chapter3 6 chemical reactivity and mechanisms '13 bw modified

8/13/2019 Chapter3 6 Chemical Reactivity and Mechanisms '13 BW Modified

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Chapter 6. Chemical Reactivity

and Mechanisms

Junha Jeon

Department of Chemistry

University of Texas at Arlington

Arlington, Texas 76019

Chem 2321, Fall 13

Can you remember?

Nature of Atomic Orbital Overlap: Two Theories: How is a covalent bond formed from the overlap of atomic overlap?

Valence bond theory

Molecular orbital theory

Valence Bond (VB) Theory

A bond: simply viewed as the sharing of electron density between two atoms as a

result of the constructive interference of their atomic orbitals.

For example,

sigma (") bond

In fact, all single bond are sigma (") bond.

H H H H H H

Valence Bond Theory

A bond: simply viewed as the sharing of electron density between two atoms as a

result of the constructive interference of their atomic orbitals.

For example,

sigma (") bond

In fact, all single bond are sigma (") bond.

H H H H H H

Molecular Orbital (MO) Theory

Valence bond theory is good, yet not perfect (only use constructive interference).

Molecular orbitals: mathematically combined atomic orbitals that extend over the

entire molecule. The mathematical method is called the linear combination of

atomic orbitals (LCAO).

MOs are a more complete analysis of bonds because they include both

constructive and destructive interference.

The number of MOs created must be equal to the number of atomic orbitals that

were used.

H2MOs

A Bond to Make

stabilizationH H

HH


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A Bond to Break

destabilizationH H

HH

A Bond to Break

destabilization

: need kinetic energy

H H

HH

6.1 Enthalpy

Enthalpy (#Hor q)

!The kinetic energy exchange between the reaction and its surroundings at

constant pressure! (q = heat)

!The amount of energy necessary to break the bond homolytically.

Enthalpy

Homolytic bond cleavage:

Heterolytic bond cleavage:

Enthalpy


Heterolytic bond cleavage:

Enthalpy


Enthalpy (#Hor q)

! The kinetic energy exchange between the reaction and its surroundings at

constant pressure!


!Bond dissociation energy (BDE), #H


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Bond Dissociation Energies Bond Dissociation Energies

Bond Dissociation Energies Bond Dissociation Energies

Bond Dissociation Energies Heat of Reaction (#H)

Most reactions involve multiple bonds breaking and forming.

!If during a chemical reaction the reaction temperature decreases, the

reaction causes the surrounding temperature to decreases.

: Exothermic process the system gives energy to the surroundings (#H

is negative).

!If during a chemical reaction the reaction temperatureincreases, the

reaction causes the surrounding temperature to increases.

: Endothermic process the system receives energy to the surroundings

(#H is positive).


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Heat of Reaction (#H)



reaction causes the surrounding temperature (e.g. solvent) to decreases.


is negative).




(#H is positive).




reaction causes the surrounding temperature (e.g. solvent) to decreases.


is negative).




(#H is positive).






is negative).




(#H is positive).






(#H is positive).




is negative).


!Reaction coordinate: the progress of the reaction

Predicting #Hof a Reaction


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Predicting #Hof a Reaction Predicting #Hof a Reaction


+ 623 762


+ 623 762

!H = 139 kJ/mol

6.2 Entropy

What is the ultimate measure for determining whether or not a reaction can

occur?

Enthalpy and Entropy must both be considered when predicting whether a

reaction will occur.

!Entropy ($S): defined as molecular disorder, randomness, freedom

: particularly statistical thermodynamics (probability)

! Entropy may most accurately be thought of as the number of states that a

molecules energy can be distributed over.

6.2 Entropy


occur?








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6.2 Entropy


occur?







6.2 Entropy


occur?







Entropy: Free Expansion of a Gas

a higher state of entropy

!Molecules exhibit vibrational, rotational, and translational motion.

!If the energy of molecules can be distributed in a higher number of vibrational,

rotational, and translational states, the sample will have a greater entropy.

Motion within molecule

Vibrational motion: motion that changes the shape of the molecule stretching, bending, and rotation of bonds

Motion of whole molecule

Rotational motion: whole molecule spins around an axis in three dimensionalspace

Translational motion: whole atom/molecule changes its location in threedimensional space











Entropy

!#$% total entropy change will determine whether a process is spontaneous

(favors the forward direction, an increase):

consider #Ssys:the reaction, #Ssurr:usuallysolvent

The Second Law of Thermodynamics

! For chemical reactions, we must consider the entropy change for both the

system (the reaction) and the surroundings (the solvent usually).


