Chapter 4 The Study of Chemical Reactions

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Chapter 4The Study of Chemical Reactions
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Modified from Jo Blackburn Richland College,
Dallas, TX Dallas County Community College
District ã 2003, Prentice Hall
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Tools for Study

• To determine a reactions mechanism, look at
• Equilibrium constant
• Free energy change
• Enthalpy
• Entropy
• Bond dissociation energy
• Kinetics
• Activation energy gt

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Chlorination of Methane
H
H
heat or light
C
H
H
Cl2
H
Cl

C
H
Cl

H
H

• Requires heat or light for initiation.
• The most effective wavelength is blue, which is
  absorbed by chlorine gas.
• Lots of product formed from absorption of only
  one photon of light (chain reaction).
  
  gt

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Equilibrium constant

• Keq products reactants
• For chlorination Keq 1.1 x 1019
• Large value indicates reaction goes to
  completion.
  gt

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Factors Determining ?G?

• Free energy change depends on
• enthalpy
• entropy
• ?H? (enthalpy of products) - (enthalpy of
  reactants)
• ?S? (entropy of products) - (entropy of
  reactants)
• ?G? ?H? - T?S? gt

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Enthalpy

• DHo heat released or absorbed during a
  chemical reaction at standard conditions.
• Exothermic, (-DH), heat is released.
• Endothermic, (DH), heat is absorbed.
• Reactions favor products with lowest enthalpy
  (strongest bonds).
  gt

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Entropy

• DSo change in randomness, disorder, freedom of
  movement.
• Increasing heat, volume, or number of particles
  increases entropy.
• Spontaneous reactions maximize disorder and
  minimize enthalpy.
• In the equation DGo DHo - TDSo the entropy
  value is often small.
  gt

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Bond Dissociation Energy

• Bond breaking requires energy (BDE)
• Bond formation releases energy (-BDE)
• Table 4.2 gives BDE for homolytic cleavage of
  bonds in a gaseous molecule.

We can use BDE to estimate ?H for a reaction.

gt
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D
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• The energy diagrams for these reactions are shown
  below

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.
.
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D
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D
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Kinetics

• Answers question, How fast?
• Rate is proportional to the concentration of
  reactants raised to a power.
• Rate law is experimentally determined.
  
  gt

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Reaction Order

• For A B ? C D, rate kAaBb
• a is the order with respect to A
• a b is the overall order
• Order is the number of molecules of that reactant
  which is present in the rate-determining step of
  the mechanism.

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Reaction-Energy Diagrams

• For a one-step reactionreactants ? transition
  state ? products
• A catalyst lowers the energy of the transition
  state.

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Activation Energy and Temperature Dependence of
Rates
Reaction rates are related to temperature by the
Arrhenius equation
Arrhenius equation
A Frequency Factor Ea Activation
Energy e-Ea/RT Fraction of collisions in which
the particles have the minimum Ea
to react.
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• Frequency Factor (A) fraction of collisions with
  proper orientation for reaction to occur.
• Activation Energy (Ea) minimum kinetic energy
  molecules must possess to overcome repulsions
  between their electron clouds when they collide.

The Arrhenius equation implies that the reaction
rate depends upon the fraction of molecules
with kinetic energy of at least Ea.
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Activation Energy

• Minimum energy required to reach the transition
  state.
• At higher temperatures, more molecules have the
  required energy.

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Energy Diagram for a Two-Step Reaction

• Reactants ? transition state ? intermediate
• Intermediate ? transition state ? product

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Rate-Determining Step

• Reaction intermediates are stable as long as they
  dont collide with another molecule or atom, but
  they are very reactive.
• Transition states are at energy maximums.
• Intermediates are at energy minimums.
• The reaction step with highest Ea will be the
  slowest, therefore rate-determining for the
  entire reaction.
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Rate, Ea, and Temperature
gt
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Halogenation of Higher Alkanes
Free Radical Halogenation of Alkanes
In higher alkanes, the replacement of different
hydrogen atoms leads to different products
The minor product was formed from substitution of
a 10 hydrogen. The major product was formed from
substitution of a 20 hydrogen.
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Chlorination of Propane
1? C
2? C

• There are six 1? Hs and two 2? Hs. We expect
  31 product mix, or 75 1-chloropropane and 25
  2-chloropropane.
• Typical product mix 40 1-chloropropane and 60
  2-chloropropane.
• Therefore, not all Hs are equally reactive.
  
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Free Radical Halogenation of Alkanes
Relative radical stabilities control product
distribution.
Methyl radical lt 10 lt 20 lt 30
Increasing radical stability
The more highly substituted the radical, the
greater its stability.
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Reactivity of Hydrogens

• To compare hydrogen reactivity, find amount of
  product formed per hydrogen 40 1-chloropropane
  from 6 hydrogens and 60 2-chloropropane from 2
  hydrogens.
• 40 ? 6 6.67 per primary H and60 ? 2 30
  per secondary H
• Secondary Hs are 30 ? 6.67 4.5 times more
  reactive toward chlorination than primary Hs.
  gt

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Chlorination Energy Diagram

• Lower Ea, faster rate, so more stable
  intermediate is formed faster.

gt
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Free Radical Halogenation of Alkanes
In the analogous reaction using bromine, the
product ratios are different, even though the
mechanism is exactly the same.
The 973 product ratio shows that Br abstracts a
20 hydrogen 97 times as fast as a 10 hydrogen. We
say that bromine is much more selective
than chlorine, and chlorine is much more reactive
than bromine.
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Bromination of Propane
1? C
2? C

• There are six 1? Hs and two 2? Hs. We expect
  31 product mix, or 75 1-bromopropane and 25
  2-bromopropane.
• Typical product mix 3 1-bromopropane and 97
  2-bromopropane !!!
• Bromination is more selective than chlorination.
  gt

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Bromination Energy Diagram

• Note larger difference in Ea
• Why endothermic?

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Bromination vs. Chlorination
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Endothermic and Exothermic Diagrams
gt
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Hammond Postulate

• Related species that are similar in energy are
  also similar in structure. The structure of a
  transition state resembles the structure of the
  closest stable species.
• Transition state structure for endothermic
  reactions resemble the product.
• Transition state structure for exothermic
  reactions resemble the reactants.
  
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Radical Inhibitors

• Often added to food to retard spoilage.
• Without an inhibitor, each initiation step will
  cause a chain reaction so that many molecules
  will react.
• An inhibitor combines with the free radical to
  form a stable molecule.
• Vitamin E and vitamin C are thought to protect
  living cells from free radicals.
  gt

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Reactive Intermediates

• Free radicals
• Carbocations (or carbonium ions)
• Carbanions
• Carbene
  gt

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Carbocation Structure

• Carbon has 6 electrons, positive charge.
• Carbon is sp2 hybridized with vacant p orbital.
  gt

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Carbocation Stability

• Stabilized by alkyl substituents 2 ways
• (1) Inductive effect donation of electron
  density along the sigma bonds.
• (2) Hyperconjugation overlap of sigma bonding
  orbitals with empty p orbital.
  gt

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Free Radicals

• Also electron-deficient
• Stabilized by alkyl substituents
• Order of stability3? gt 2? gt 1? gt methyl
  gt

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Carbanions

• Eight electrons on C6 bonding lone pair
• Carbon has a negative charge.
• Destabilized by alkyl substituents.
• Methyl gt1? gt 2 ? gt 3 ?
  gt

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Carbenes

• Carbon is neutral.
• Vacant p orbital, so can be electrophilic.
• Lone pair of electrons, so can be nucleophilic.
  gt

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End of Chapter 4