Bond Making and Bond Breaking

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Bond Making and Bond Breaking
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Common Reactive Intermediates
Bond Making and Bond Breaking

• Radicals and carbocations are electrophiles
  because they contain an electron deficient
  carbon.
• Carbanions are nucleophiles because they contain
  a carbon with a lone pair.

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Energy Diagrams

• For the general reaction

• The energy diagram would be shown as

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Activation Energy

• The activation energy is the minimum amount of
  energy a collision of the reactants must supply
  in order to react.
• The larger the Ea, the greater the amount of
  energy that is needed for a reaction to occur,
  and the slower the reaction rate.
• The structure of the transition state is
  somewhere between the structures of the starting
  material and product.
• Transition states are drawn in brackets, with a
  superscript double dagger ().

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Drawing a Transition State
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Equilibrium vs Rate
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Stepwise Reactions

• Consider the following two step reaction

• An energy diagram must be drawn for each step.
• The two energy diagrams must then be combined to
  form an energy diagram for the overall two-step
  reaction.
• Each step has its own energy barrier, with a
  transition state at the energy maximum.

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Step 1 Energy Diagram
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Step 2 Energy Diagram
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Overall Energy Diagram
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Kinetics

• Kinetics is the study of reaction rates,
  specifically the effect of reactant
  concentrations on rate.
• Temperature, Ea, reactant structure, and
  catalysts all also can affect reaction rates.
• Collision theory provides an explanation for
  these effects.

Rate collision frequency x orientation factor x
energy factor
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Predictions of Collision Theory

• For an elementary reaction, the higher the
  concentration of the reactants, the faster the
  rate.
• The higher the temperature, the faster the rate.
• The lower the Ea the faster the reaction.
• A rate law or rate equation shows the
  relationship between the reaction rate and the
  concentrations of the reactants. It is
  experimentally determined.

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Kinetics and Mechanism

• A rate equation contains concentration terms for
  all reactants in a one-step mechanism. Collision
  theory requires this.
• A rate equation contains concentration terms for
  only the reactants in the rate-determining step
  of a multi-step reaction.

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Catalysts

• Some reactions do not proceed at a reasonable
  rate unless a catalyst is added.
• A catalyst is a substance that speeds up the rate
  of a reaction. It is recovered unchanged in a
  reaction, and it does not appear in the product.

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Enzymes

• Enzymes are biochemical catalysts composed of
  amino acids held together in a very specific
  three-dimensional shape.
• An enzyme contains a region called its active
  site which binds an organic reactant, called a
  substrate. The resulting unit is called the
  enzyme-substrate complex.
• Once bound, the organic substrate undergoes a
  very specific reaction at an enhanced rate. The
  products are then released.