WADE7 Lecture 04

7/27/2019 WADE7 Lecture 04

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Chapter 4

Copyright 2010 Pearson Education, Inc.

Organic Chemistry, 7th Edition

L. G. Wade, Jr.

The Study of

Chemical Reactions


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Chapter 4 2

Introduction

Overall reaction: reactants products

Mechanism: Step-by-step pathway.

To learn more about a reaction:

Thermodynamics

Kinetics


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Chapter 4 3

Chlorination of Methane

Requires heat or light for initiation.

The most effective wavelength is blue, which isabsorbed by chlorine gas.

Many molecules of product are formed fromabsorption of only one photon of light (chainreaction).


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Chapter 4 4

The Free-Radical Chain

Reaction Initiation generates a radical intermediate.

Propagat ion: The intermediate reacts with

a stable molecule to produce anotherreactive intermediate (and a product

molecule).

Terminat ion: Side reactions that destroythe reactive intermediate.


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Chapter 4 5

Initiation Step: Formation of

Chlorine Atom

A chlorine molecule splits homolytically into

chlorine atoms (free radicals).


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Chapter 4 6

Propagation Step: Carbon

Radical

The chlorine atom collides with a methane

molecule and abstracts (removes) an H,forming another free radical and one of the

products (HCl).


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Chapter 4 7

Propagation Step: Product

Formation

The methyl free radical collides with another

chlorine molecule, producing the organicproduct (methyl chloride) and regenerating the

chlorine radical.


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Chapter 4 8

Overall Reaction


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Chapter 4 9

Termination Steps

A reaction is classified as a termination step whenany two free radicals join together producing anonradical compound.

Combination of free radical with contaminant orcollision with wall are also termination steps.


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Chapter 4 10

More Termination Steps


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Chapter 4 11

Lewis Structures of Free

Radicals

Free radicals are unpaired electrons. Halogens have 7 valence electrons so one of them

will be unpaired (radical). We refer to the halides asatoms not radicals.


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Chapter 4 12

Equilibrium Constant

Keq = [products][reactants]

For CH4 + Cl2

CH3Cl + HCl

Keq = [CH3Cl][HCl] = 1.1 x 1019

[CH4][Cl2]

Large value indicates reaction goes tocompletion.


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Chapter 4 13

Free Energy Change

DG = (energy of products) - (energy of reactants)

DG is theamount of energy available to do work.

Negative values indicate spontaneity.

DGo = -RT(lnKeq) = -2.303 RT(log10Keq)

where R= 8.314 J/K-mol and T= temperature inkelvins.


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Chapter 4 14

Factors Determining DG

Free energy change depends on:

Enthalpy

DH= (enthalpy of products) - (enthalpy of reactants)

Entropy

DS = (entropy of products) - (entropy of reactants)

DG = DH - TDS


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Chapter 4 15

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 lowestenthalpy (strongest bonds).


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Chapter 4 16

Entropy

DSo = change in randomness, disorder,

or freedom of movement.

Increasing heat, volume, or number ofparticles 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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Chapter 4 17

Calculate the value ofDG for the chlorination of methane.

DG =2.303RT(logKeq)

Keq for the chlorination is 1.1 x 1019, and log Keq = 19.04

At 25 C (about 298 Kelvin), the value ofRTis

RT= (8.314 J/kelvin-mol)(298 kelvins) = 2478 J/mol, or 2.48 kJ/mol

Substituting, we have

DG = (2.303)(2.478 kJ/mol)(19.04) =108.7 kJ/mol (25.9 kcal>mol)

This is a large negative value forDG, showing that this chlorination has a large driving force that

pushes it toward completion.

Solved Problem 1

Solution


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Chapter 4 18

Bond-Dissociation Enthalpies

(BDE) Bond-dissociation requires energy (+BDE).

Bond formation releases energy (-BDE).

BDE can be used to estimate DHfor a reaction. BDE for homolytic cleavage of bonds in a

gaseous molecule. Homo lyt ic cleavage: When the bond breaks, each atom gets

one electron. Heteroly t ic cleavage: When the bond breaks, the most

electronegative atom gets both electrons.


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Chapter 4 19

Homolytic and Heterolytic

Cleavages


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Chapter 4 20

Enthalpy Changes in Chlorination

CH3-H + Cl-Cl CH3-Cl + H-Cl

Bonds Broken DH (per Mole) Bonds Formed DH (per Mole)

Cl-Cl +242 kJ H-Cl -431 kJ

CH3-H +435 kJ CH3-Cl -351 kJ

TOTALS +677 kJ TOTAL -782 kJ

DH = +677 kJ + (-782 kJ) = -105 kJ/mol


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Chapter 4 21

Kinetics

Kinetics is the study of reaction rates.

Rate of the reaction is a measure of how the

concentration of the products increase whilethe concentration of the products decrease.

A rate equation is also called the rate law andit gives the relationship between the

concentration of the reactants and thereaction rate observed.

Rate law is experimentally determined.


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Chapter 4 22

Rate Law

For the reaction A + B C + D,

rate = kr[A]a[B]b

a is the order with respect to A

b is the order with respect to B

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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Chapter 4 23

Activation Energy

The value ofkdepends on temperature asgiven by Arrhenius:

RTEa

Aek

/

r

where A = constant (frequency factor)

Ea = activation energy

R= gas constant, 8.314 J/kelvin-mole

T= absolute temperature

Ea is the minimum kinetic energy needed to react.


