6.1.1 - Aromatic compounds Flashcards
Arenes
Aromatic molecules containing 1 or more benzene rings
-OH
Hydroxy -
NO2
Nitro -
F
Fluoro -
NH2
-amine
Benzoic acid
Benzene ring attached to COOH group
Acid group is always highest priority (1’ priority)
Phenyl -
Prefix if attached to an akyl chain w/ more than 6 carbons OR w/ a carbon chain w/ a functional group
Kekule structure
Benzene is v. reactive
Diff C-C bond lengths
Problems w/ Kekule’s structure
C-C bonds are all same lengths
Benzene does not undergo addn. reactions readily (resistance to reaction)
Enthalpy of hydrogenation 152kJ/mol less exothermic than expected
Delocalised electrons
Electrons shared between more than two atoms
Which model of benzene do we use now
Delocalised model
Using a labelled diagram. disprove Kekule
p-orbitals overlap sideways —> Delocalised pi-system above and below
Planar carbon ring
All bonds 120 degrees
Why do electrophiles react more easily w cyclohexane than w/ benzene
In benzene, pi-electrons are delocalised
In cyclohexane, the pi-electrons are localised
So benzene has a lower electron density
And so benzene polarises the electrophile less
Priorities
COOH>…> hydrocarbons > F,Cl,Br,I> NO2
How did the scientist disprove Kekule
Using diffraction to find the carbon-carbon lengths
Why does phenol react more readily w/ electrophiles then benzene does
Lone pair from -OH group in phenol is drawn into the delocalised benzene ring
so electron density increases
and the electrophile is more strongly attracted
Electron donating groups
-OH
-NH2
-CH3
2 (6),4 directing
Electron withdrawing groups
-NO2
-COOH
3’, (5) directing
Less reactive so only one substitution
Why is bromine not attracted to benzoic acid
-COOH is an electron withdrawing group
therefore pi-system becomes less dense
and will not attract electrophiles as strongly
Electrophilic substitution w/ bromine and phenols
White ppt
Decolourises bromine water
Nitration of benzene (nitrobenzene)
HNO3
H2SO4 (Conc. catalyst)
50 degrees
HNO3 + H2SO4 —> NO2^+ + HSO4 ^- + H2O
H+ + HSO4^- –> H2SO4
Dinitrobenzene
2 HNO3
Conc. H2SO4
70 degrees
Bromination of benzene
Occurs at RTP (electrophilic subsitution) in the presence of a halogen carrier
Overall: C6H6 + Br2 –> C6H5Br + HBr
Br2 + FeBr3 —> FeBr4^- + Br^+
H^+ + FeBr4^- —> FeBr3 + HBr
General formula for electrophilic substitution w/ benzene
AX + C6H6 —> C6H5A + HX
A - Electrophile
X - halogen
Halogen carriers
Benzene is non polar and so needs a halogen carrier to gnerate an electrophile e.g. FeBr3/AlBr3
The higher the elctronegativity …
The more the electrophile
Why isn’t phenol v. soluble
Non polar benzene ring
What does phenol dissociate into
Phenoxide ion and H^+
Reactions w/ phenol
Only strong bases (no carbonates)
Reactive metals e.g. Na
Br2 –> 2,4,6 tribromophenol
Functional group priorities
-COOH (oic acid) ester (-oate) acid chloride (-oyl chloride) -amide A,K,A -amine
Tests for phenol
pH paper and Na2CO3 (aq) - pH paper turns red, no reaction
FeCl3 (aq) in NH3 sol. - solution turns purple
Bromine water - decolourises, white ppt may form
Phenol w/ dilute vs conc HNO3
One substitution vs 3 substitutions
