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Chapter 4
The Study of
Chemical Reactions
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College Distri...
Chapter 4 2
Tools for Study
• To determine a reaction’s
mechanism, look at:
Equilibrium constant
Free energy change
Ent...
Chapter 4 3
Chlorination of Methane
• Requires heat or light for initiation.
• The most effective wavelength is blue, whic...
Chapter 4 4
Free-Radical Chain Reaction
• Initiation generates a reactive intermediate.
• Propagation: the intermediate re...
Chapter 4 5
Initiation Step
A chlorine molecule splits homolytically
into chlorine atoms (free radicals)
=>
Cl Cl + photon...
Chapter 4 6
Propagation Step (1)
The chlorine atom collides with a
methane molecule and abstracts
(removes) a H, forming a...
Chapter 4 7
Propagation Step (2)
The methyl free radical collides with
another chlorine molecule, producing
the other prod...
Chapter 4 8
Overall Reaction
C
H
H
H
H Cl+ C
H
H
H
+ H Cl
C
H
H
H
+ Cl Cl C
H
H
H
Cl + Cl
C
H
H
H
H + Cl Cl C
H
H
H
Cl + H...
Chapter 4 9
Termination Steps
• Collision of any two free radicals
• Combination of free radical with
contaminant or colli...
Chapter 4 10
Equilibrium constant
• Keq = [products]
[reactants]
• For chlorination Keq = 1.1 x 1019
• Large value indicat...
Chapter 4 11
Free Energy Change
• ∆G = free energy of (products -
reactants), amount of energy available to
do work.
• Neg...
Chapter 4 12
Problem
• Given that -X is -OH, the energy difference for
the following reaction is -1.0 kcal/mol.
• What per...
Chapter 4 13
Factors Determining ∆G°
• Free energy change depends on
enthalpy
entropy
∀∆H° = (enthalpy of products) - (e...
Chapter 4 14
Enthalpy
• ∆Ho
= heat released or absorbed during
a chemical reaction at standard
conditions.
• Exothermic, (...
Chapter 4 15
Entropy
• ∆So
= change in randomness, disorder,
freedom of movement.
• Increasing heat, volume, or number of
...
Chapter 4 16
Bond Dissociation Energy
• Bond breaking requires energy (+BDE)
• Bond formation releases energy (-BDE)
• Tab...
Chapter 4 17
Which is more likely?
Estimate ∆H for each step using BDE.
CH4 HCl+ +Cl CH3
CH3 + Cl2 CH3Cl + Cl
or
Cl+CH4 CH...
Chapter 4 18
Kinetics
• Answers question, “How fast?”
• Rate is proportional to the concentration
of reactants raised to a...
Chapter 4 19
Reaction Order
• For A + B → C + D, rate = k[A]a
[B]b
a is the order with respect to A
a + b is the overall...
Chapter 4 20
Activation Energy
• Minimum energy required to reach
the transition state.
• At higher temperatures, more
mol...
Chapter 4 21
Reaction-Energy Diagrams
• For a one-step reaction:
reactants → transition state → products
• A catalyst lowe...
Chapter 4 22
Energy Diagram for a
Two-Step Reaction
• Reactants → transition state → intermediate
• Intermediate → transit...
Chapter 4 23
Rate-Determining Step
• Reaction intermediates are stable as long
as they don’t collide with another molecule...
Chapter 4 24
Rate, Ea, and Temperature
X + CH4 HX + CH3
X E a Rate @ 300K Rate @ 500K
F 1.2 kcal 140,000 300,000
Cl 4 kcal...
Chapter 4 25
Conclusions
• With increasing Ea, rate decreases.
• With increasing temperature, rate
increases.
• Fluorine r...
Chapter 4 26
Chlorination of Propane
• There are six 1° H’s and two 2° Η’s. We
expect 3:1 product mix, or 75% 1-
chloropro...
Chapter 4 27
Reactivity of Hydrogens
• To compare hydrogen reactivity, find
amount of product formed per hydrogen:
40% 1-c...
Chapter 4 28
Predict the Product Mix
Given that secondary H’s are 4.5 times as
reactive as primary H’s, predict the
percen...
Chapter 4 29
Free Radical Stabilities
• Energy required to break a C-H bond
decreases as substitution on the carbon
increa...
Chapter 4 30
Chlorination Energy Diagram
Lower Ea, faster rate, so more stable
intermediate is formed faster.
=>
Chapter 4 31
• There are six 1° H’s and two 2° Η’s. We
expect 3:1 product mix, or 75% 1-
bromopropane and 25% 2-bromopropa...
Chapter 4 32
• To compare hydrogen reactivity, find amount of
product formed per hydrogen: 3% 1-
bromopropane from 6 hydro...
Chapter 4 33
Bromination Energy Diagram
• Note larger difference in Ea
• Why endothermic?
