Lecture 4
AN OVERVIEW
OF ORGANIC REACTIONS
Organic chemistry, JohnMcMurry
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AN OVERVIEW OFORGANICREACTIONS
• Kinds of organic reactions
• Mechanisms
• Radical reactions
• Polar reactions
• Describing a reaction: Equilibria, energy changes, energy
diagrams, intermediates and transition states
• A comparison between biological reactions and
laboratory reactions
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Kinds of Organic Reactions
• Addition reactions: Two reactants combining to forma
single product
• Elimination reactions: Single reactant splitting into two
products
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Kinds of Organic Reactions
• Substitution reactions: Two reactants exchanging parts to
form two newproducts
• Rearrangement reactions: Single reactant yielding an
isomeric product through reorganization of its bonds and
atoms
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Example
• Classify each of the following reactions asan
addition, elimination, substitution, or
rearrangement
• a)CH3Br + KOH → CH3OH +KBr
• b) CH3CH2Br → H2C═CH2 +HBr
• c) H2C═CH2+ H2 → CH3CH3
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Example
• Classify each of the following reactions asan
addition, elimination, substitution, or
rearrangement
• a)CH3Br + KOH → CH3OH +KBr
• b) CH3CH2Br → H2C═CH2 +HBr
• c) H2C═CH2+ H2 → CH3CH3
• Solution:
• a)CH3Br + KOH→ CH3OH + KBr(Substitution)
• b) CH3CH2Br → H2C═CH2+ HBr (Elimination)
• c) H2C═CH2+ H2 → CH3CH3 (Addition)
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How Organic ReactionsOccur:
Mechanisms
• Reaction mechanism: Overall description of a reactionprocess:
Must account for all reactantsand products
• Bond formation or breakage can be symmetrical orunsymmetrical
• Symmetrical cleavage – Homolytic;
1 e movement
• Unsymmetrical cleavage – Heterolytic;
2 e movement
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How Organic ReactionsOccur:
Mechanisms
• Radical reactions: Processes that involve
symmetrical bond-breaking and bond-making
• Radical: Neutral chemical species containing odd
number of electrons
• Has a single, unpaired electron in one of itsorbitals
• Polar reactions: Processes involving unsymmetrical
bond-breaking and bond-making
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Radical Reactions
• Not as common in comparison to polar reactions
• Radical substitution: CH4+ Cl2 → CH3Cl + HCl
• Radical addition:
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Radical Substitution
• Initiation
• Propagation
• Termination
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Radical Addition
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Polar groups
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Polar Reactions
• Polarizability: Tendency of atoms in a molecule to undergo
polarization
• Larger atoms, easily polarizable
• Smaller atoms, less polarizable
• Nucleophile: “Like” (+)
• Electrophile: “Like” (-)
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Some Nucleophiles andElectrophiles
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Example
• An electrostatic potential map of boron trifluoride
is shown
• Is BF3likely to be a nucleophile or anelectrophile?
• Draw a Lewis structure for BF3
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Example
• An electrostatic potential map of boron trifluoride
is shown
• Is BF3likely to be a nucleophile or anelectrophile?
• Draw a Lewis structure for BF3
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Polar Reaction Example: HBr + Ethylene
• Electrophilic addition reaction
• pi part results from p–p overlap
• σ results from sp2–sp2 overlap
• Double bond is more accessible to approachingreactants
than a single bond
• More electron-rich
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Carbon–Carbon Single and DoubleBonds
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Mechanism
Carbocation: Substance that contains a
trivalent, positively charged carbon atom
having six electrons in its outershell
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Example
• Write the mechanism:
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Describing a Reaction: Equilibria and
Energy Changes
• At equilibrium:
 C  D 

K eq =
a
b
 A  B 
c
d
• If Keq > 1 Reaction takes place from left to right
• If Keq = 1 constant concentration of reactants andproducts
• If Keq < 1 Reaction takes place in the reversedirection
• Standard free energy change at 1 atm pressure and 298K:
ΔG° = - RT ln Keq
Where,
R= 1.987 cal/(K · mol) = 8.314 J/(K · mol)
T= Temperature in Kelvin
ln Keq = Natural logarithm of Keq
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Describing a Reaction: Energy Changes
Gibbs free energy (ΔG): Change in energy: ΔG= Gproducts – Greactants
• Exergonic: ΔG< 0, spontaneous, release energy (Energy is lost)
• Endergonic: ΔG> 0, Energy is absorbed; needed
ΔG°= Δ H°- T Δ S°
Enthalpy change (ΔH) (heat of reaction): Total bonding energychange
in areaction
• Exothermic: Reaction that releases heat
• Endothermic: Reaction that absorbsheat
Entropy change (ΔS): Change in the amount of molecular randomness
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Explanation of Thermodynamic Quantities:
ΔG°= Δ H°- T Δ S°
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Example
• Which reaction is likely to be more exergonic,one
with Keq = 1000 or one with Keq = 0.001?
• Solution:
• ΔG° = -RT ln Keq
• Large Keq is related to a large negativeΔG°
• A favorable reaction
• Therefore, a reaction with Keq = 1000 is more exergonic
than a reaction with Keq =0.001
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Describing a Reaction: EnergyDiagrams
and Transition States
Highest energy point
Energy required to
go from reactant to
transition state
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Some Hypothetical Energy Diagrams
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Example
• Which reaction is faster, one with ΔG‡ = +45 kJ/mol
or one with ΔG‡ = +70kJ/mol?
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Example
• Which reaction is faster, one with ΔG‡ = +45 kJ/mol
or one with ΔG‡ = +70kJ/mol?
• Solution:
• Larger value for ΔG‡ indicates a slower reaction
• Therefore, reaction with ΔG‡ = +45 kJ/mol is fasterthan
a reaction with ΔG‡ = +70 kJ/mol
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Describing a Reaction:
Intermediates
• If a reaction occurs in more than one step, itmust
involve species that are neither the reactant nor
the final product called reaction intermediate
• Each step has its own free energy of activation
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An Energy Diagram
for the Reaction of Ethylene with HBr
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Example
• Sketch an energy diagram for a two-step reaction in
which both steps are exergonic
• Second step has a higher-energy transition state than
the first
• Label the parts of the diagram corresponding to
reactant, product, intermediate, overall ΔG‡, andoverall
ΔG°
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Example
• Solution:
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Biological Reactions vesus Laboratory Reactions
• Laboratory reactions
• Often done using relatively simple reagents; in an organicsolvent
• Catalyst might be used
• Biological reactions
• Involve relatively complex reagents called coenzymes; occur in an
aqueous medium inside cells
• Enzymes provide an alternative mechanism that makeslife
possible
• Active site: Pocket in an enzyme where a substrate is bound and
undergoes reaction
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Models of Hexokinase inSpace-filling and
Wire-frame Formats
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