Chapter 6 Chemical Reactivity and Reaction...

Chapter 6

Chemical Reactivityand

Reaction Mechanisms

Chemical Reactivity

Enthalpy

A simple chemical reaction can be broken down into bond creating and

bond breaking components:

A-B + Y-Z → A-Y + B-Z

A-B → A· + B· ∆Hº = +BDE Y-Z → Y· + Z· ∆Hº = +BDEA· + Y· → A-Y ∆Hº = -BDEB· + Z· → B-Z ∆Hº = -BDE

BDE= Bond dissociation energy

∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed

Homolytic Bond Dissociation Energies(∆Hº): Y−Z → Y· + Z·


H

CH3C

H

H Br Br

H

CH3C

H

Br H Br++

423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol

For the process:A-B + Y-Z → A-Y + B-Z

BDE=


H

CH3C

H

H Br Br

H

CH3C

H

Br H Br++

423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol

∆Hºrxn = 423 +192-294-366

= -45 kJ/mol

Enthalpy(H)

Reaction coordinate

An Exothermic Reaction

∆H0 =⊖

A + B

C + D

Enthalpy(H)

Reaction coordinate

And Endothermic Reaction

∆H0 =⊕

A + B

C + D

Chemical Reactivity

Entropy(a measure of the disorder associated with a system)

Entropy

Entropy change is favorable when the result is a more

random system.

This is called a Positive Entropy Change.

The Concept of Entropy(The Degree of Randomness)

A change in degree of randomness is a change in the number of ways of arranging particles.

More order Less Order

solid liquid gas

solid + liquid ions in solution

solid + solid gases + ions in solution

Fewer degrees of More degrees ofmovement movement

fewer particles more particles

Chemical Reactivity

Gibbs Free Energy

ΔG = ΔH - TΔS

Enthalpy Factor Entropy Factor

Freeenergy

(G)

Reaction coordinate

∆G0 =⊖

A + B

C + D

Exergonic process

A + B C + D

Freeenergy

(G)

Reaction coordinate

∆G0 =⊕

A + B

C + D

Endergonic Process

A + B C + D

Chemical ReactivityEquilibria

!G under Nonstandard Conditions

"!!G = !G° only when the reactants and products are in their standard states

"! there normal state at that temperature

"!partial pressure of gas = 1 atm

"!concentration = 1 M

"!under nonstandard conditions, !G = !G° + RTlnQ

"!Q is the reaction quotient

"! at equilibrium !G = 0

"!!G° = !RTlnK

ΔG < 0 for a spontaneous processΔG > 0 for a nonspontaneous processΔG = 0 for a process at equilibrium

ΔG = ΔH - TΔS

!G under Nonstandard Conditions

"!!G = !G° only when the reactants and products are in their standard states

"! there normal state at that temperature

"!partial pressure of gas = 1 atm

"!concentration = 1 M

"!under nonstandard conditions, !G = !G° + RTlnQ

"!Q is the reaction quotient

"! at equilibrium !G = 0

"!!G° = !RTlnK

∆Gº (kJ/mol) Keq% product at equilibrium

-17 1000 99.9%

-11 100 99%

-6 10 90%

0 0 50%

+6 0.10 10%

+11 0.01 1%

+17 0.001 0.1%

Relationship Between ∆G and Keq

Chemical Reactivity

Kinetics and Rates

First-Order Kinetics Second-Order Kinetics

Rate of a Reaction

What Affects the Rate of a Reaction?

Energy of ActivationTemperature

Steric Considerations

Thermodynamics vs. Kinetics Thermodynamics vs. Kinetics

Eactivation

Faster Slower

∆Eact

∆Eact

∆Eact

∆Eact

Exe

rgo

nic

En

de

rgo

nic

Effect of a Catalyst

∆E

(g) (l) (l)

∆Hº ≈ −108 kJ/mol

H C

H

C

H

H H

H + Br Br H C

H

C

Br

H H

H + H Br

(g) (l) (l) (g)


HC C

H

H

HBr Br H C

Br

C

Br

H H

H+

HC C

H

H

HH H H C

H

C

H

H H

H+

(g) (l) (g)


(g) (l) (l)


H

C C

H

H

H

Br Br H C

Br

C

Br

H H

H+

Ni

H C

H

C

Br

H H

HH

C C

H

H

HH Br+

(l) (g) (g)

∆Hº ≈ +57 kJ/mol

HC C

H

H

BrH C C H H Br+

(l) (g) (g)

∆Hº ≈ +198 kJ/mol

Chemical Reactivity

Transition States vs.Intermediates

Potentialenergy

Reaction coordinate

Reactant

Product

Transition State

High-energy stateCannot be isolated

Bonds being broken or formed

Potentialenergy

Reaction coordinate

Reactant

Product

Transition State Transition

State

Intermediate

Longer-livedBonds not being

broken or formed

R

R' R"

LGNu

- R

R'R"

Nu LG -LGR

R'R"Nu:

-

Some reactions proceed through a single transition state.

