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249 Cards in this Set
- Front
- Back
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nomenclature of benzene derivatives follows same rules used for
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other substituted hydrocarbons
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nitro group can be represented as a resonance hybrid of two equiv dipolar structures
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toluene
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styrene
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phenol
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anisole
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when none of the substituents qualifies as a principal group
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the substituents are cited & numbered in alphabetical order
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If a substituent is eligible for citation as a principal group
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it is assumed to be at carbon-1 of the ring
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o-xylene
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m-cresol
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catechol
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resorcinol
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hydroquinone
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when a benzene derivative contains 2+ substituents on the ring
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only #s may be used to designate the positions of substituents - usual nomenclature rules followed
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is it ever simpler to name a benzene ring as a substituent group?
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yes
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benzene ring or substituted benzene ring cited as sub referred to generally as
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aryl group
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unsub benzene ring as a substituent
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phenyl group
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Ph-Ch2 group
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benzyl group
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bp of benzene derivatives
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similar to those of other hydrocarbons w similar shapes & mm
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mp of benzene & cyclohexane
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unusually high bc of symmetry
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Addition of a C atom adds
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20-30 C to the bp
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mp of p-disub benzene derivatives
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typically much higher than those of corresponding o/m isomers
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bc isomer w highest mp is usually one that most easily crystallized
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many p-sub cmpds can be separated from o/m isomers by recrystallization
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benzene / aromatic density
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not as dense as water, more dense than alkanes, alkenes of about same mm
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benzene & hydrocarbon deriv insoluble in
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h2o
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benzene derivatives w sub that form H-bonds to water
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more soluble
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most useful absorptions in IR spectra of benzene derivatives
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C-C stretch absorptions of the ring, lower freq than C=C absorption of alkenes
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C-C stretch occurs @ lower freq than alkene C=C bc
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C-C bonds in benzene rings have bond order of 1.5
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overtone & combination bands
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1660-2000 cm-1 (help determine substitution patterns)
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proton NMR spectrum of benzene
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singlet @ chem shift of 7.4
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chem shifts are greater than those of alkenes by
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1.5-2 ppm
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NMR absorptions @ large chem shifts
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particularly characteristic of most benzene derivatives
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pi-e density in benzene lies
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in 2 doughnut-shaped regions above & below plane of the ring
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In an NMR experiment
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benzene mlcs in soln are moving about randomly- assume all possible orientations rel to applied field Bo, but particular orientation dominates chem shift
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ring current
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circulation of pi e around the ring
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Ring current induces
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magnetic field Bi, forms closed loops thru the ring
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Induced field opposes applied field
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along axis of ring, but augments applied field outside ring (region occupied by benzene protons)
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Augment field outside ring
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correspondingly higher freq required for absorption, chem shifts of aromatic protons inc
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ring current & large chem shift characteristic of cmpds that are
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aromatic by Huckel 4n+2 rule
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Basis of both ring current & aromaticity
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overlap of p orbitals in cont cyclic array
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when protons in sub benzene derivative nonequiv
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split each other
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can splitting occur across 1+ C-C bond?
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yes
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leaning
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chem shifts of two coupled protons similar, intensities differ from 1:1 ideal
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Two leaning doublet pattern typical of
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disub benzene rings in which 2 diff ring sub have a para relationship
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protons ortho to the more electropositive group
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have a smaller chem shift
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benzylic protons
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protons on C adjacent to benzene ring
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chem shift of benzylic protons
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2-3 ppm
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OH absorptions of phenols
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lower field than those of alcohols & undergo exchange in D2O
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In C NMR spectra, chem shifts of aromatic C
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in C-C db region 110-160 ppm
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Exact value depend on
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ring substituents present
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Chem shift of benzene
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128.5 ppm
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Quaternary ring C
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higher chem shift (bears no H)
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Bc proton-decoupling technique enhances size of peaks of C that bear H
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peaks for C that do not bear H considerably smaller
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chem shifts of benzylic C
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18-30 region
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simple aromatic hydrocarbons: 2 absorption bands in UV spectra
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rel strong @ 210 nm, weaker near 260 nm
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Substituent groups on the ring alter
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both lambda max values & intensities of both peaks, esp if sub has an unshared e pair or 2p orbitals that can overlap w pi -e system of aromatic ring
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Most extensive conj associated w inc in
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both lambda max & intensity
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VSEPR rules predict that O of p-ethylanisole like O of water should be tetrahedral & sp3, but O is sp2
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allows one e pair to occupy a 2p orbital, which has same size, shape & orientation as the C 2p orbitals of the ring
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EAS: H of aromatic ring sub by
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electrophile (lewis acid)
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all EAS rxns occur by..
