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170 Cards in this Set
- Front
- Back
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allylic group
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group on a C adjacent to a db
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benzylic group
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group on a C adjacent to a benzene ring or substituted benzene ring
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many cases, allylic & benzylic groups are
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unusually reactive
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allylic carbocations are
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resonance stabilized
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allyl cation
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resonance stabilized
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resonance stabilization of the benzyl cation
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resonance structures of the benzyl cation symbolize the overlap of
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2p orbitals of the benzylic C & benzene ring to form bonding MOs
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e deficiency & resulting pos charge on benzylic carbocation are shared
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not only by benzylic C but also by alternate C of the ring
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resonance structures of the benzyl cation
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account for distribution of pos charge calculated from MO theory
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resonance stabilizations of the benzyl & allyl cations
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about the same
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structures & stabilities of allylic & benzylic carbocations have important consequences for rxns in which
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they are involved as reactive intermediates
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Rxns in which benzylic or allylic carbocations are formed as intermediates are generally
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considerably faster than analogous rxns involving comparably substituted nonallylic or nonbenzylic carbocations
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the greater reactivities of allylic & benzylic halides result from
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the stabilities of the carbocation intermediates that are formed when they react
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benzylic cation more stable relative to its alkyl halide sm than is
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tert-butyl cation
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bc of possibility of resonance
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o & p sub groups on benzene that activate EAS further accelerate Sn1 rxns @ benzylic position
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carbocation derived from ionization of p-methoxy derivative not only has same types of resonance structures as unsub cmpd
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but also an additional structure in which charge can be delocalized onto sub group itself
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most alcohols require
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forcing conditions or Lewis acid catalysts to react w HCl to give alkyl chlorides, but such conditions are unnecessary when benzylic alcohols react w HCl
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Addition of hydrogen halides to conj dienes
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also reflects stability of allylic carbocations
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protonation of a conj diene gives
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the allylic carbocation rather than its nonallylic isomer bc the allylic carbocation is formed more rapidly
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consequence of involvement of allylic carbocations as reactive intermediates is that
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in many cases more than one prod can be formed
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more than one prod is possible bc the pos charge (& e deficiency)
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is shared btwn two C
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Nuc can react at either of the e deficient C atoms & if 2 C are not equiv
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2 diff prod result
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2 prod are derived from one allylic carbocation that has
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2 resonance forms
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prod derived from rxn of water @ ring C
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are not formed
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Products are not aromatic and thus
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lack stability associated w the aromatic ring
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Allylic radical
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has an unpaired e at an allylic position
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Allylic radicals are resonance stabilized &
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more stable than comparably substituted nonallylic radicals
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Allyl radicalc
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Benzylic radical
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unpaired e @ a benzylic position
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benzyl radical
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bc allylic & benzylic radical are esp stable
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they are more readily formed as reactive intermediates
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Initiation step
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dissociation of mlclr bromine into bromine atoms - promoted by heat or light
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in the first propagation step
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a bromine atom abstracts the one benzylic H in preference to either the 6 nonbenzylic H or the 5 H of the aromatic ring
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It is in this propagation step that
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the selectivity for sub of the benzylic H occurs
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reason for selectivity
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weaker benzylic C-H bond (greater stability of benzylic radical that is formed)
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second propagation step
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benzylic radical reacts w another mlc of bromine to generate a mlc of prod as well as another bromine atom, which can react again
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free rad halogenation is used
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to halogenate alkanes industrially
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bc free-rad halogenation of alkanes w diff types of H give smixtures of prod
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rxn not useful in the lab
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When benzylic H is present
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undergoes sub so much more rapidly than ordinary H that a single prod obtained
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Bc the allylic radical is also relatively stable
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a similar sub occurs preferentially @ the allylic position of an alkene but a competing rxn occurs in the case of an alkene that is not observed w benzylic sub (addition of halogen to the alkene db by an ionic mech)
