4. In direct competition, electron-withdrawing groups exert a somewhat greater influence than electron-donating groups. Arylation of para-disubstituted compounds XC6H4Y showed that substitution ortho to the group X became increasingly preferred as the electron-withdrawing character of X increases (with Y held constant). 5. Substituents have a much smaller effect than in electrophilic or nucleophilic substitution, hence the partial rate factors are not great. 6. Although hydrogen is the leaving group in most free-radical aromatic substitutions, ipso attack and ipso substitution (e.g., with Br, NO2,or CH3CO as the leaving group) have been found in certain cases
4. Reactivity in the Attacking Radical
TABLE5.3. Some Common Free Radicals in Decreasing Order of Activity
The E values represent activation energies for the reaction
（2） Alkenes.
（3）Alkyl Side Chains of Aromatic Rings.
(4)Compounds Containing Electron-Withdrawing Substituents. Z-CH2-CH3
Z is COOH, COCl, COOR, SO2Cl, or CX3.
F · 1：1.4
Br ·1.1600
With certain large radicals there is a steric factor that may change the selectivity pattern. For example, in the photochemical chlorination of isopentane in H2SO4 with N-chloro-di-tert-butylamine and N-chloro-tertbutyl-tert-pentylamine, the primary hydrogens are abstracted 1.7 times faster than the tertiary hydrogen.
R H
ClF3 -75oC F
X2, hv
Me 41%
R X
Me ClF3 -75oC F 47%
BrF3, F3COF, Br2O, ROX,CCl4, BrCCl3, NBS, X-NR2 · H2SO4
14-2 Halogenation at Silicon
14-3 Allylic and Benzylic Halogenation
β
α
(5) Stereoelectronic Effects.
H
OMe
Hale Waihona Puke OMe O OH2. Reactivity at a Bridgehead
3.Reactivity in Aromatic Substrates
TABLE 5.2. Partial Rate Factors for Attack of Substituted Benzenes by Phenyl Radicals Generated from Bz2O2
1.1 Free-Radical Mechanisms in General
ROOR， ROOH，RCOOOOC,R-N=N-R, R-O-Cl, Cl2, Br2, RCH=O,R2C=O
The following are some general characteristics of free-radical reactions：
1. Reactions are fairly similar whether they are occurring in the vapor or liquid phase, though solvation of free radicals in solution does cause some differences. 2. They are largely unaffected by the presence of acids or bases or by changes in the polarity of solvents, except that nonpolar solvents may suppress competing ionic reactions. 3. They are initiated or accelerated by typical free-radical sources, such as the peroxides, referred to, or by light. In the latter case, the concept of quantum yield applies. Quantum yields can be quite high, for example, 1000, if each quantum generates a long chain, or low, in the case of nonchain processes. 4. Their rates are decreased or the reactions are suppressed entirely by substances that scavenge free radicals, for example, nitric oxide, molecular oxygen, or
1.4 Neighboring-Group Assistance in Free-Radical Reactions
CH3 Br * CH3 CH3
Br2, hv
CH3 Br * Br CH3 CH3
2. REACTIVITY
2.1 Reactivity for Aliphatic Substrates
ADVANCED ORGANIC CHEMISTRY Part II
Shanxi Normal University 2014
CHAPTER 5
Substitution Reactions: Free Radicals 1. MECHANISMS
1.1 Free-Radical Mechanisms in General 1.2 Free-Radical Substitution Mechanisms 1.3 Mechanisms at an Aromatic Substrate 1.4 Neighboring-Group Assistance in Free-Radical Reactions
benzoquinone. These substances are called inhibitors.
1.2 Free-Radical Substitution Mechanisms
First Step: OR
Second Step:
OR
1.3 Mechanisms at an Aromatic Substrate
5. The Effect of Solvent on Reactivity
CH3 H3C CH3 CH3
Cl2
H3C H3C
CH3 Cl CH3
CH3 + H3C CH2Cl CH3
REACTIONS HYDROGEN AS LEAVING GROUP A. Substitution by Halogen 14-1 Halogenation at an Alkyl Carbon
N-chlorosuccinimide(NCS), arylselenyl chlorides (ArSeCl), diselenides (ArSeSeAr),TsNSO, Cl2O
14-4 Halogenation of Aldehydes
B. Substitution by Oxygen
14-5 Hydroxylation at an Aromatic Carbon
2. REACTIVITY
2.1 The Effect of Substrate Structure 2.2 The Effect of the Leaving Group 2.3 The Effect of the Attacking Nucleophile
3. REACTIONS
1. MECHANISMS
14-6 Formation of Cyclic Ethers
14-7 Formation of Hydroperoxides
14-8 Formation of Peroxides
14-9 Acyloxylation
C. Substitution by Sulfur
D. Substitution by Nitrogen 14-11 The Direct Conversion of Aldehydes to Amides
14-12 Amidation and Amination at an Alkyl Carbon