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similar mech
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First step in mech of benzene bromination
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formation of complex btwn Br2 & FeBr3
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Formation of complex results in
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formal pos charge on one of bromines
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Pos charged bromine
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better e acceptor, better LG than bromine in Br2 itself
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-FeBr4 is a __ base than Br-
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weaker
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-FeBr4 is essentially the prod of
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a Lewis acid-base association rxn of Br- w FeBr3
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In FeBr4 an e pair on Br- has already been donated to
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Fe, is less available to act as a base than a naked e pair on Br- itself
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This step results in formation of resonance stabilized carbocation but
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disrupts aromatic stabilization of the benzene ring
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Harsh conditions (high reagent conditions, high temp, & strong Lewis acid cat) required bc
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2nd step does not occur under milder conditions used to bring about bromine addition to alkene
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rxn completed when
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bromide ion (complexed to FeBr3) acts as base to remove ring proton, regenerate cat. FeBr3 & give products bromobenzene & HBr
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Rxn of bromide @ e-deficient C itself doesn't occur
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bc the resulting addition product would not be aromatic - by losing a B-proton instead the carbocation can form bromobenzene a stable aromatic cmpd
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EAS steps
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generation of an electrophilic, nuc rxn of pi e of the aromatic ring w electrophile to form a resonance-stabilized carbocation intermediate
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the electrophile approaches the pi e cloud fo the ring
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above or below the plane of the mlc
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In the carbocation intermediate
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the C @ which the electrophile reacts becomes sp3 hybridized & tetrahedral
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loss of a proton from the carbocation intermediate to form
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the substituted aromatic cmpd
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The proton is lost from the C @ which
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substitution occurs
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This C again becomes
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part of the aromatic pi e system
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electrophile in nitration
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+NO2 (the nitronium ion) formed by acid-cat removal of the elements of water from HNO3
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rxn of the benzene pi e w the electrophile
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to form a carbocation intermediate
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Loss of a proton from the carbocation
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to give a new aromatic cmpd
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sulfonation
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Sulfur trioxide
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fuming liquid that reacts violently w water to give H2SO4
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Source of SO3 for sulfonation
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usually a soln of SO3 in conc H2SO4 called fuming sulfuric acid or oleum
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in one sulfonation mech, the electrophile is
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neutral sulfur trioxide
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When sulfur trioxide reacts w the benzene ring pi e
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an oxygen accepts the e pair displaced from sulfur
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Sulfonic acids such as benzenesulfonic acid
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are rather strong acid
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Is sulfonation reversible?
