chapter 6 chemical reactivity and reaction...

Chapter 6

Chemical Reactivity and

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

BDE= Bond dissociation energy

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

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


For the process: A-B + Y-Z → A-Y + B-Z

Bond Dissociation Energies

H

CH3C

H

H Br Br

H

CH3C

H

Br H Br++

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

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


∆Hºrxn = 423 +192-294-366

= -45 kJ/mol

H

CH3C

H

H Br Br

H

CH3C

H

Br H Br++

423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol423 kJ/mol 192 kJ/mol 294 kJ/mol 366 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 of movement movement

fewer particles more particles

Chemical Reactivity

Gibbs Free Energy Change

ΔG = ΔH - TΔS

Enthalpy Factor Entropy Factor

Free energy

(G)

Reaction coordinate

∆G0 =⊖

Exergonic (Spontaneous) process

A + B C + D

Free energy

(G)

Reaction coordinate

∆G0 =⊕

Endergonic (non-Spontaneous) Process

A + B C + D

Chemical Reactivity Equilibria

!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 Note: As Keq increases, i.e., ∆G becomes more negative

and the reaction becomes more favorable.

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

ΔG = ΔH - TΔS

ΔH ΔS Low Temperature

High Temperature

- + Spontaneous Spontaneous

+ - Non- Spontaneous

Non- Spontaneous

- - Spontaneous Non- Spontaneous

+ + Non- Spontaneous Spontaneous

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

-17 1000 99.9%

-11 100 99%

-6 10 90%

0 1 50%

+6 0.1 10%

+11 0.01 1%

+17 0.001 0.1%

Relationship Between ∆G and Keq

Chemical Reactivity

Kinetics and Rates

Potentialenergy

Reaction coordinate

Thermodynamics vs. Kinetics

Reactants

Products

thermodynamics initial and final states

spontaneity

kinetics intermediate states speed of reaction

Eactivation

First-Order Kinetics Second-Order Kinetics

Rate of a Reaction

What Affects the Rate of a Reaction?

Energy of Activation Temperature

Steric Considerations

Potentialenergy

Reaction coordinate

Reactants ➝ Products

Reactants

Products

Influence of a Catalyst

uncatalyzed reaction

Ea

Ea

catalyzed reaction

(g) (l) (l)

∆Hº ≈ −20 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 ≈ +86 kJ/mol

HC C

H

H

BrH C C H H Br+

(l) (g) (g)

∆Hº ≈ +98 kJ/mol

Chemical Reactivity

Transition States vs. Intermediates

Potentialenergy

Reaction coordinate

Reactant

Product

Transition State High-energy state Cannot be isolated

Bonds being broken or formed

Potentialenergy

Reaction coordinate

Reactant

Product

Transition State Transition

State

Intermediate

Longer-lived Bonds 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.

Potential

energy

Effect of a Catalyst

Energy

Reaction coordinate

Eact

Eact

Intermediate

Kinetic product

Thermodynamic product

Reaction 1

Product #1

Reaction 2

Product #2

Potentialenergy

Reaction coordinate

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

of the reactant(s).

Ea

The Hammond Postulate

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

reaction.

Products

Reactants

Products

Potentialenergy

Reaction coordinate

The Hammond Postulate

In 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.

Products

Ea

Reactants

Products

Chemical Reactivity

Nucleophiles and Electrophiles

Nucleophilic Centers An 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 Centers An 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 Attack Loss of a Leaving Group

Proton Transfer Carbocation Rearrangements

Patterns of Chemical Reactions Nucleophilic Attack

BrH

O

H

Br O

H

H

Patterns of Chemical Reactions Loss of a Leaving Group

Br O Br

O

Br++ Br

Patterns of Chemical Reactions Proton Transfer

H

H

Patterns of Chemical Reactions Proton Transfer

OH

HO

H+

O

HO

H

H+

CC

O

HO H

CC

O

H

O H

Patterns of Chemical Reactions

Nucleophilic Attack Loss of a Leaving Group

Proton Transfer Carbocation Rearrangements

Examples

H3CCH2

H

CO

O

CH3C

H3CCH2

HC

O

O

CH3CH2

H3CCH2

H3C

1a

1b

H3CCH2

CH

CH3

HO

CH3

H3CCH2

HCCH3

HO

CH3

1c HS

O

O

CH3

O

H

SO

O

CH3

O

1d

1e C

C

O

H3C

H3CH

H

OH

H

H

C

C

O

H3C

H3CH

H

OH

H

H

C C

O

H3C CH3

HH

OCH2

CH3

C C

O

H3C CH3

HH

OCH2

CH3

3C

C

O

H3C

H3CH

HC

C

OH

H3C

H3CH

H

HOH2O, HO-

C

C

O

H3C

H3CH

H

δ-δ+

H O C

C

O

H3C

H3CH

H

HO

H

O

Hδ+δ-

H

O

4

C

C

O

H3C

H3CH

HC

C

OH

H3C

H3CH

H

HOH2O, H3O+

δ-

C

C

O

H3C

H3CH

H

H O

H

H

C

C

OH

H3C

H3CH

H

OH

HOH

H

O

H

HH

C

C

O

H3C

H3CH

HO

H

HH

C

C

O

H3C

H3CH

H

HO

H

H

Patterns of Chemical Reactions Carbocation 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.

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 carbocation 3º carbocation


H

C

C

C

CH3H3C

CH3

H H

C

C

C

CH3H3C

CH3

H


formation of a more stable resonance structure


Examples

4aCH3

CH3

H

CH3

CH3

4b

H3C C

CH3

C

CH3

CH3

OH

H3C C

CH3

C

CH3

CH3

OH

H3C C

OH

C

CH3

CH3

CH3

5a

H3C C

CH3

CH

C

CH3

CH3

H

H

5bH

H

H

H

H

5c

O H

H

H

H3C C

CH3

CH

C

CH3

CH3

H

H

H

H

H

H

H

O H

H

HO

H

OH

H

H

H

H

H

chapter 6 chemical reactivity and reaction...

Documents