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one rxn can be promoted over the other
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if the rxn conditions are chosen carefully
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Addition of bromine predominant rxn if
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free-radical sub is suppressed by avoiding conditions that promote free-rad rxns (heat, light or free-rad initiators) & rxn carried out in solvents of slightly polarity tha tpromote ionic mech for bromine addition
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Free-rad sub occurs
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when rxn promoted by heat, light or free-rad initiators, an apolar solvent such as CCl4 is used & bromine is added slowly so that its conc remains very low
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useful reagent employed to accomplish experimental convenience
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N-bromosuccinimide in CCl4 under free-rad conditions (heat or light & peroxides), allylic bromination takes place & addition to the db is not observed
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initiation step in allylic and benzylic bromination w NBS
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is formation of a bromine atom by homolytic cleavage of the N-Br bond in NBS itself
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The ensuing substitution rxn has
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3 propagation steps
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First
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bromine atom abstracts an allylic H from the alkene mlc
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HBr thus formed reacts w
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the NBS in the second propagation step by an ionic mech to produce a Br2 mlc
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the last propagation step is
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the rxn of this bromine mlc w the radical formed
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the unique role of NBS is
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to maintain the very low conc of bromine by reacting w HBr
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The Br2 conc remains low bc
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it can be generated no faster than the HBr mlc & an allylic radical are generated
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Every time a bromine mlc is formed
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an allylic radical is also formed w which a bromine can react
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the low solubility of NBS in CCl4 is crucial to
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the success of allylic bromination w NBS
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When solvents that dissolve NBS are used
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diff rxns are observed
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CCl4 must be
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used as the solvent in allylic or benzylic bromination w NBS
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During the rxn, the insoluble NBS, which is more dense than CCl4
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disappears from the bottom of the flask & less dense by-prod succinimide forms a layer on the surface of the CCl4
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Many steps of the mechanism
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occur @ the surface of the insoluble NBS
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mix of prod are formed in some allylic bromination rxns bc
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as resonance structures indicate the unpaired e in the free-rad intermediate is shared by 2 diff C
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The prototype for allylic anions is
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the allyl anion
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allyl anion
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benzyl anion
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allylic & benzylic anions are more stable than
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their nonallylic & nonbenzylic counterparts
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Reasons for stabilities of anions
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resonance stabilization, & polar effect of db (in the allyl anion) or phenyl ring (in benzyl anion)
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The polar effect of both groups
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stabilizes anions
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altho these cmpds are very weak acids
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their acidities are much greater than the acidities of alkanes that do not contain allylic or benzylic H
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Free benzylic or allylic carbanions
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are rarely involved as reactive intermediates
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A number of rxns involve species that have
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carbanion character
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Two of these are the
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rxns of Grignard & related organometallic reagents & E2 elim
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Grignard reagents resemble
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allylic carbanions
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Allylic Grignard reagents undergo
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a rapid equilibration in which the -MgBr group moves back & forth btwn the 2 partially neg C
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the TS for this rxn can be envisioned as
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an ion pair consisting of an allylic carbanion and a +MgBr cation
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Bc the allylic carbanion is resonance stabilized
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this TS has relatively low E & equil occurs rapidly
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allylic rearrangement
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simultaneous movement of a group G & db so that one allylic isomer converted into another - not resonance structures, two distinct species in equil
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rapid allylic rearrangement of an unsymm Grignard reagent means
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the reagent is actually a mix of 2 diff reagents
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same mix of reagents obtained from
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either of two allylically related alkyl halides
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when grignard reagents undergo subsequent rxn
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mix of prod obtained & same mix obtained regardless of alkyl halide used to form grignard reagent
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SN2 & E2 rxns of alkyl halides are
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competing rxns
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major factors that determine which rxn is dominant
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structure of alkyl halide
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Structural effect in alkyl halide that tends to promote greater fraction of elimination
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enhanced acidity of B-hydrogens
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Greater ratio of elim to sub observed when
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B-hydrogens of alkyl halide have higher than normal acidity
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Why does acidic B-hydrogen inc rate of E2 rxn?