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Yes
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The SO3H group is replaced by H when
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sulfonic acids are heated w steam
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alkylation
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rxn that results in the transfer of an alkyl group
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Friedel-Crafts alkylation
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an alkyl group is transferred to an aromatic ring in the presence of an acid cat
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electrophile in FC alkylation formed by
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complexation of the Lewis acid AlCl3 w the halogen of an alkyl halide
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If the alkyl halide is secondary or tertiary
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complex can further react to form carbocation intermediate
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The electrophile in FCA
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either alkyl halide Lewis acid complex or carbocation derived from it
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loss of a proton to chloride ion
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completes the alkylation
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bc primary carbocations are too unstable to be involved as intermediates
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it is prob the complex of the alkyl halide & AlCl3 that rearrranges - has enough carbocation character that it behaves like a carbocation
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a monoalkylation prod can be obtained in good yield if
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a large excess of the aromatic sm is used
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if alkenes and alcohols are used as the alkylating agents in FCA, the carbocation electrophiles are generated
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from alkenes by protonation & from alcohols by dehydration
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when benzene reacts w an acid chloride in the presence of a lewis acid such as AlCl3
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a ketone is formed
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acylation rxn
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acyl group transferred from one group to another
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FCA
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an acyl group is introduced into an aromatic ring in presence of a Lewis acid
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electrophile in FCA
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carbocation called acylium ion, formed when acid chloride reacts w Lewis acid AlCl3
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Weaker lewis acids such as FeCL3 & ZnCl2 can be used to form acylium ions in FCA
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of aromatic cmpds that are more reactive than benzene
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ketones are weakly
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basic
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ketone prod of FCAc reacts w Lewis Acid
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in Lewis acid-base association to form a complex that is catalytically inactive
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Consequences of formation of this complex
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at least one equiv of lewis acid must be used to ensure its presence throughout the rxn, & complex must be destroyed before ketone prod can be isolated
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Type of rxn can only occur at
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an adjacent ortho position bc rxn @ other positions would produce highly strained prod
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When 5 or 6 membered rings are involved
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this process is much faster than rxn of acylium ion w the phenyl ring of another mlc
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proximity effect
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kinetic advantage of intramlclr rxns
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The multiply sub prod observed in FCA are not a problem in FCacy
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bc the ketone prod of acylation are much less reactive than the benzene sm
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Alkylation rxn is useful for preparing
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certain alkylbenzenes
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ACylation rxn is excellent method for
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synth of aromatic ketones
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When a monosub benzene undergoes an EAS rxn
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3 possible disub prod might be obtained
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Nitration of bromobenzene could give
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ortho, meta or para bromonitrobenzene
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If a sub benzene undergoes further sub mostly @ the ortho and para positions
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the original sub is called an O, p directing group
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Bromine is an o p directing group bc
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all electrophilic sub rxns of bromobenzene occur @ the o and p positions
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Other electrophilic sub rxns of nitrobenzene also give
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mostly the meta isomers
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nitro group is a
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meta directing group
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EAS rxns at 1 position of a benzene derivative
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are much faster than the same rxns @ another position
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Substitution rxns at the diff ring positions are
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in competition
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all o p directing substituents are either
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alkyl groups or groups that have unshared e pairs on atoms directly attached to the benzene ring
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atom directly attached to the benzene ring has
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unshared e pairs
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Reaction of E+ at the para position of anisole gives a
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carbocation intermediate w the following four important resonance structures
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the unshared e pair of the methoxy group can
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delocalize the pos charge on the carbocation
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Espec important structure bc it contains
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more bonds than others & every atom has an octet
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if the electrophile reacts w anisole @ the meta position
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the carbocation intermediate that is formed has fewer resonance structures than the ion
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The charge cannot be delocalized onto the
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OCH3 group when rxn occurs @ the meta position
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For the O to delocalize the charge
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it must be adjacent to an e deficient C
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Pos charge is shared on
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alternate C of the ring
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When meta sub occurs
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the pos charge is not shared by the C adjacent to the O
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Rxn of an electrophile @ either o or p positions of anisole
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gives a carbocation w more resonance structures (more stable carbocation)
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RLS in many EAS rxns is
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formation of the carbocation intermediate
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Prod derived from the more rapidly formed carbocation
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the more stable carbocation are the ones observed
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Substituents containing atoms w unshared e pairs adjacent to the benzene ring are
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o p directors in EAS rxns bc their e pairs can be involved in the resonance stabilization of the carbocation intermediates
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Rxn of E+ @ position o or para to an alkyl group gives
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an ion that has one tertiary carbocation resonance structure
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Rxn of the electrophile meta to the alkyl group gives
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an ion w all resonance forms w secondary carbocations
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bc rxn @ the o or p position gives
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the more stable carbocation, alkyl groups are o p groups
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m directing groups are all
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polar groups that do not have an unshared e pair on an atom adjacent to the benzene ring
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Bc repulsion btwn 2 like charges & so E of interaction inc w dec separation
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the resonance structure is less important than the others
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By Hammond's postulate the more stable carbocation intermediate should be
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formed more rapidly
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Nitro group is a meta director bc
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the ion that results from meta sub is more stable than the one that results from para sub
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Substituents that have pos charges adjacent to the aromatic ring are
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meta directors bc meta substitution gives the carbocation intermediate in which like charges are further apart
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Not all meta-directing groups have full pos charges like the nitro group but
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all of them have bond dipoles that place a substantial amt of pos charge next to the benzene ring
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aromatic substitution rxn of a benzene derivative bearing an o, p directing group would give
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2 x o as p prod if substitution were completely random bc there are 2 o positions
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which sub prod is major one in rxn mixture?
