Chapter 17 test 1 Flashcards
Nucleophilic Aromatic Substitution
- A nucleophile replaces a leaving group on the aromatic ring
- Electron-withdrawing substituents MUST activate the ring for nucleophilic substitution
Activation For Nucleophilic Aromatic Substitution
- Activation for Nucleophilic Aromatic Substitution occurs in an opposite sense to Electrophilic Aromatic Substitution.
- Requires strongly electron withdrawing groups to activate the ring. Often see fluoride as the leaving group.
When does the benzyne mechanism operate?
The benzyne mechanism operates when the halobenzene is unactivated toward nucleophilic aromatic substitution (when there are no electron withdrawing groups), and forcing conditions are used with a strong base.
What is characteristic of the product distribution of the benzyne mechanism?
50/50 meta/para distribution
What groups can survive KMnO4’s “brutal” oxidation?
- NO2
- COOH
- SO3H
Benzylic Oxidation
hot, concd. KMnO4 and H2O
an alkylbenzene–> a benzoic acid salt –> protonation gives a COOH
Benzylic Bromination vs Benzylic Chlorination
- Br2 reacts only at the benzylic position
- Cl2 is more reactive and not as selective as bromination, so results in mixtures
- The benzylic position is the most reactive
What is used for benzylic bromination?
Elemental bromine or NBS
+
hv
Why doesnt Br2add to the double bond in the benzene ring?
Br2does not add to the benzene ring in benzylic bromination because the benzene ring is relatively unreactive unless it has powerful activating substituents.
- SN1 at the Benzylic position
Benzylic carbocations are resonance-stabilized, easily formed.
Benzyl halides undergo SN1 reactions.
SN2 at the Benzylic Position
SN2 reactions of benzyl halides efficiently convert aromatic methyl groups to functional groups
- Halogenation, followed by substitution, gives the functionalized product
Why are benzylic halides so reactive via SN2?
The transition state is stabilized by the ring
