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57 terms

Kaplan MCAT OChem Ch. 6: Aromatic Compounds

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aromatic
stable ring system
aromatic
cyclic, conjugated polyenes that possess 4n + 2pi electrons
aromatic
adopt planar conformations to allow max overlap of their conjugated pi orbitals
aromatic
alternating single and multiple (double or triple)
Huckel's rule
4n + 2pi electrons, its indicator
Huckel's rule
if cyclic conjugated polyene follows this, it's aromatic compound w/ extra stability resulting from filled bonding orbitals
antiaromatic
have electrons in higher energy nonbonding or antibonding orbitals, to be avoided
aromatic
neutral compounds, anions, and cations all may be this
aliphatic
any compound that's not aromatic
antiaromatic
if compound fits all characterstics of an aromatic compound EXCEPT Huckel's (if has only 4 pi electrons) => cyclic conjugated polyenes that are destabilized
aromatic
2, 6, 10, 14, 18
aryl compounds or arenes
aromatic compounds are referred to as this
arenes
symbolized as Ar
arene as sub
benzene represented as this = Ph
Aliphatic compounds
called alkyl, represented as R
alkyl benzenes
substituted rings
ortho
1,2
meta
1,3
para
1,4
polycyclic
multiple connected or heteocyclic rings (rings made up w/ atoms other than just carbon)
pyridine
lone pair can function as base and pull proton out of solution
pyrrole
lone pair can't act as base because it gives 4n + 2pi electrons => be would energetically unfavorable if leave aromatic, so won't react
physical prop of aromatic compounds
similar to those of other hydrocarbons
chem. properties of aromatic compounds
affected by aromaticity
chem. properties of aromatic compounds
planar shape of benzene => six pi orbitals overlap => delocalizing electron density
chem. properties of aromatic compounds
all six carbons are sp2 hybridized
chem. properties of aromatic compounds
each six orbitals overlaps equally w/ its two neighbors => delocalized electrons form two pi electron clouds, one above and one blow plane of ring
chem. properties of aromatic compounds
delocalization stabilizes => fairly unreactive

don't undergo addition rxns
aromatic compound rxns
EAS and Reduction
type of EAS rxns aromatic compounds
halogenation

sulfonation

nitration

acylation (Friedel-Crafts Reactions)

substituent effects
type of reduction rxns for aromatic compounds
catalytic reduction
EAS (aromatic)
undergoes sub instead of addition (this would disrupt aromaticity) => end product will restore aromaticity
EAS (aromatic)
H atom on ring is replaced by another group
halogenation (aromatic)
rings react w/ Br or Cl in presence of LA such as FeCl3, FeBr3, AlBr3, or AlCl3
halogenation (aromatic)
F, highly reactive, tends to produce multisub products
halogenation (aromatic)
Iodine's lack of reactivity require special conditions
sulfonation (aromatic)
ring reacts w/ fuming sulfuric acid (hot mix of sulfuri acid and sulfur trioxide) => sulfonic acids
sulfonation (aromatic)
since benzene isn't very reactive => intense reaction conditions
sulfonation (aromatic)
reversible
nitration (aromatic)
mix of nitric and sulfuric acid => nitronium, NO2+, a strong electrophile
nitration (aromatic)
nitronium + aromatic rings => nitro compounds
nitration (aromatic)
product is least reactive sub aromatic compound
acylation (Friedel-Crafts Rxns) (aromatic)
an acyl group is turned into carbocation (great electrophile) by a LA catalyst such as AlCl3
acylation (Friedel-Crafts Rxns) (aromatic)
carbocation acyl group incorporated into aromatic ring
Friedel-Crafts Alkylation (aromatic)
can occur, but less useful because product is attacked faster than the starting material => overalkylation
substitution effects (aromatic)
strongly influence susceptibility of ring to EAS
substitution effects (aromatic)
can be grouped into two major categories: enhance substitution (activating) or inhibit substitution (deactivating)
substitution effects (aromatic)
in whether group tends to donate or withdraw electron density
activator
groups that donate electron density
deactivator
group that withdraws electron density
activator
if attached => new group added at ortho or para position
deactivator
if attached => new group added at meta
deactivators
exception, halogens added at ortho and para position
ortho v. para q
often at para
catalytic reduction (aromatic)
ring reduced to cyclohexane by catalytic hydrogenation, but because of benzene's stability, occurs under vigorous condition (elevated temp. and pressure)
catalytic reduction (aromatic)
ruthenium or rhodium are most common catalysts
catalytic reduction (aromatic)
platinum or palladium may be used to require higher pressure