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para
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FCac of toluene gives essentially
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all para substitution prod & almost no ortho prod
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The electrophile cannot react @ the o position w/o developing
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VDW repulsions w the methyl group that is already on the ring
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nitration of toluene gives
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twice as much o-nitrotoluene as p-nitrotoluene
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Nitration of toluene @ either o or p position is
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so fast that it occurs on every encounter of the reagents
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Ready availability of o-nitrotoluene makes it
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a good sm for certain o-sub benzene derivatives
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If EAS rxn yields a mix of o and p isomers
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a problem of isomer separation arises that mus tbe solved if the rxn is to be useful
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para isomer of o, p pair typically has
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the higher melting point
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Activating group
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a substituted benzene derivative reacts more rapidly than benzene itself
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deactivating group
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substituted benzene derivative reacts more slowly than benzene itself
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A given substituent group is either
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activating in all EAS rxns or deactivating in all such rxns
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all meta-directing groups are
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deactivating groups
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all o, p directing groups except for halogens
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are activating groups
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Halogens are
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deactivating groups
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Directing effects are concerned w
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the relative rates of substitution @ diff positions of the same cmpd
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Activating or deactivating effects are concerned w
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the relative rates of substitution of diff cmpds
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Consider the effect of the substituent on the
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stability of the intermediate carbocation, then apply Hammond's postulate by assuming that the stability of this carbocation is related to the stability of the TS for its formation
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resonance effect of a sub group
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ability of the sub to stabilize the carbocation intermediate in electrophilic substitution by delocalization of e from the substituent into the ring
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The resonance effect is the same effect responsible for the
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o, p directing effects of substituents w unshared e pairs i.e. OCH3 & halogen
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resonance effect of the methoxy group stabilizes the carbocation
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polar effect
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tendency of sub group by virtue of its electronegativity to pull e away from the ring
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When a ring substituent is electronegative
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it pulls e of the ring toward itself & creates e deficiency or pos charge in the ring
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In carbocation intermediate of an electrophilic sub rxn
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pos end of the bond dipole interacts repulsively w the pos charge in the ring, raising the E of the ion
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e donating resonance effect of a substituent group w unshared e pairs
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if it were dominant, would stabilize pos charge & would activate further substitution
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If such a group is electroneg, its EWD polar effect, if dominant
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would destabilize pos charge & would deactivate further substitution
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Whether a substituted derivative of benzene is activated or deactivated toward further substitution
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depends ont he balance of the resonance and polar effects of the substituent group
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Anisole undergoes elec sub more rapidly than benzene bc
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the resonance effect of the methoxy group far outweights its polar effect
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The benzene mlc
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has no sub to help stabilize the carbocation intermediate by resonance
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Carbocation intermediate (& TS) derived from the elec sub of anisole is
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more stable relative to sm than the carbocation (& TS) derived from the elec sub of benzene
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In a given rxn, the o and p sub of anisole
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are faster than the sub of benzene
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The methoxy group activates
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the benzene ring toward o and p substitution
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altho the o and p positions of anisole are highly activated toward sub
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the meta position is deactivated
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When sub occurs in the meta position
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the methoxy group cannot exert its resonance effect & only its rate retarding polar effect is operative
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Whether a group activates or deactivates further sub depends on
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the position on the ring being considered
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The methoxy group activates
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o, p sub & deactivates meta sub
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bc o p sub is the observed mode of substitution
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the methoxy group is considered to be an activating group
