Topic 3: Aromatic Compounds Flashcards
criteria for compound to be aromatic
cyclic
planar
conjugated (with one stroke)
(4n+2) pi electrons
how many pi electrons do anti-aromatic compounds have
4n pi electrons
difference between aromatic and anti-aromatic compounds
aromatic: all p orbitals overlap smoothly > stable
anti-aromatic: pi electron system does not allow for effective delocalisation > highly unstable > distort its structure to avoid being anti-aromatic
what are heteroaromatic compounds
aromatic compounds containing heteroatom such as N, O or S
sometimes atom in ring may look sp3 (non-aromatic) but if got lone pair > can participate in conjugation > considered sp2 (aromatic)
requirement for ring to be conjugated
all atoms connected in ring must be sp2 or sp hybridised > have empty p orbital to overlap
if sp3 but have LP > can also take part in conjugation
effects of CH3, OH and NH2 substituents in electrophilic aromatic substitution
ortho, para activating
effects of halide substituents in electrophilic aromatic substitution
ortho, para deactivating
effects of NO2, CN, CHO, CO2R, COR, COOH on electrophilic aromatic substitution
meta deactivating
effect on EDG on aromatic compounds
stabilised benzene ring by increasing electron density in the pi electron cloud > more stable and more reactive towards electrophiles
what are diazonium salts and how are they prepared
highly reactive intermediates that can readily undergo nucleophilic aromatic sub
prepared via treatment of phenylamine with NaNO2 and acid (HX)
why can diazonium salts undergo SNAr
diazonium group (N2+) excellent leaving group > generates highly reactive cation > easily react with nucleophile
generation of reactive cation makes ring highly susceptible to nucleophilic attack
no need for strong EWG
reactions with diazonium salts
with e- rich aromatic compounds > azo dyes
with H2O and heat > phenol
with BF4-, elimination of N2 gas > fluorobenzene
with CuX (X= Cl, B4, CN) > substitution of X on benzene
reaction with H3PO2 > reduction to form benzene
how to convert chlorobenzene to phenylamine
treatment with NaNH2 (strong base) in liquid NH3
intermediate is benzyne (extremely strained, highly reactive with Nu)
requirements for SNAr
presence and position of strong EWG: reaction only occurs when got strong EWG at para/ortho of leaving group (meta no reaction)
use of heteroatom nucleophile
difference between electro and nucleo sub
electro: better to have EDG > make benzene pi centre more nucleophilic > more prone to electrophilic attack
nucleo: better to have EWG > make benzene centre more electrophilic > more prone to nucleophilic attack
