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Radical reactions Forming of a radical. Homolysis:. Radical reactions Forming of a radical. Homolytic cleavage by Photolysis. Radical reactions Forming of a radical. Reactions of molecules with other radicals. Radical reactions Forming of a radical. Radical reactions Stability.
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Homolysis: Radical reactionsForming of a radical
Homolytic cleavage by Photolysis Radical reactionsForming of a radical
Reactions of molecules with other radicals Radical reactionsForming of a radical
Radical substitution reactionsHalogenation Chlorine rather unselective !!! (number of hydrogens versus radical position) Bromine less reactive- More selective !!!
Stereochemistry: Radical substitution reactionsHalogenation
Allylic and benzylic substitution Radical substitution reactionsHalogenation
Allylic radical substitution Radical substitution reactionsHalogenation If polar solvent -> electrophilic addition (stabilization of carbocation) • If apolar solvent -> radical substitution • If low Br2 concentration -> allylic radical substitution • If high Br2 concentration -> radical addition to double bond
Allylic radical substitution Radical substitution reactionsHalogenation If substrate not chosen carefully -> 3 products
Allylic radical substitution -> NBS is better reagent if selective bromination wanted (in apolar solvent) NBS keeps concentration of Br2 low -> just allylic radical substitution (no radical addition to double bomd) Radical substitution reactionsHalogenation
Benzylic radical substitution -> NBS is better reagent if effective bromination wanted Radical substitution reactionsHalogenation Cl2 is more reactive than Br2 -> chlorination can occur at different positions
Addition to conjugated dienes Radical addition reactionsAddition of HBr to alkenes
Radical addition reactionAddition of oxygen -> autooxidation
Radical addition reactionAddition of oxygen -> autooxidation
Autooxidation of unsaturated fats and oils -> origins of rancidity (unpleasant smell and taste of badly stored fats Radical addition reactionAddition of oxygen -> autooxidation
Autooxidation of unsaturated fats and oils -> origins of rancidity (unpleasant smell and taste of badly stored fats Radical addition reactionAddition of oxygen -> autooxidation -> Oxidation (acid) and chain shortening
Antioxidants and health Radical addition reactionAddition of oxygen -> autooxidation Vitamin E
Many natural products are produced that way -> Enzymes (oxidases, peroxidases,…) catalyze reactions Radical reactionsPhenolic oxidative coupling
Many natural products are produced that way -> Enzymes (oxidases, peroxidases,…) catalyze reactions Radical reactionsPhenolic oxidative coupling
Electrophilic ReactionsAromatic substitution Electrophilic reactions with aromatic substrates -> substitution
Electrophilic ReactionsAromatic substitution Halogenation
Electrophilic ReactionsAromatic substitution Sulfonation
Electrophilic ReactionsAromatic substitution Electrophilic alkylations -> Friedel-Craft alkylation
Electrophilic ReactionsAromatic substitution Electrophilic alkylations -> Friedel-Craft alkylation -> Reaction mechanism dependent on structure of alkyl
Electrophilic ReactionsAromatic substitution Electrophilic alkylations -> Friedel-Craft alkylation -> alkylation of aromatic ring can be achieved by any system that generates carbocation
Electrophilic ReactionsAromatic substitution Electrophilic alkylations -> Friedel-Craft alkylation -> Limitation of alkylation by rearrangement reaction of carbocation
Electrophilic ReactionsAromatic substitution Electrophilic acylation -> Friedel-Craft Acylation
Electrophilic ReactionsAromatic substitution Electrophilic acylation -> Friedel-Craft Acylation
Electrophilic ReactionsAromatic substitution Effect of substituents -> influence rate and position of substitution Electron withdrawing substituents -> deactivate and hinder further substitution (-> destabilize ortho and para -> substitution preferable in meta position) Electron donating substituents -> activate for further substitution (-> preferable in ortho and para) Substituent has maximum effect on reaction if it is in ortho or para position -> electron releasing substituents (ortho and para) -> electron withdrawing substituents (destabilize arenium cation ->meta)
Electrophilic ReactionsAromatic substitution Effect of substituents -> influence rate and position of substitution
Electrophilic ReactionsAromatic substitution Effect of substituents -> Inductive effect Nitration of Toluene -> E = NO2 Nitration of Toluene -> 59% ortho, 37% para, 4 % meta Nitration of Nitrobenzene -> 93% meta, 6% ortho, 1% para
Electrophilic ReactionsAromatic substitution Effect of substituents -> Resonance Effect -> substitutents with strong electron releasers
Electrophilic ReactionsAromatic substitution Effect of substituents
Electrophilic ReactionsAromatic substitution Effect of substituents -> reaction is regioselective
Electrophilic ReactionsAromatic substitution Converting substituents (introduced by electrophilic substitution) -> into other functional groups
Electrophilic Reactionssubstitution on polycyclic aromatic compounds Monosubstitution -> predominant at position 1 Effect of substituents on further substitution Electron releasing substituent Electron withdrawing substituent