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Gibbs Free Energy

!The spontaneity of a process depends only on #Stot

!#Ssyscan be measured or estimated.

!#$% measurement of #Ssurr?

Gibbs Free Energy



!#$% measurement of #Ssurr?

Gibbs Free Energy

1. The sign of #Ssurr depends on the direction of the heat flow:

At constant temperature, an exothermic process in the system causes heat to flow

into surroundings, increasing the random motions and thus the entropy of

surroundings.

2. The magnitue of #Ssurr depends on the temperature:

The transfer of a given quantity of energy as heat produces a much greater percent

change in the randomness of the surroundings at a low temperature than it does at

a high temperature.

Gibbs Free Energy



!#$% measurement of #Ssurr? see, slide28 for #Gsys

Gibbs Free Energy


!Multiply both sides by negative temperature (T): The Gibbs Free Energy, #G:

simply #G = T#Stot

Gibbs Free Energy

!Spontaneity of a process:

#Stot:positive

The Gibbs Free Energy,#G: negative


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Gibbs Free Energy


#Stot:positive


Gibbs Free Energy


#Stot:positive


"The second law of thermodynamics

Gibbs Free Energy: Example

1. Predict the sign (+ or ) for #Ssys.

2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the

sign (+ or -) of $Hsys.

3. Predict the sign (+ or -) for $Ssurr.

4. Predict the sign (+ or -) for $G.

5. How will the spontaneity of the reaction depend on temperature?


1. Predict the sign (+ or ) for #Ssys. Negative


sign (+ or -) of $Hsys.







sign (+ or ) of #Hsys. (overall, three pi bonds to one pi and two sigma bonds.)

(hint: see the next page.)

Enthalpy


Enthalpy (#Hor q)

! The kinetic energy exchange between the reaction and its surroundings at

constant pressure!


!Bond dissociation energy (BDE), #H

slide 13


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sign (+ or ) of #Hsys. (overall, three pi bonds to one pi and two sigma bonds.)




sign (+ or ) of #Hsys. Negative (exothermic)








3. Predict the sign (+ or ) for #Ssurr.







3. Predict the sign (+ or ) for #Ssurr.







3. Predict the sign (+ or ) for #Ssurr. Positive







3. Predict the sign (+ or ) for #Ssurr. Positive, for #Ssys. Negative.




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3. Predict the sign (+ or ) for #Ssurr. Positive, for #Ssys. Negative.

4. Predict the sign (+ or ) for #G.






3. Predict the sign (+ or ) for #Ssurr. Positive

4. Predict the sign (+ or ) for #G.

5. Depending on temperature!!

Gibbs Free Energy: Exergonic vs. Endergonic

!The spontaneity of a process: negative #G"Exergonic process

: favor the products

!The spontaneity of a process: positive #G"Endergonic process

: favor the reactants

6.4 Equilibria

Consider an exergonic process with a () $G. Will every molecule of A and B be

converted into products?

No. An equilibrium will eventually

be reached.

A spontaneous process will simply

favor the products meaning there

will be more products than

reactants.

Equilibria




be reached.

A spontaneous process will simplyfavor the products meaning there


reactants.

Equilibria




be reached.

A spontaneous process will simplyfavor the products meaning there


reactants


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Equilibria

Why doesnt an exergonic process react 100% to give products? Why will some

reactants remain? (moles of reactants are present.)

In any reaction, collisions are necessary.

!As [A] and [B] decrease, collisions between A and B will occur less often.

!As [C] and [D] increase, collisions between C and D will occur more often.

The more often C and D collide, the more often collisions will occur with

enough free energy for the reverse reaction to take place.

!Recall that EQUILIBRIUM is dynamic and occurs when the forward and

reverse reaction rates are equal.

Equilibria










Equilibria










Equilibria










Equilibria










Equilibria








!Recall that equilibriumis dynamic and occurs when the forward and reverse

reaction rates are equal.


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Equilibria

!Equilibrium is also the state with the

lowest free energy overall.

!Every system seeks to achieve

a minimum of free energy.

