Lecture 16 Alkenes Structure, Stability and Formation

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Lecture 16 - Alkenes - Structure, Stability and
Formation

• Chapter 5
• Structure of alkenes - requirements for p bond
  formation
• Isomerism in alkenes - cis and trans and E and
  Z
• Relative stabilities of alkenes - degree of
  substitution
• Cycloalkenes
• Preparation of alkenes - Elimination reactions
• Dehydration of Alcohols with acids

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Structure and bonding in ethylene Figure 5.1
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cis-trans isomerism
1-butene
2-methylpropene
cis-2-butene
trans-2-butene
Cinnamaldehyde (trans alkene - E)
cis alkene (Z)
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Interconversion of cis and trans-2-butene Figure
5.2
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Heats of combustion of isomeric C4H8
alkenes Figure 5.4
Generally, the more substituted an alkene, the
more stable
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Molecular models of cis-2-butene and
trans-2-butene Figure 5.5
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Cycloalkenes - trans not necessarily more stable
than cis
C-12 cis and trans equal in energy
Sterculic acid (natural product)
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Preparation of Alkenes - Elimination reactions
Dehydration of Alcohols
Usually results in more highly substituted alkene
being major product - Zaitsev rule
(regioselectivity)