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

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

Radical Chemistry (II) Organic Chemistry II Fall 1999 Radical Chemistry (II) general reactions:  923-924 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:  926-7 Focus On practice:  925, Problem 21.4 & 21.5 OrgChem-Chap21 Chapter 19

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,  932-933 practice:  933, Problem 21.8 OrgChem-Chap21 Chapter 19

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:  937-8 Focus On OrgChem-Chap21 Chapter 19

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 21.14 & 21.15 OrgChem-Chap21 Chapter 19

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 21.16 & 21.17 OrgChem-Chap21 Chapter 19