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in TS of E2 rxn, base removes B-proton & TS of rxn has carbanion character @ B-carbon atom
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partially formed carbanion stabilized in same way that fully formed carbanion is
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more stable TS results in faster rxn
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Another reason benzylic E2 rxns faster
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alkene db, partially formed in TS, conj w benzene ring
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SN2 rxns of allylic & benzylic halides are
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relatively fast
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allylic & benzylic SN2 rxns are accelerated bc
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the energies of their TS are reduced by 2p orbital overlap
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In the TS of the SN2 rxn
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the C at which sub occurs is sp2 hybridized: incoping nuc & departing LG partially bonded to a 2p orbital on this C
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Overlap of 2p orbital w 2p orbitals of adjacent db or phenyl ring provides
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additional bonding that lowers the E of the TS & accelerates the rxn
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orbital overlap
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allylic & benzylic alcohols are oxidized selectively by
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suspension of activated manganese (IV) dioxide, MnO2
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primary allylic alcohols oxidized to
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aldehydes
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secondary allylic alcohols oxidized to
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ketones
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Acitvated MnO2 is obtained by
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oxidation-reduction rxn of potassium permanganate, KMnO4, w Mn2+ salt such as MnSO4 under either alkaline or acidic conditions followed by thorough drying
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Allylic & benzylic oxidation of alcohols takes place on
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surface of the MnO2, which is insoluble in solvents used for the rxn
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Water competes w alcohol for sites on MnO2 and thus
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must be removed by drying to produce an active oxidant
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Rxn is selective bc
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allylic & benzylic alcohols react much more rapidly than ordinary alcohols
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In first step of mech
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OH group of alcohol rapidly adds to MnO2 to give ester
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Next step (RLS)
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Mn(IV) accepts an e to become Mn(III) & H atom transferred from allylic or benzylic C to an O of the oxidant
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prod has an unpaired e on the
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allylic or benzylic C & is therefore resonance stabilized radical
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Allylic/benzylic selectively occurs bc
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analogous radical intermediate in oxidation of an alcohol that is not allylic or benzylic is less stable & formed more slowly
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In final step rapid, Mn(III) is reduced to more stable Mn(II)
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& strong C=O db formed to give aldehyde prod, which is washed away from oxidant surface by solvent
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treatment of alkylbenzene derivatives w strong oxidizing agents under vigorous conditions converts
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alkyl side chain int oa carboxylic acid group
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Oxidants commonly used for this purpose
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Cr(VI) derivatives i.e. Na2Cr2O7 (sodium dichromate) or CrO3, K2MnO4 (potassium permanganate) or O2 & special catalysts
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alkyl side chain, regardless of length
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converted into a CA group
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Oxidation of alkyl side chains requires
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presence of a benzylic H
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tert-butylbenzene, which has no benzylic H
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resistant to benzylic oxidation
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Benzylic oxidations occur in many cases bc
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resonance stabilized benzylic intermediates i.e. benzylic radicals are involved
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conditions for side-chain oxidation
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generally vigorous: heat, high conc oxidant and/or long rxn times
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1-phenylethanol readily oxidized to acetophenone under
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milder conditions - normal oxidation of secondary alcohols to ketones - but converted into benzoic acid under more vigorous conditions
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essential oil
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possessed key characteristic i.e. odor or flavor of natural material from which it comes
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isoprene rule
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terpenes all consist of repeating units w same C skeleton as 5-C diene isoprene
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isoprene
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basis of terpene or isoprenoid classification
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connectivity of C skeleton
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C-4 is
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either C of dimethyl branch
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many terpenes: isoprene units connected in
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1-4 arrangement, so C-4 of one skeleton connected to C-1 of other
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prime on one number, absence on other mean
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connection is between diff isoprene units
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some cmpds derived from conventional terpene structures by
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skeletal rearrangements
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criteria by which to recognize terpenes
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multiple of 5 C atoms in main C skeleton
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main connectivity C of isoprene C skeleton
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within each 5 carbon unit
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terpene C skeletons contain
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multiples of 5 C atoms
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monoterpenes
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terpenes w 10 C atoms in carbon chains
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sesquiterpenes
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15 C
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diterpenes
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20 C
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repetitive isoprene skeleton in all terpenes has a common origin in
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two simple five C cmpds
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pyrophosphate group
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alkyl pyrophosphates are esters of
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inorganic acid pyrophosphoric acid
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pyrophosphate & phosphate are
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nature's LG
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biosynth of simple monoterpene geraniol: first step
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IPP & DMAP bound to enzyme prenyl transferase
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DMAP loess pyrophosphate LG in
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SN1-like process
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carbocation formed is
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relatively stable allylic cation
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carbocations like other electrophiles
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can react w pi e of a db, which acts as a nuc
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Rxn of carbocation w db of IPP gives
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new carbocation
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Loss of a proton from a B-C of this carbocation gives
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the monoterpene geranyl pyrophosphate
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geraniol is formed in the rxn of
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water w geranyl pyrophosphate
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