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Geometrical isomers exist

• Make problem 3.5

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Nomenclature Z vs E

• Note Z/E is the correct nomenclature! You will
  notice that cis/trans are used incorrectly
  throughout the book.

11
Nomenclature
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The Cahn-Ingold-Prelog system
13
Continued

• Multiple bonds are treated as multiplied single
  bonds

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Z or E ?

• Make problem 3.12

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Stability of double bonds

• Generally spoken, E-double bonds are more stable
  than Z-double bonds as a result of steric
  interactions

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Stability of double bonds (II)
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Stability of double bonds (III)

• The more substituted, the more stable the double
  bond
• The more the electrons are localized in
  s-orbitals, the lower they are in energy
• Make problem 3.14

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Cycloalkenes

• Make problem 3.18

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Cyclooctene

• Cyclooctene is the smallest ring in which an
  E-double bond can exist

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Bredts rule

• Bredts rule a double bond at a bridgehead
  cannot exist

21
But..

• Of course there are exceptions to this rule. The
  smallest system where a bridgehead double bond is
  possible, is shown above
• Make problems 3.20 and 3.21

22
Acetylenes ethyne
H-C?C-H

• The 2p-orbitals that are not used for
  hybridization are involved in additional bond
  formation

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The triple bond consists of a s-bond

• Again, the sp-orbitals are combined to form a
  s-bond

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.and two p-bonds

• Combination of both p-orbitals on the two atoms
  leads to two bonding p-orbitals that are
  perpendicular two each other

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The different bonds compared
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Nomenclature
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Nomenclature (II)

• Make problem 3.23

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Cycloalkynes

• Cyclic acetylenes are stable starting from
  nine-membered rings

29
Degree of unsaturation

• Make problem 3.26

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Problems

• Make problems 3.30, 3.37, 3.39, 3.40, 3.43