1. Chapter 21 The Radical Chemistry
Organic Chemistry II
Fall 1999
Chapter 21 The Radical Chemistry
free radicals (radicals): R• (odd e-),  919
formation: homolytic cleavage, radical ions ( 918)
reactive & unstable intermediates: e- poor
stability: bond energy (R-H),  920 & Table 21.1
hyperconjugation & resonance stabilization: cf.  273
geometry: sp2/sp3 (rapid inversion),  921 bot.
OrgChem-Chap21
Chapter 19

2. Radical Chemistry (I) generation of radicals: initiation
Organic Chemistry II
Fall 1999
Radical Chemistry (I)
generation of radicals: initiation
peroxide: R-O-O-R (30 kcal/mole),  922
fragmentation: carboxylate radical,  924 bottom
azo compounds: R-N=N-R (AIBN),  922 bottom
photochemical reactions: Cl-Cl,  923 top
carbonyl compounds: R-C(O)-R’
OrgChem-Chap21
Chapter 19

3. Radical Chemistry (II)
Organic Chemistry II
Fall 1999
Radical Chemistry (II)
general reactions: 
with radicals: coupling / disproportionation (rare)
with normal molecules: H-abstraction / addition
stable radicals: phenoxy radicals,  925
resonance / steric hindrance: isolated as a solid
discovery of a radical:  Focus On
practice:  925, Problem 21.4 & 21.5
OrgChem-Chap21
Chapter 19

4. Halogenation chain reaction: mechanism,  928 Fig. 21.1
Organic Chemistry II
Fall 1999
Halogenation
CH4 + Cl2  CH3-Cl + H-Cl hn
chain reaction: mechanism,  928 Fig. 21.1
an equation for the overall reactions:  929 top
hard to control: multiple chlorination,  929 middle
selectivity: Cl2 (2:1,  929) < Br2 (0:1,  930)
exothermic / endothermic:  931 top & Figure 21.2
NBS: allylic/benzylic bromination, 
practice:  933, Problem 21.8
OrgChem-Chap21
Chapter 19

5. Radical Reactions (I) dehalogenation: Bu3SnH,  934 & Fig. 21.3
Organic Chemistry II
Fall 1999
Radical Reactions (I)
dehalogenation: Bu3SnH,  934 & Fig. 21.3
autoxidation: air oxidation (•O-O•),  935
regioselectivity: tetralin ( 935) & ether ( 936)
antioxidants: BHA/BHT,  936 bottom
lipid autoxidation & vitamin E:  Focus On
OrgChem-Chap21
Chapter 19

6. Radical Reactions (II)
Organic Chemistry II
Fall 1999
Radical Reactions (II)
addition to alkenes: anti-Markovnikov product
HBr with hn or R-O-O-R: Br•,  939 mechanism
slow radical reactions with HCl / HI:  940 middle
R-S-H / CCl4 (CBr4) / X2:  940 bottom
practice:  941, Problem & 21.15
OrgChem-Chap21
Chapter 19

7. Radical Reactions (III)
Organic Chemistry II
Fall 1999
Radical Reactions (III)
dissolving metal reductions: M  M+ + e-
Birch reduction of arenes:  942 top & Fig. 19.4
substituent effect: a-anions to EWGs;  942 bottom
conjugated alkenes:  943 bottom &  944 top
selective formation enolates vs others:  945 top
alkyne reductions: E-alkenes,  945 bottom
practice:  946, Problem & 21.17
OrgChem-Chap21
Chapter 19