6.1.1 - Aromatic compounds

Aromatic molecules containing 1 or more benzene rings

Hydroxy -

Nitro -

Fluoro -

-amine

Benzene ring attached to COOH group Acid group is always highest priority (1’ priority)

Prefix if attached to an akyl chain w/ more than 6 carbons OR w/ a carbon chain w/ a functional group

Benzene is v. reactive Diff C-C bond lengths

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

Electrons shared between more than two atoms

Delocalised model

p-orbitals overlap sideways —> Delocalised pi-system above and below Planar carbon ring All bonds 120 degrees

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

COOH>…> hydrocarbons > F,Cl,Br,I> NO2

Using diffraction to find the carbon-carbon lengths

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

-OH -NH2 -CH3 2 (6),4 directing

-NO2 -COOH 3’, (5) directing Less reactive so only one substitution

-COOH is an electron withdrawing group therefore pi-system becomes less dense and will not attract electrophiles as strongly

White ppt Decolourises bromine water

HNO3 H2SO4 (Conc. catalyst) 50 degrees HNO3 + H2SO4 —> NO2^+ + HSO4 ^- + H2O H+ + HSO4^- –> H2SO4

2 HNO3 Conc. H2SO4 70 degrees

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

AX + C6H6 —> C6H5A + HX A - Electrophile X - halogen