Potentialenergy

LGR

R'R"

R

R' R"

+ R

R'R"

Nu

LGR

R'R"

R

R' R"

+Nu:-

Some reactions proceed through two 0r more reactive intermediates and transition states.

Pote

ntia

len

ergy

Effect of a Catalyst

Potentialenergy

Reaction coordinate

In an exergonic reaction, the transition state is closer to the energy and structure

of the reactant(s).

Ea

Reactants

Products

The Hammond Postulate

Changing the energy of the products does very little to influence the rate of the

reaction.

Products

Potentialenergy

Reaction coordinate

Reactants

Products

The Hammond PostulateIn an endergonic reaction,

the transition state is closer to the energy and structure

of the product(s).

Ea

Changing the energy of the products lowers the energy of activation and increases

the rate of the reaction.

ProductsEa

Chemical Reactivity

Nucleophiles and Electrophiles

Nucleophilic CentersAn electron-rich atom which is capable of

donating a pair of electrons (i.e., a “Lewis base”)

May or may not have a negative charge:

Must consider electronegativity of atoms:

HO

H HO

CO

H

H

H

H

CLi

H

H

H

δ+....δ-

δ-....δ+

Electrophilic CentersAn electron-deficient atom which is capable of

accepting a pair of electrons (i.e., a “Lewis acid”)

May or may not have a positive charge:

The electrophilic center may be an empty orbital:

CCl

H

H

HC

CH3

H3C

H3C

δ+....δ- C

CH3

H3C

CH3

≡

CCH3

H3C

CH3

Patterns of Chemical Reactions

Nucleophilic AttackLoss of a Leaving Group

Proton TransferCarbocation Rearrangements

Patterns of Chemical ReactionsNucleophilic Attack

BrH

O

H

Br O

H

H

Patterns of Chemical ReactionsLoss of a Leaving Group

Br O Br

O

Br++ Br

Patterns of Chemical ReactionsProton Transfer

H

H

Patterns of Chemical ReactionsProton Transfer

OH

HO

H+

O

HO

H

H+

CC

O

HO H

CC

O

H

O H

Patterns of Chemical ReactionsCarbocation Rearrangements

Where do carbocations come from ?

C LG C LG+

H

C

H H

C HH

HC

H

HH

Two views of the methyl cation, CH3+. The carbon and three hydrogens all lie in the same plane and the carbon is sp2 hybridized. One unoccupieid, unhybridized p orbital remains perpendicular to the plane of the four atoms.

The orbitals of the three C-H bonds lie perpendicular to the

p orbital, resulting in zero overlap and zero

stabilization.

Stabilization by Hyperconjugation

Two views of the ethyl cation, CH3CH2+. The carbon and three hydrogens of the positive carbon all lie in the same plane and the carbon

is sp2 hybridized. One unoccupied, unhybridized p orbital remains perpendicular

to the plane of the four atoms. The second carbon has the expected sp3 hybridization.

One of the three C-H bonds of the adjacent methyl group lies in the same plane as the unoccupied p orbital of of the positive carbon.

Some of the electron density flows from the sigma bond toward the

electron deficient carbon, resulting in stabilization of the carbocation.

Note: This stabilization is also present in two other conformers as

the C-C bond rotates.

H

C

H CH3

C CH

H

H

H

H

CH

HC

H

HH

Stabilization by Hyperconjugation


Increasing Stability

Methyl Primary Secondary Tertiary

H

H H

H

C H

H

C C

C

C C

As more adjacent carbons with additional attached groups are added, additional

hyperconjugation and stabilization of the resulting carbocation occurs.


Increasing Stability

Methyl Primary Secondary Tertiary

H

H H

H

C H

H

C C

C

C C

Carbocations may “rearrange” from 1º--->2º--->3º

to form more stable structures !!!


H3CH3C CH3

HH

H3CH3C CH3

H3C

H3CH3C CH3

H3CH3C CH3

CH3

hydride shift

methyl shift

2º carbocations 3º carbocations

2º carbocation 3º carbocation

H

C

C

C

CH3H3C

CH3

H H

C

C

C

CH3H3C

CH3

H

2º carbocation 3º carbocation

formation of a more stable resonance structure


Chapter 6 Chemical Reactivity and Reaction...

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