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

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

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