Ch'6 Organic Reactions

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Ch.6 Organic Reactions
Fluoxetine Hydrochloride (Prozac)
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Writing Organic Reactions
Both equations are correct
Inorganic by-products are often omitted
Reagents may be shown on the left or on top of
the arrow
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Writing Organic Reactions
Consecutive steps are indicated by numbering
reagents
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Types of Organic Reactions
1. Substitution an atom/group of atoms is
replaced by another atom/group of atoms General
equation -
Generally heteroatom
Involves s-bond breakage and formation
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Types of Organic Reactions
2. Elimination atoms are removed from the
starting material and a p-bond is formed
generally H generally halogen
These reactions are often base-promoted
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Types of Organic Reactions
3. Addition atoms are added to the starting
material
A p-bond is broken, two s-bonds are
formed Addition and elimination reactions are
opposite
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This reaction is a(n) A) Elimination
B) Substitution C) Addition
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Intro to Reaction Mechanisms
Reaction mechanism detailed description of how
bonds are broken and formed during the conversion
of starting material to product
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Intro to Reaction Mechanisms
Changes in bonding are shown using curved
arrows Bond cleavage/formation can be homolytic
or heterolytic
Arrows with half-heads are used to show movement
of one e-
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Intro to Reaction Mechanisms
Heterolytic bond making/breaking is much more
common Double-headed arrows show e- movement
The more electronegative atom accepts the
electron pair
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Which of the following correctly shows the e-
pair movement in the above reaction?
A B
C D
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Intro to Reaction Mechanisms
Common reactive intermediates in organic
chemistry -
radical carbocation carbanion
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Note that the p orbital exists, but contains no
electrons. What sort of chemical species would a
carbocation react with?
Although this is a pyramidal structure, we will
also encounter planar carbanions in the future.
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Methylenes
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Breaking a chemical bond A) always releases
energy B) always requires energy C) can release
or require energy
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Thermodynamics
Thermodynamics area of chemistry describing
energy and equilibrium
DGo DHo TDSo
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Bond Dissociation Energies
Bond dissociation energy enthalpy change (DHo)
for homolysis of a bond
The stronger the bond, the higher its BDE
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If the BDE for an H-Cl bond is 104 Kcal/mol,
calculate DHo for
CH3CH2 - H Cl Cl ? CH3CH2 Cl H -
Cl
DHo (98 58) (81 104) - 29 Kcal/mol
This reaction is exothermic and causes an
increase in the temperature of its surroundings.
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Bond Dissociation Energies
Dont memorize the numbers, but know these trends
1. BDEs decrease down a row
2. Heteroatom-heteroatom bonds are usually
weak (low BDE)
3. C-H bonds generally have a higher BDEs than
C-C bonds
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Which has the higher BDE?
A B
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Which has the higher BDE?
A B
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Kinetics
Kinetics study of reaction rates The rate of a
reaction is determined by the activation energy
(Ea) the difference in energy between the
reactants and the transition state
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Reaction Energy Diagrams
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Reaction Energy Diagrams
Some reactions involve multiple steps -
This substitution can take place by a two-step
mechanism -
Step 1
Step 2
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Reaction Energy Diagrams
overall DHo
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Catalysts
Catalyst substance that speeds up a reaction by
lowering the activation energy. Not consumed
during the reaction.
Note that the energies of reactants and products
are unchanged
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Catalysts
Common catalysts are acids and transition metals
Only a small amount (relative to starting
material) is required
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Enzymes
Enzyme biochemical catalyst made up of amino
acids
Enzymes have very specific 3D shapes Unlike many
ordinary catalysts (like H2SO4), enzymes can be
very specific
hexokinase, an enzyme involved in
carbohydrate metabolism