K > 1

K = 1

K < 1

Equilibria

!Equilibrium is also the state with the

lowest free energy overall.

!Every system seeks to achieve

a minimum of free energy.

K > 1

K = 1

K < 1

Equilibria

!Keqand $G

R= gas constant (8.314 J/molK)

K > 1

K = 1

K < 1

Equilibria

!Keqand $G

R= gas constant (8.314 J/molK)

Thermodynamics

!Keq, $G, $H, and $S: thermodynamic terms

!Thermodynamics: the study of how energy is distributed under the influence of

entropy.

Thermodynamics



entropy.


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Thermodynamics



entropy.

6.5 Kinetics

!Spontaneous process:

thermodynamically favorable, i.e. favoring formation of products

That does not tell us anything about the rate or kinetics for the process.

!Some spontaneous processes: fast (e.g.explosions)

!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]

!The study of reaction rates: kinetics

Kinetics







Kinetics







Kinetics







Kinetics







!Reaction rate: the number of collisions that will result in product production in a

given period of time


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Kinetics







!Reaction rate: the number of collisions that will result in product production in a

given period of time

Rate Equations

!Reaction constant: specific to each reaction

!Concentration: proportional to a frequency of collisions of molecules that lead

to a reaction

Rate Equations

!Reaction constant: specific to each reaction

!Concentration: proportional to a frequency of collisions of molecules that lead

to a reaction

Rate Equations

!The degree to which a change in [reactant] will affect the rate is known as the

order.

!The order is represented by xand yin the rate law equation (experimentally

determined).

Rate Equations


order.


determined).

Rate Equations


order.


determined).


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Factors Affecting the Reaction Rate

0. The concentrations of the reactants

1. Energy of activation

2. The temperature

3. Geometry and sterics

Factors Affecting the Reaction Rate

0. The concentrations of the reactants


2. The temperature

3. Geometry and sterics

related to the rate constant (k)

Factors Affecting the Reaction Rate (Rate Constant, k)


: energy barrier between the reactants and the products the minimum

amount of energy required for a reaction to occur between two reactants that

collide



!A certain threshold kinetic energyof molecules (Ea)




!Distribution of kinetic energy






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!Catalysts (and enzymes)

catalyst: a compound that

can speed up the rate of a

reaction without itself being

consumed by the reaction

alternating a reaction pathway

not changing $Greactants,

$gproducts and the position of

equilibrium












$gproducts and the position of

equilibrium












$Gproducts and the position of

equilibrium


2. Temperature

: a measure of a systems averagekinetic energy


3. Geometric and Steric

: the geometry of the reactants and the orientation of their collision can have

an impact on the frequency of collisions that lead to a reaction.


3. Geometric and Steric

: the geometry of the reactants and the orientation of their collision can have

an impact on the frequency of collisions that lead to a reaction.

We are going to discuss this factor in chapter 7.


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6.6 Energy Diagrams: Kinetics vs.Thermodynamics Kinetics vs.Thermodynamics

!C + D is kineticallyand thermodynamicallyfavorable!

Kinetics vs.Thermodynamics

!C + D is thermodynamicallyfavorable!

!E + F is kineticallyfavorable!




At high temperature, C + D vs. E + F ??




At high temperature, C + D vs. E + F ??

Transition States vs. Intermediates

!Transition states: at all localenergy maxima

!Intermediates: at all localenergy minima


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!Transition states: at all local energy maxima

transiently exist and cannot be isolated!

bonds are in the process of being

broken and/or formed simultaneously.


!Transition states: at all local energy maxima

transiently exist and cannot be isolated!

bonds are in the process of being

broken and/or formed simultaneously.


!Intermediates: at all localenergy minima

intermediates generally exist long enough to be observed.

bonds are notin the process of breaking or forming.

The Hammond Postulate

!Two points on an energy diagram that are close in energy should be similar in

structure.

The Hammond Postulate

!Two points on an energy diagram that are close in energy should be similar in

structure.

