aromatics Flashcards
what is the structure of benzene
C6H6
delocalised aromatic ring
empirical structure of benzene
CH
evidence for the delocalised structure of benzene
B- Bond lengths= all same length if kekule was correct would be puckered (double bonds shorter)
E- electrophilic sub=resists reaction with Br2 (doesn’t decolourise- no = bonds)
E-enthalpy change of hydrogenation = less exothermic than predicted, more stable than expected - not a tri-ene
explain the delocalised structure of benzene
S-sideways overlap of p orbitals
A- above and below planar ring
F-forms a Pi system
E-electrons delocalised (mobile)
L- bond lengths equal
I-intermolecular between C-C and C=C
A- angle is 120 (trig planar)
R- ring is planar
Naming aromatics
C6H6 - benzene (if CH3 attached then methylbenzene)
Follows normal rules except:
add No2- nitrobenzene
add NH2- phenylamine
add ethene- phenylethene
add OH - phenol
Reactions of benzene basics
electron rich pi system- attracts electrophiles
Pi system makes it more stable
tends to maintain delocalised pi system in reactions
Undergoes electrophillic sub
Why are alkenes more reactive than benzene
More electeon dense than Pi system
due to localized bond instead of delocalised
this allows alkenes to induce dipoles which benzene can’t
electrophillic substitution def
H atom on benzene is replaces by an electrophile
the electeophile is attracted to electron rich pi system and accepts pair of electrons forming Covalent bond
Nitration of benzene
Reagents: Concentrated nitric acid + benzene
conditions: concentrated sulfuric acid (catalyst), heat under reflux at 50oc (above 100 u get further sub)
overall equation: C6H6 + HNO3 –> C6H6NO2 + H2O
mechanism: electrophilic sub
step 1) generation of electrophile (NO2+)
HNO3 + H2SO4 –> NO2+ + HSO4- + H20
step 2 (DRAWING) arrow from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as NO2 to half circle with + in centre (hetero fission) –> final product + H+
step 3) catalyst regeneration- H+ + HSO4- –> H2SO4
halogenation of benzene
Reagents: halogen + benzene
conditions: iron(111) or aluminium (111) halogen e.g. AlBr3 ‘halogen carrier’ catalyst
overall equation: C6H6 + X2 —> C6H6X + HX
mechanism: Electrophilic substitution
step 1) generation of electrophile (X+) - X2 + FeX3 —> X+ + FeX4-
step 2 (DRAWING) arrow from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as X to half circle with + in centre (hetero fission) –> final product + H+
step 3) regeneration of catalyst- FeX4- + H+ —> FeX3 + HX
The alkylation of benzene
Reagents: haloalkane and benzene
conditions: AlCl3 (halogen carrier cat)
overall equation: benzene + haloalkane –> benzene with alkyl group + HCl/Br ect
mechanism: Electrophilic substitution
1 generation of electrophile (alkyl+) haloalkane + AlX3 —> alkyl+ + AlX4-
step 2 (DRAWING) arrow from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as alkyl to half circle with + in centre (hetero fission) –> final product + H+
step 3) regeneration of catalyst- AlX4- + H+ —> AlX3 + HX
acylation of benzene
Reagents: acyl chloride e.g. CH3COCl and benzene
conditions: AlCl3- halogen carrier
overall equation: benzene + acyl chloride –> benzene with acyl group + HCl
mechanism: Electrophilic substitution
1 generation of electrophile (acyl+) haloalkane + AlCl3 —> acyl+ + AlCl4-
step 2 (DRAWING) arrow from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as acyl to half circle with + in centre (hetero fission) –> final product + H+
step 3) regeneration of catalyst- AlCl4- + H+ —> AlCl3 + HCl
nitration of phenol
Reagents: Dilute nitric acid
conditions: room temp
overall equation: phenol + HNO3 —> 2-nitrophenol / 4-nitrophenol + H2O
mechanism: ELECTROPHILLIC SUB
step 1 (DRAWING) arrow from bond to H in HNO3 (induced dipoles) then from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as NO2 to half circle with + in centre (hetero fission) –> final product + H2O
Bromination of phenol
Reagents: Bromine water
conditions: room temperature
overall equation: phenol + Br2 –> bromo-phenol (white precipitate forms) + HBr
bromine water decolourises
mechanism: Electrophilic substitution
step 1 (DRAWING) arrow from bond to Br in Br2 (induced dipoles) then from circle to electrophile –> unstable intermediate, arrow from H bonded to same carbon as Br to half circle with + in centre (hetero fission) –> final product + HBr
reactions of of phenol as an acid
phenol = weak acid
partially dissociates into phenoxide ion
reacts with basic substances to form phenoxide salts
reacts with metals, metal hydroxides
doesn’t react with carbonates, oxides or ammonia - as only reacts with strong bases
H on OH replaced by metal ion, charges on O and metal ion shown
phenol molecular and simplest formular
molecular - C6H6O
simplest- C6H5OH
common uses of phenols
antiseptics, disinfectants and plastics
why is phenol more reactive than benzene
in phenol- the lone pair of electrons on P orbital of Oxygen is donated
becoming partially delocalised into the Pi system
making the Pi system more electron dense than in benzene
it is now sufficiently electron dense to induce a dipole and attract electrophile more strongly unlike in benzene
What is a directing group?
groups attached to to the ring that determine where other electrophiles attach
activating groups- electron donating groups
these donate electrons to the pi system making it more electron dense and more susceptible to attack (reaction)
activates ring and is position 2 (ortho) and 4 (para) directing
e.g. OH NH2 NHR - MUST KNOW
this is why phenylamine doesn’t require a catalyst to react with Br2
deactivating groups- electron withdrawing groups
withdraw electrons making the Pi system less electron dense making the pi system less electron dense and so less susceptible to attack (reaction)
actives ring and is position 3 (meta) directing
e.g. NO2
this is why nitrobenzene does require a catalyst to react with br2