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Chapter 4 24

Activation Energy (Continued)

At higher temperatures, more molecules havethe required energy to react.


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Chapter 4 25

Energy Diagram of an Exothermic

Reaction

The vertical axis in this graph represents the potentialenergy.

The transition state is the highest point on the graph,and the activation energy is the energy differencebetween the reactants and the transition state.


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Chapter 4 26

Rate-Limiting 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 theslowest, therefore rate-determining for the

entire reaction.


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Chapter 4 27

Energy Diagram for the

Chlorination of Methane


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Chapter 4 28

Rate, Ea,and Temperature

X + CH4 HX + CH3

X Ea(per Mole) Rate at 27 C Rate at 227 C

F 5 140,000 300,000

Cl 17 1300 18,000

Br 75 9 x 10-8 0.015

I 140 2 x 10-19 2 x 10-9


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Chapter 4 29

Consider the following reaction:

This reaction has an activation energy (Ea) of +17 kJ/mol (+4 kcal/mol) and a DH of +4 kJ/mol (+1

kcal/mol). Draw a reaction-energy diagram for this reaction.

We draw a diagram that shows the products to be 4 kJ higher in energy than the reactants. The barrieris made to be 17 kJ higher in energy than the reactants.

Solved Problem 2

Solution


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Chapter 4 30

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).


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Chapter 4 31

Primary, Secondary, and Tertiary

Hydrogens


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Chapter 4 32

Chlorination Mechanism


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Chapter 4 33

Bond Dissociation Energies for

the Formation of Free Radicals


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Chapter 4 34

Tertiary hydrogen atoms react with Cl about 5.5 times as fast as primary ones. Predict the product

ratios for chlorination of isobutane.

There are nine primary hydrogens and one tertiary hydrogen in isobutane.

(9 primary hydrogens) x (reactivity 1.0) = 9.0 relative amount of reaction

(1 tertiary hydrogen) x (reactivity 5.5) = 5.5 relative amount of reaction

Solved Problem 3

Solution


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Chapter 4 35

Even though the primary hydrogens are less reactive, there are so many of them that the primary

product is the major product. The product ratio will be 9.0:5.5, or about 1.6:1.

Solved Problem 3 (Continued)

Solution


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Chapter 4 36

Stability of Free Radicals

Free radicals are more stable if they are

highly substituted.


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Chapter 4 37

Chlorination Energy Diagram

LowerEa, faster rate, so more stableintermediate is formed faster.


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Chapter 4 38

Rate of Substitution in the

Bromination of Propane


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Chapter 4 39

Energy Diagram for the

Bromination of Propane


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Chapter 4 40

Hammond Postulate

Related species that are similar inenergy are also similar in structure.

The structure of the transition state

resembles the structure of the closeststable species.

Endotherm ic react ion: Transition state

is product-like. Exotherm ic react ion: Transition state is

reactant-like.


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Chapter 4 41

Energy Diagrams: Chlorination

Versus Bromination


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Chapter 4 42

Endothermic and

Exothermic Diagrams


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Chapter 4 43

Radical Inhibitors

Often added to food to retard spoilageby radical chain reactions.

Without an inhibitor, each initiation stepwill cause a chain reaction so that manymolecules 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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Chapter 4 44

Radical Inhibitors (Continued)

A radical chain reaction is fast and has manyexothermic steps that create more reactive radicals.

When an inhibitor reacts with the radical, it creates astable intermediate, and any further reactions will beendothermic and slow.


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Chapter 4 45

Carbon Reactive Intermediates


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Chapter 4 46

Carbocation Structure

Carbon has 6 electrons, positively charged.

Carbon is sp2 hybridized with vacantp orbital.


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Chapter 4 47

Carbocation Stability


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Chapter 4 48

Carbocation Stability

(Continued)

Stabilized by alkyl

substituents in two ways:

1. Induc t ive effect: Donation

of electron density along the

sigma bonds.

2. Hyperconjugat ion:

Overlap of sigma bonding

orbitals with emptyp orbital.


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Chapter 4 49

Free Radicals

Also electron-deficient. Stabilized by alkyl substituents.

Order of stability:3 > 2 > 1 > methyl


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Chapter 4 50

Stability of Carbon Radicals


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Chapter 4 51

Carbanions

Eight electrons oncarbon: 6 bonding

plus one lone pair. Carbon has anegative charge.

Destabilized by alkylsubstituents.

Methyl >1 > 2 > 3


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Chapter 4 52

Carbenes

Carbon is neutral.

Vacantp orbital, so can be electrophilic. Lone pair of electrons, so can be nucleophilic.


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Chapter 4 53

Basicity of Carbanions

A carbanion has a negative charge on its

carbon atom, making it a more powerful baseand a stronger nucleophile than an amine.

A carbanion is sufficiently basic to remove aproton from ammonia.


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Chapter 4 54

Carbenes as Reaction

Intermediates

A strong base can abstract a proton from tribromomethane(CHBr3) to give an inductively stabilized carbanion.

This carbanion expels bromide ion to give dibromocarbene. Thecarbon atom is sp2 hybridized with trigonal geometry.

A carbene has both a lone pair of electrons and an emptyporbital, so it can react as a nucleophile or as an electrophile.

WADE7 Lecture 04

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