6.7 Nucleophiles and Electrophiles

Three major reaction categories:

1. Ionic (polar) reactions

2. Radical reactions (chapter 11)

3. Pericyclic reactions (chapter 17)


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Nucleophiles and Electrophiles


the participation of ions as reactants, intermediates, or products

the force of attraction between opposite charges



the participation of ions as reactants, intermediates, or products

the force of attraction between opposite charges

!Nucleophile (nucleophilic reagent): a reagent that forms a bond to its reaction

partner (the electrophile) by donating both bonding electrons. (nucleus lover)

!Nucleophilic center: an electron-rich atom that is capable of donating a pair of

electron


electrophile nucleophile

an electrophilecenter





electron



a nucleophilecenter





electron

!Polarizability: the ease of distortion of the electron cloud of a molecular entity

by an electric field (such as that due to the proximity of a charged reagent)

affecting the strength of nucleophilicity. ROH vs.RSH





electron

!Polarizability: the ease of distortion of the electron cloud of a molecular entity

by an electric field (such as that due to the proximity of a charged reagent)

affecting the strength of nucleophilicity. ROH vs.RSH


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!Electrophile (electrophilic reagent): a reagent that forms a bond to its reaction

partner (the nucleophile) by accepting both bonding electrons from its reaction

partner. (electron lover)

!Electrophilic center: an electron-deficient atom that is capable of accepting a

pair of electrons


a nucleophilecenter






pair of electrons



a nucleophilecenter






pair of electrons

carbocation


Nucleophiles and Electrophiles 6.8 Mechanism and Arrow Pushing

!Acid-base reaction


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Four Patterns of Ionic Mechanism

1. Nucleophilic attack

2. Loss of a leaving group

3. Proton transfers (acid/base)

4. Rearrangements



a lone pair:

a nucleophilic center

empty porbital:

an electrophilic center



a lone pair:


empty porbital:




a lone pair:


inductive effect (carbon

with a &+ charge):




a lone pair:


inductive effect (carbon

with a &+ charge):


1. Nucleophilic Attack

A. Stepwise view: Resonance contribution

B. One-step electron flow


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2. Loss of Leaving Group 2. Loss of Leaving Group

2. Loss of Leaving Group

or

!Acid-base reaction (protonated)

!Acid-base reaction (deprotonated)

3. Proton Transfer

or

3. Proton Transfer

A. One-step electron flow

B. Stepwise view: Resonance contribution

3. Proton Transfer


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4. Rearrangement

Carbocations can be stabilized by neighboring groups through slight orbital

overlap called hyperconjugation.

4. Rearrangement

Carbocations can be stabilized by neighboring groups (e.g. alkyl group) through

slight orbital overlap called hyperconjugation.

4. Rearrangement

Carbocations can be stabilized by neighboring groups (e.g. alkyl group) through

slight orbital overlap called hyperconjugation.

4. Rearrangement

Carbocation stability: depending upon # of alkyl groups attached directly to the

carbocation

4. Rearrangement

Carbocation rearrangement

1. Hydride (H) Shift

2. Methyl Shift

4. Rearrangement



2. Methyl Shift


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4. Rearrangement



2. Methyl Shift

4. Rearrangement

What is the driving force of the Carbocation rearrangement?


2. Methyl Shift

4. Rearrangement

What is the driving force of the Carbocation rearrangement? Stability of C+


2. Methyl Shift

4. Rearrangement

Carbocation rearrangement?

4. Rearrangement


4. Rearrangement



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4. Rearrangement

Carbocation rearrangements generally do not occur when the carbocation is

already tertiary unless a rearrangement will produce a resonance-stabilized

carbocation;

4. Rearrangement



carbocation;

4. Rearrangement



carbocation;

6.9 Combining the Patterns of Arrow Pushing'

Combining the Patterns of Arrow Pushing'

!Two arrow-pushing patterns simultaneously

!Concerted process: Two or more primitive changes are said to be concerted

(or to constitute a concerted process) if they occur within the same elementary

reaction.

1. The arrow starts on a pair of electrons (a bonded pair or a lone pair)

2. The arrow ends on a nucleus (as the formation of a lone pair) .

6.10 Drawing Curved Arrows


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1. The arrow starts on a pair of electrons (a bonded pair or a lone pair)

2. The arrow ends on a nucleus (as the formation of a lone pair) .


3. Avoid breaking the octet rule. Never give C, N, O, or F more than 8 valence

electrons.

4. Draw arrows that follow the four key patterns.


3. Avoid breaking the octet rule. Never give C, N, O, or F more than 8 valence

electrons.

4. Draw arrows that follow the four key patterns.


chapter3 6 chemical reactivity and mechanisms '13 bw modified

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