Chapter 4 The Study of Chemical Reactions

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Chapter 4The Study of Chemical Reactions
Organic Chemistry, 5th EditionL. G. Wade, Jr.
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

• Thermodynamics
• Equilibrium constant
• Free energy change
• Enthalpy
• Entropy
• Bond dissociation energy
• Kinetics
• Activation energy

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Chlorination of Methane

• 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).

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Free-Radical Chain Reaction

• Initiation generates a reactive intermediate.
• Propagation the intermediate reacts with a
  stable molecule to produce another reactive
  intermediate (and a product molecule).
• Termination side reactions that destroy the
  reactive intermediate.

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Initiation Step

• A chlorine molecule splits homolytically into
  chlorine atoms (free radicals)

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Propagation Step (1)

• The chlorine atom collides with a methane
  molecule and abstracts (removes) a H, forming
  another free radical and one of the products
  (HCl).

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Propagation Step (2)

• The methyl free radical collides with another
  chlorine molecule, producing the other product
  (methyl chloride) and regenerating the chlorine
  radical.

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Termination Steps

• Collision of any two free radicals
• Combination of free radical with contaminant or
  collision with wall.

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Overall Reaction
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Additional Propagation Steps
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Equilibrium constant

• Keq products reactants
• For chlorination Keq 1.1 x 1019

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Free Energy Change

• DG free energy - the amount of energy
  available to do work.
• The DG free energy is
• (free energy of products) (free energy of
  reactants)

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Free Energy Change

• ?G and Keq are related by the equation
• Keq e-?G/RT
• ?Go -RT(lnKeq)
• where R 1.987 cal/K-moland T temperature in
  kelvins
• Since chlorination has a large Keq, the free
  energy change is large and negative and the
  reaction goes to completion.

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Free Energy Change
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Problem

• Given that -X is -OH, the energy difference for
  the following reaction is -1.0 kcal/mol.
• What percentage of cyclohexanol molecules will be
  in the equatorial conformer at equilibrium at
  25C?

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Factors Determining ?G?

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

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Enthalpy

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

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Entropy

• DSo change in randomness, disorder, freedom of
  movement.
• ?S? (entropy of products) - (entropy of
  reactants)
• 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.

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Enthalpy (?H) andBond 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.
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?H for Halogenation Rxn

• Estimate DH for each step using BDE.

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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.

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Reaction Order

• For A B ? C D, rate krAaBb
• kr is the rate constant
• a is the order with respect to A
• a b (the sum of the powers) is the overall
  order
• Order is the number of molecules of that reactant
  which is present in the rate-determining step of
  the mechanism.
• The order must be experimentally determined and
  is dependent upon the mechanism of the reaction.

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Rate Constant

• The value of kr depends on conditions of the
  reaction, especially temperature as given by the
  Arrhenius equation
• kr Ae-Ea/RT
• A a constant (the frequency factor)
• Ea activation energy
• R Gas Constant (1.987cal/Kelvin-mole)
• T Temperature in Kelvins
  

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Activation Energy

• At higher temperatures, more molecules have the
  required energy.

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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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Energy Diagram for a Two-Step Reaction

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

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Conclusions

• With increasing Ea, rate decreases.
• With increasing temperature, rate increases.
• Activation Energies for halogenation
• F 1.2 kcal (reacts explosively)
• Cl 4 kcal (reacts at a moderate rate)
• Br 18 kcal (must be heated to react)
• I 34 kcal (does not react detectably)

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Chlorination of Propane
1? C
2? C

• Reactivity Analysis
• 1? Hs 6 2? Hs. 2 so expect 31
  product mix, (75 1-chloro and 25 2-chloro).
• Typical product mix 40 1-chloropropane and 60
  2-chloropropane.
• Therefore, not all Hs are equally reactive.

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Reactivity of Hydrogens

• What is our reactivity?
• 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

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Free Radical Stabilities

• Energy required to break a C-H bond decreases as
  substitution on the carbon increases.
• Stability 3? gt 2? gt 1? gt methyl?H(kcal)
  91, 95, 98, 104

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Free Radicals

• Electron-deficient orbital
• Stabilized by alkyl substituents (induction)
• Order of stability3? gt 2? gt 1? gt methyl

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Predict the Product Mix

• Given that secondary Hs are 4.5 times as
  reactive as primary Hs, predict the percentage
  of each monochlorinated product of n-butane
  chlorine.

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Chlorination Energy Diagram

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

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Bromination of Propane
1? C
2? C

• Reactivity Analysis
• 1? Hs 6 2? Hs 2. so expect 31 product
  mix,
• (75 1-bromo and 25 2-bromo).
• Typical product mix 3 1-bromopropane and 97
  2-bromopropane !!!
• Bromination is more selective than chlorination.
  

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Reactivity of Hydrogens

• What is our reactivity?
• 3 ? 6 0.5 per primary H and97 ? 2 48.5
  per secondary H
• Secondary Hs are 48.5 ? 0.5 97 times more
  reactive toward bromination than primary Hs.

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Bromination Energy Diagram

• Note larger difference in Ea
• Why endothermic?

gt
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Bromination vs. Chlorination
gt
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Endothermic and Exothermic Diagrams
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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.

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Reactive Intermediates

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

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Carbocation Structure

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

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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.

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Free Radicals

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

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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 ?

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Carbenes

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

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End of Chapter 4