Arenes Flashcards
Physical properties of benzene
Structure: Trigonal planar, non-polar
1. low MP and BP
2. Soluble in organic solvents, insoluble in water
3. poor electrical conductivity
( electrons in benzene delocalises within benzene molecule. electrons need to delocalise across benzene molecules)
Explain the resonance stability of benzene
- Each carbon atom is sp2 hybridised, hence each C atom has an unhybridised p orbital
- The p orbital will all form a continuous sideway overlap
- The pi electrons delocalise across all 6 C atoms
- Delocalised pi electron cloud formed, making benzene very stable
Why does benzene undergo substitution and not addition?
- Substitution preserves the stability of benzene
- Addition disrupts the stability of benzene, which is energetically unfavourable
Electrophilic substitution with NO2+ to form nitrobenzene
- concentrated HNO3, concentrated H2SO4 catalyst, 55 degrees
Electrophilic substitution with halogen
Cl2(g), anhydrous AlCl3/FeCl3(s), warm OR
Br2(g), anhydrous AlBr3/FeBr3(s), warm
Alkylation (Step-up reaction)
RX, anhydrous AlX3/FeX3, warm
Electrophilic substitution (Nitration) for methylbenzenes
concentrated HNO3, concentrated H2SO4 catalyst, 30 degrees
Electrophilic Substitution (Halogenation) for methylbenzenes
Cl2(g), anhydrous AlCl3/FeCl3(s), room temperature OR
Br2(g), anhydrous AlBr3/FeBr3(s), room temperature
FRS for methylbenzenes
Monosub: Limited Cl2/Br2, UV light
Multisub: Excess Cl2/Br2, UV light (remove all H in alkyl)
Side-chain oxidation for methylbenzenes
Acidic condition:
- KMnO4, dilute H2SO4, heat under reflux
Basic condition:
- KMnO4, dilute NaOH, heat under reflux
- methylbenzene: benzoic acid (white solid) + H2O
- for ethylbenzene and longer, benzoic acid (white solid) + CO2, H2O
Why electrophilic substitution for methylbenzene requires milder conditions?
- presence of e-donating alkyl group increases electron density of benzene ring
- methyl benzene more susceptible to electrophilic attack than benzene and is more reactive
electron-donating groups are () groups
electron-withdrawing are () groups
electron-donating are (activating) groups
electron-withdrawing are (deactivating groups)