=>
Chapter 4 34
Bromination vs. Chlorination
=>
Chapter 4 35
Endothermic and
Exothermic Diagrams
=>
Chapter 4 36
Hammond Postulate
• Related species that are similar in energy are
also similar in structure. The structure o...
Chapter 4 37
Radical Inhibitors
• Often added to food to retard spoilage.
• Without an inhibitor, each initiation step
wil...
Chapter 4 38
Reactive Intermediates
• Carbocations (or carbonium ions)
• Free radicals
• Carbanions
• Carbene
=>
Chapter 4 39
Carbocation Structure
• Carbon has 6 electrons,
positive charge.
• Carbon is sp2
hybridized
with vacant p orb...
Chapter 4 40
Carbocation Stability
• Stabilized by alkyl
substituents 2 ways:
• (1) Inductive effect:
donation of electron...
Chapter 4 41
Free Radicals
• Also electron-
deficient
• Stabilized by alkyl
substituents
• Order of stability:
3° > 2° > 1...
Chapter 4 42
Carbanions
• Eight electrons on C:
6 bonding + lone pair
• Carbon has a negative
charge.
• Destabilized by al...
Chapter 4 43
Carbenes
• Carbon is neutral.
• Vacant p orbital, so
can be electrophilic.
• Lone pair of
electrons, so can b...
Chapter 4 44
End of Chapter 4
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Wade04 el estudio de las reacciones quimicas

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Wade04 el estudio de las reacciones quimicas

  1. 1. Chapter 4 The Study of Chemical Reactions Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Organic Chemistry, 5th Edition L. G. Wade, Jr.
  2. 2. Chapter 4 2 Tools for Study • To determine a reaction’s mechanism, look at: Equilibrium constant Free energy change Enthalpy Entropy Bond dissociation energy Kinetics Activation energy =>
  3. 3. Chapter 4 3 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). => C H H H H + Cl2 heat or light C H H H Cl + HCl
  4. 4. Chapter 4 4 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. =>
  5. 5. Chapter 4 5 Initiation Step A chlorine molecule splits homolytically into chlorine atoms (free radicals) => Cl Cl + photon (hν) Cl + Cl
  6. 6. Chapter 4 6 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). C H H H H Cl+ C H H H + H Cl =>
  7. 7. Chapter 4 7 Propagation Step (2) The methyl free radical collides with another chlorine molecule, producing the other product (methyl chloride) and regenerating the chlorine radical. C H H H + Cl Cl C H H H Cl + Cl =>
  8. 8. Chapter 4 8 Overall Reaction C H H H H Cl+ C H H H + H Cl C H H H + Cl Cl C H H H Cl + Cl C H H H H + Cl Cl C H H H Cl + H Cl => Cl Cl + photon (hν) Cl + Cl
  9. 9. Chapter 4 9 Termination Steps • Collision of any two free radicals • Combination of free radical with contaminant or collision with wall. C H H H Cl+ C H H H Cl Can you suggest others? =>
  10. 10. Chapter 4 10 Equilibrium constant • Keq = [products] [reactants] • For chlorination Keq = 1.1 x 1019 • Large value indicates reaction “goes to completion.” =>
  11. 11. Chapter 4 11 Free Energy Change • ∆G = free energy of (products - reactants), amount of energy available to do work. • Negative values indicate spontaneity. • ∆Go = -RT(lnKeq) where R = 1.987 cal/K-mol and T = temperature in kelvins • Since chlorination has a large Keq, the free energy change is large and negative. =>
  12. 12. Chapter 4 12 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 25°C? =>
  13. 13. Chapter 4 13 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° =>
  14. 14. Chapter 4 14 Enthalpy • ∆Ho = heat released or absorbed during a chemical reaction at standard conditions. • Exothermic, (-∆H), heat is released. • Endothermic, (+∆H), heat is absorbed. • Reactions favor products with lowest enthalpy (strongest bonds). =>
  15. 15. Chapter 4 15 Entropy • ∆So = 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 ∆Go = ∆Ho - T∆So the entropy value is often small. =>
  16. 16. Chapter 4 16 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. A B A + B We can use BDE to estimate ∆H for a reaction. =>
  17. 17. Chapter 4 17 Which is more likely? Estimate ∆H for each step using BDE. CH4 HCl+ +Cl CH3 CH3 + Cl2 CH3Cl + Cl or Cl+CH4 CH3Cl + H H Cl2+ HCl Cl+ 104 103 58 84 => 104 84 58 103
  18. 18. Chapter 4 18 Kinetics • Answers question, “How fast?” • Rate is proportional to the concentration of reactants raised to a power. • Rate law is experimentally determined. =>
