Organic Reactions

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Organic Reactions

• Kinds of Reactions
• Mechanisms (polar, non-polar)
• Bond Dissociation Energy
• Reaction Profiles

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Types of Reactions

• Addition Reactions
• Elimination Reactions

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Types of Reactions

• Substitution
• Polar
• Non-polar

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Rearrangement
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Definitions

• Mechanism Complete step-by-step of exactly which
  bonds break and which bonds form and in what
  order.
• Thermodynamics The study of the energy changes
  that occur in chemical transformations. This
  allows for comparison of stability of reactants
  and products.
• Kinetics The study of reaction rates,
  determining which products are formed most
  rapidly. One can predict how the rate will
  change with changing conditions.

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Reaction Profile (Exothermic)
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2nd Order Reaction
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1st Order Reaction
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Bond BreakingNon-polar and Polar
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Bond FormingNon-polar and Polar
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Non-polar Reaction Involves Free Radicals
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Free Radicals are Neutral, but Electron-Deficient
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Free Radical Chlorination
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Experimental Evidence Helps to Determine Mechanism

• Chlorination does not occur at room temperature
  in the dark.
• The most effective wavelength of light is blue
  that is strongly absorbed by Cl2 gas.
• The light-initiated reaction has a high quantum
  yield (many molecules of product are formed from
  each photon of light).

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Free Radical Species are Constantly Generated
Throughout the ReactionPropagation
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Termination Reaction of any 2 Radicals
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Enthalpy of Reaction (DHo) Measures Difference in
Strength of Bonds Broken and Bonds FormedBond
Dissociation Energy
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DHo Sbonds broken-Sbonds formed
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DHrxn -105 kJ/mol
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Why Not This Mechanism?
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Chlorination of Propane
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Hs are not abstracted at the same rate.
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Chlorination of Methylpropane
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Tertiary Hs removed 5.5 times more readily than
primary Hs in chlorination reactions
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3o Radicals are Easiest to Form
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Stability of Free Radicals
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Bromination is Very Selective
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RDS in Bromination is highly endothermic
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Consider the free radical monochlorination of
2,2,5-trimethylhexane. Draw all of the unique
products (ignore stereoisomers use zig-zag
structures please) and predict the ratio or
percent composition of the products.The relative
reactivity of H abstraction in a chlorination
reaction 1o 2o 3o 1 4.5 5.5
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Chlorofluorocarbons and the Depletion of Ozone
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Polar ReactionsNucleophiles Electrophiles
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Nucleophiles are BasesElectrophiles are Acids
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Addition of HBr to Ethylene
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Reactions Often Go Through Intermediates
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Transition State
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Addition Reaction is a Two-Step Mechanism
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How Many Mechanistic Steps?How Many
Intermediates?How Many Transition States?Which
Step is Rate-Determining?