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42 Cards in this Set
- Front
- Back
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Aryl halide
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Halogen bound to carbon of a benzene ring or another aromatic ring
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Vinylic halide
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halogen bound to carbon of a double bond
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Allylic group
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on carbon adjacent to the double bond
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Benzylic group
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on carbon adjacent to an aromatic ring
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Do aryl halides undergo nucleophilic substitution reactions by the Sn1 or Sn2 mechanisms?
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No
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Phenol
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Hydroxy bound to aromatic ring
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Catechol
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Two adjacent hydroxy groups on aromatic ring
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What catalysts dramatically inc reactivity of aryl & vinylic haldies in substitution reactions?
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transition-metal catalysts
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Simple vinylic & aryl halides under sn2 conditions
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Inert
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Why don't vinylic halides undergo Sn2 reactions?
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hybridization is sp during transition & VDW
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Why don't aryl halides undergo sn2 reactions?
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hybridization is sp during transition, VDW, backside approach would place nuc through plane of ring, & derivative would yield a twisted & highly strained double bond
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Base promoted B-elim rxn vinylic halides
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alkynes
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Ph-CH=CH-BR + KOH @ 200 C
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Ph-CH=-CH-H + K+ Br- + H2O
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Ph-CH(Br)-CH(Br)-Ph + 2 KOH in C2H5OH
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Ph-C=-C-Ph + 2K+ Br- 2H2O
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Do vinylic eliminations require harsh conditions?
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yes heat or strong bases & often enhanced acidity (i.e. benzylic)
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Vinylic & aryl halides under Sn1/e1 conditions
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inert (some forced under extreme conditions but uncommon)
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H2C=C(CH3)-Br + C2H5OH --> 55C
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no rxn
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Ph-Br + C2H5OH --> 55C
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no rxn
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Why are vinylic & aryl halides inert under sn1 conditions?
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unstable vinylic cation (sp hybridized, vacant 2p orbital not conjugated w pi e- system of db system) & e withdrawing polar effect of db discourages formation pos charge & carbon-halogen bonds stronger in vinylic halides (sp2 vs sp3 in alkyl)
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Aryl cation
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carbocation w e deficient carbon part of aromatic ring: carbon prefers linear but impossible too much strain - vacant orbital must remain sp2 = high E (& EW db)
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Difference between aryl cation & cation in EAS
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Carbocation in EAS stabilized by resonance, cation orthogonal (@ right angle) to pi e system
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nucleophilic substitution reactions
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Aryl halides w 1+ nitro group o/p to halogen under mild conditions
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NAS obeys rate law
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second order: K[aryl halide][nucleophile]
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speeds up NAS
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more nitro groups o/p to halogen LG
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Reactivities of aryl hlides in NAS
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Ar-F >> Ar-Cl ~ Ar-Br ~ Ar-I
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NAS mechanism
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nuc reacts @ halide-bearing carbon below/above plane aromatic ring to yield resonance stabilized Meisenheimer complex w neg charge delocalized thru pi e system of ring (RLS) & Nitro group --> loss of halide ion
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Why do o/p nitro groups stabilize Meisenheimer?
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Resonance
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Why is strength of carbon-halogen bond (/basicity of halide) not important in determining reaction rate of NAS?
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Loss of halide is not rate limiting
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Frontside substitution of NAS stereochemistry
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retention
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Heck Reaction Mechanism
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Does Heck reaction occur w/o Palladium catalyst?
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No
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Stereochemistry Heck reaction
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Retention
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Transition metals
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d block or B groups of periodic table
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Why is it convenient to think of e in 4s & 3d orbitals as valence electrons?
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similar energies
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Ligands
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Coordination compounds or transition-metal complexes: can be neutral or complex (have charge) - All are Lewis bases (donate e- pairs)
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L-type ligand
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dissociates from metal w bonding e pair & becomes neutral mlc - covalent
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X-type ligand
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dissociates from metal w bonding e pair & becomes negative ion - covalent
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Dative bond
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L-type bond emphasized by leaving bonding e pair on ligand & depicting bond as arrow from e to metal
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How do alkenes or aromatic rings act as ligands?
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By donating pi e to a metal
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