  19. 19. Chapter 4 19 Reaction Order • For A + B → C + D, rate = k[A]a [B]b 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. • The value of k depends on temperature as given by Arrhenius: ln k = -Ea + lnA RT =>
  20. 20. Chapter 4 20 Activation Energy • Minimum energy required to reach the transition state. • At higher temperatures, more molecules have the required energy. => C H H H H Cl
  21. 21. Chapter 4 21 Reaction-Energy Diagrams • For a one-step reaction: reactants → transition state → products • A catalyst lowers the energy of the transition state. =>
  22. 22. Chapter 4 22 Energy Diagram for a Two-Step Reaction • Reactants → transition state → intermediate • Intermediate → transition state → product =>
  23. 23. Chapter 4 23 Rate-Determining Step • Reaction intermediates are stable as long as they don’t 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. =>
  24. 24. Chapter 4 24 Rate, Ea, and Temperature X + CH4 HX + CH3 X E a Rate @ 300K Rate @ 500K F 1.2 kcal 140,000 300,000 Cl 4 kcal 1300 18,000 Br 18 kcal 9 x 10-8 0.015 I 34 kcal 2 x 10-19 2 x 10-9 =>
  25. 25. Chapter 4 25 Conclusions • With increasing Ea, rate decreases. • With increasing temperature, rate increases. • Fluorine reacts explosively. • Chlorine reacts at a moderate rate. • Bromine must be heated to react. • Iodine does not react (detectably). =>
  26. 26. Chapter 4 26 Chlorination of Propane • There are six 1° H’s and two 2° Η’s. We expect 3:1 product mix, or 75% 1- chloropropane and 25% 2-chloropropane. • Typical product mix: 40% 1-chloropropane and 60% 2-chloropropane. • Therefore, not all H’s are equally reactive. => 1° C 2° C CH3 CH2 CH3 + Cl2 hν CH2 Cl CH2 CH3 + CH3 CH Cl CH3
  27. 27. Chapter 4 27 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 and 60% ÷ 2 = 30% per secondary H • Secondary H’s are 30% ÷ 6.67% = 4.5 times more reactive toward chlorination than primary H’s. =>
  28. 28. Chapter 4 28 Predict the Product Mix Given that secondary H’s are 4.5 times as reactive as primary H’s, predict the percentage of each monochlorinated product of n-butane + chlorine. =>
  29. 29. Chapter 4 29 Free Radical Stabilities • Energy required to break a C-H bond decreases as substitution on the carbon increases. • Stability: 3° > 2° > 1° > methyl ∆H(kcal) 91, 95, 98, 104 =>
  30. 30. Chapter 4 30 Chlorination Energy Diagram Lower Ea, faster rate, so more stable intermediate is formed faster. =>
  31. 31. Chapter 4 31 • There are six 1° H’s and two 2° Η’s. We expect 3:1 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. => 1° C 2° C CH3 CH2 CH3 + CH2 Br CH2 CH3 +Br2 heat CH3 CH Br CH3 Bromination of Propane
  32. 32. Chapter 4 32 • To compare hydrogen reactivity, find amount of product formed per hydrogen: 3% 1- bromopropane from 6 hydrogens and 97% 2- bromopropane from 2 hydrogens. • 3% ÷ 6 = 0.5% per primary H and 97% ÷ 2 = 48.5% per secondary H • Secondary H’s are 48.5% ÷ 0.5% = 97 times more reactive toward bromination than primary H’s. => Reactivity of Hydrogens
  33. 33. Chapter 4 33 Bromination Energy Diagram • Note larger difference in Ea • Why endothermic? =>
  34. 34. Chapter 4 34 Bromination vs. Chlorination =>
  35. 35. Chapter 4 35 Endothermic and Exothermic Diagrams =>
  36. 36. Chapter 4 36 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. =>
  37. 37. Chapter 4 37 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. =>
  38. 38. Chapter 4 38 Reactive Intermediates • Carbocations (or carbonium ions) • Free radicals • Carbanions • Carbene =>
  39. 39. Chapter 4 39 Carbocation Structure • Carbon has 6 electrons, positive charge. • Carbon is sp2 hybridized with vacant p orbital. =>
  40. 40. Chapter 4 40 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. =>
  41. 41. Chapter 4 41 Free Radicals • Also electron- deficient • Stabilized by alkyl substituents • Order of stability: 3° > 2° > 1° > methyl =>
  42. 42. Chapter 4 42 Carbanions • Eight electrons on C: 6 bonding + lone pair • Carbon has a negative charge. • Destabilized by alkyl substituents. • Methyl >1° > 2 ° > 3 ° =>
  43. 43. Chapter 4 43 Carbenes • Carbon is neutral. • Vacant p orbital, so can be electrophilic. • Lone pair of electrons, so can be nucleophilic. =>
  44. 44. Chapter 4 44 End of Chapter 4

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