Chapter 4 the study of chemical reactions
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Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 4 The Study of Chemical Reactions. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall. Tools for Study. To determine a reaction’s mechanism , look at: Equilibrium constant

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Chapter 4 the study of chemical reactions l.jpg

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chapter 4The Study of Chemical Reactions

Jo Blackburn

Richland College, Dallas, TX

Dallas County Community College District

ã 2003,Prentice Hall


Tools for study l.jpg
Tools for Study

  • To determine a reaction’s mechanism, look at:

    • Equilibrium constant

    • Free energy change

    • Enthalpy

    • Entropy

    • Bond dissociation energy

    • Kinetics

    • Activation energy =>

Chapter 4


Chlorination of methane l.jpg
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).=>

Chapter 4


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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. =>

Chapter 4


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Initiation Step

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

=>

Chapter 4


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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).

=>

Chapter 4


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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.

=>

Chapter 4


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=>

Overall Reaction

Chapter 4


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Termination Steps

  • Collision of any two free radicals

  • Combination of free radical with contaminant or collision with wall.

Can you suggest others? =>

Chapter 4


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Equilibrium constant

  • Keq = [products] [reactants]

  • For chlorination Keq = 1.1 x 1019

  • Large value indicates reaction “goes to completion.”=>

Chapter 4


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Free Energy Change

  • DG = free energy of (products - reactants), amount of energy available to do work.

  • Negative values indicate spontaneity.

  • DGo = -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. =>

Chapter 4


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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 25°C?

=>

Chapter 4


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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 =>

Chapter 4


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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). =>

Chapter 4


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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. =>

Chapter 4


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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.=>

Chapter 4


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104

103

58

84

104

84

=>

103

58

Which is more likely?

Estimate DH for each step using BDE.

Chapter 4


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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.=>

Chapter 4


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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 + lnART =>

Chapter 4


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=>

Activation Energy

  • Minimum energy required to reach the transition state.

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

Chapter 4


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=>

Reaction-Energy Diagrams

  • For a one-step reaction:reactants  transition state  products

  • A catalyst lowers the energy of the transition state.

Chapter 4


Energy diagram for a two step reaction l.jpg

=>

Energy Diagram for a Two-Step Reaction

  • Reactants  transition state  intermediate

  • Intermediate  transition state  product

Chapter 4


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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. =>

Chapter 4


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Rate, Ea,and Temperature

=>

Chapter 4


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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). =>

Chapter 4


Chlorination of propane l.jpg
Chlorination of Propane

1 C

  • There are six 1 H’s and two 2 H’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. =>

2 C

Chapter 4


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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 H’s are 30%  6.67% = 4.5 times more reactive toward chlorination than primary H’s. =>

Chapter 4


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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. =>

Chapter 4


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Free Radical Stabilities

  • Energy required to break a C-H bond decreases as substitution on the carbon increases.

  • Stability: 3 > 2 > 1 > methylDH(kcal) 91, 95, 98, 104 =>

Chapter 4


Chlorination energy diagram l.jpg
Chlorination Energy Diagram

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

=>

Chapter 4


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Bromination of Propane

1 C

2 C

  • There are six 1 H’s and two 2 H’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. =>

Chapter 4


Reactivity of hydrogens32 l.jpg
Reactivity of Hydrogens

  • 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 and97%  2 = 48.5% per secondary H

  • Secondary H’s are 48.5%  0.5% = 97 times more reactive toward bromination than primary H’s. =>

Chapter 4


Bromination energy diagram l.jpg
Bromination Energy Diagram

  • Note larger difference in Ea

  • Why endothermic?

=>

Chapter 4



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Endothermic and Exothermic Diagrams

=>

Chapter 4


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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.=>

Chapter 4


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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. =>

Chapter 4


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Reactive Intermediates

  • Carbocations (or carbonium ions)

  • Free radicals

  • Carbanions

  • Carbene =>

Chapter 4


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Carbocation Structure

  • Carbon has 6 electrons, positive charge.

  • Carbon is sp2 hybridized with vacant p orbital. =>

Chapter 4


Carbocation stability l.jpg
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.=>

Chapter 4


Free radicals l.jpg
Free Radicals

  • Also electron-deficient

  • Stabilized by alkyl substituents

  • Order of stability:3 > 2 > 1 > methyl =>

Chapter 4


Carbanions l.jpg
Carbanions

  • Eight electrons on C:6 bonding + lone pair

  • Carbon has a negative charge.

  • Destabilized by alkyl substituents.

  • Methyl >1 > 2  > 3  =>

Chapter 4


Carbenes l.jpg
Carbenes

  • Carbon is neutral.

  • Vacant p orbital, so can be electrophilic.

  • Lone pair of electrons, so can be nucleophilic. =>

Chapter 4


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End of Chapter 4

Chapter 4


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