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the deactivating effects of halogen substituents reflect
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a diff balance of resonance and polar effects
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resonance interaction of chlorine e pairs w the ring
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is much less effective than the interaction of o e pairs bc the chlorine valence e reside in orbitals w higher quantum numbers
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Bc these orbitals & the C 2p orbitals of the benzene ring have diff sizes & diff #s of nodes
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they do not overlap so effectively
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w a weak rate-enhancing resonance effect & strong rate-retarding polar effect
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chlorine is a deactivating group
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Bromine & iodine exert weaker polar effects than chlorine but
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their resonance effects are also weaker, so they are deactivating groups
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Fluorine as a second period element has a stronger resonance effect than the other halogens
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but as the most electroneg element it has a stronger polar effect as well - deactivating
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The deactivating, rate-retarding polar effects of the halogens are similar at all ring positions but offset somewhat by
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their resonance effects when substitution occurs para to the halogen
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Resonance effect of a halogen cannot come into play when
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substitution occurs @ the meta position of a halobenzene
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Meta substitution in halobenzene is deactived
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even more than para substitution
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Alkyl sub such as methyl group have no resonance effect but
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polar effect of an alkyl group toward e deficient C is an electropos, stabilizing effect
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Alkyl sub on a benzene ring stabilize carbocation intermediates in elec sub
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so they are activating groups
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Bc a nitro group has no e donating resonance effect
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the polar effect of this electroneg group destabilizes the carbocation intermediate & retards elec sub @ all positions of the ring
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nitro group is a meta directing group bc
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sub is retarded more at the o and p positions than at the meta positions
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the meta directing effect of the nitro group is not due to selective activation of the meta positions but
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to greater deactivation of the o and p positions
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When an elec sub rxn is carried out on a benzene derivative w more than one substituent
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the activating & directing effects are roughly the sum of the effects of the separate substituents
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In the FC acylation of m-xylene, both methyl groups
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direct the substitution to the same positions
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methyl groups are
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o, p directors
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Substitution @ the position o to both methyl groups is diff bc
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VDW repulsions btwn both methyls & the electrophile would be present in the TS
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Substitution occurs at a ring position that is
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para to one methyl & ortho to other
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Two meta directing groups on a ring direct further substitution to
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the remaining open meta position
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if one group is much more strongly activating than the other
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the directing effect of the more powerful activating group generally predominates
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after the first bromination, the OH & Br groups direct subsequent brominations
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to diff positions
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strong activating & directing effect of the OH group @ ortho & para
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overrides the weaker directing effect of the Br group
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activating or deactivating effects of substituents in an aromatic cmpd determine
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the conditions that must be used in an elec sub rxn
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when a deactivating group is being introduced by an elec sub rxn
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it is easy to introduce one gorup @ a time bc the products are less reactive than the reactants
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toluene can be nitrated only once bc
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the nitro group that is introduced retards a second nitration on the same ring
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when an activating group is introduced by elec sub
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the prod are more reactive than the reactants
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additional sub can occur easily under the conditions of the first substitution
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so mixtures of prod are obtained
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some deactivating substituents
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retard some rxns to the point they are not useful
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a good way to prepare a substituted cyclohexane
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prepare corresponding benzene derivative, then hydrogenate it
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cat. hydrogenation of benzene derivatives gives
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corresponding cyclohexanes & cannot be stopped at the cyclohexadiene or cyclohexane stage
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The delta H of hydrogenation of benzene can be used to
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provide another estimate of aromatic stabilization E of benzene
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bc the first hydrogenation rxn of benzene is endothermic
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E must be added for it to take place
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