Page 1

CHAPTER 15: RADICAL REACTIONS

RADICAL OVERVIEW

EXAMPLES

Main internal combustion engine pollutant Nitrogen monoxide (NO)

Metabolism is essentially food oxidation (produces electrons)

Possible alternative reactions:

Ionizing Radiation

FISHHOOK ARROWS

HETEROLYTIC CLEAVAGE

N O

2e- + 4H+ + O O 2 H2O

e- + O O O O

2e- + 2H+ + O O O H2

HO

H

H C

H

H

Br

Page 2

HOMOLYTIC CLEAVAGE

RADICAL HALOGENATION REACTION

GENERAL PATTERN

O O

H

H

Br2

Page 3

MECHANISM

INITIATION

PROPAGATION STEPS

TERMINATION STEPS

ClCl2hv

Page 4

SELECTIVITY OF REACTION, PART I

CHLORINATION AND BROMINATION DIFFERENCES

Product ratios1

Reaction with chlorine Reaction with bromine

Type 1˚ 2˚ 3˚ Type 1˚ 2˚ 3˚

Selectivity2 Selectivity

WHY TERTIARY > SECONDARY > PRIMARY

R-H à R� + H�

Type3 kcal/mol

H3C-H 105

CH3CH2-H 100

(CH3)2CH-H 96

(CH3)3C-H 95.8

1 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 529 2 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 530 3 Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.243

Page 5

EXAMPLES

1.   Give all possible monohalogenated products of this reaction, including stereoisomers.

2.   What is the ratio of products for this reaction?

Product ratio4

SELECTIVITY OF REACTION, PART II

ENERGY DIAGRAMS

Chlorination Bromination

Reaction5 Relative Rate at 27 ˚C

CH4 + F� à CH3� + HF

CH4 + Cl� à CH3� + HCl

CH4 + Br� à CH3� + HBr

4 Solomons, G., Fryhle, C.B., Organic Chemistry, 8th ed., Wiley, 2004, pp. 468 5 Wade, L.G., Organic Chemistry, 8th ed., Pearson, 2013, pp. 145

• 70% C, 30% D • 30% C, 70% D • 99% C, 1% D • 1% C, 99% D

Page 6

EARLY AND LATE TRANSITION STATES

Hammond’s Postulate: transition states more closely resemble either the reactants or products, depending on whether the step is exothermic or endothermic.

Exothermic Step: Endothermic step:

BROMINE VS. CHLORINE SELECTIVITY

1)   WHY CHLORINE IS UNSELECTIVE

H Cl HCl

Page 7

2)   WHY BROMINE IS SELECTIVE

APPLICATION: DENTAL FILLINGS

The filling used by Dr. Tawatari has 4 materials: •   Quartz nanoparticles: the bulk of the

material, which gets encapsulated by the polymer.

•   Polymer monomer #1 •   Polymer monomer #2 •   Radical initiator

Polymer monomer #1 Radical Initiator

Polymer monomer #2

O

OH

O

O

O O

OH

O O

O

OO

OO

O

O

Page 8

APPLICATION: OZONE LAYER

Link of ozone “hole” and Freon use published in 1974; Freon had been used since 1930. Freon was banned from use with aerosols in USA since 1978, worldwide ban in 1996.

Each Cl atom destroys on average 10,000 ozone molecules before it is removed.6

6 Baird, C., Cann, M., Environmental Chemistry, 5th ed., Freeman, 2012, pp. 28

1979 1984 1989 1994 1999 2000

Antarctic (South Pole) ozone concentrations

OO

O

O O hv O O

O O O OO

O

Natural ozone formation:

Natural ozone depletion:

OO

OO

Above stratosphere the majority of oxygen exists in atomic form.

In stratosphere the majority of oxygen exists in diatomic form.

Catalytic ozone depletion: