Organic III: Aromatic Chemistry Flashcards
What was kekule’s model?
- benzene was made up of a planar ring of carbon atoms with alternating single and double bonds
-each C atom bonded to 1 hydrogen
Also added that benzene was constantly flipping between 2 isomers by switching double/single bonds
Evidence against kekule’s model?
X ray diffraction studies have shown all carbon-carbon bonds in benzene are the same length - in between length of single and double bond
What is the delocalised model for benzene?
Each carbon atom in the ring forms three σ bonds (with hydrogen and adjacent carbon atoms)
- bonds form due to overlap of their atomic orbitals
- remaining p orbital on each C atom overlaps sideways with p orbitals of neighbouring C atoms above and below plane—> delocalised ring of π bonds
- leads to electrons being delocalised causing a π system - 2 ring shaped clouds of electron density ABOVE AND BELOW plane
Evidence of delocalisation?
Enthalpy change of hydrogenation data :
Hydrogenation of cyclohexene : each molecule has 1 c=c bond (enthalpy change = -120 kJ/mol)
Hydrogenation of benzene:
- Kekule structure of benzene as cyclohexa-1,3,5-triene has three double C=C bonds so predicted enthalpy change = - 360kJ/mol (3x cyclohexene as 3 double bonds)
But it was actually -208 kJ/mol
Why is benzene resistance to bromination compared to alkenes?
It is an Electrophilic Addition reaction
- involve breaking up delocalised system
Also, in benzene, attraction of electrophiles is reduced compared to in alkenes due to - charge being spread out (ELECTRONS ARE DELOCALISED) - less electron density
Combustion of benzene ?
2C6H6 (l) + 15O2 (g) → 12CO2 (g) + 6H2O (g)
- smoky flame due to too little O2 available to burn benzene completely - incomplete combustion
So carbon forms soot - Smoky flame
Halogenation of benzene ? overall conditions and reaction?
Conditions : metal halide carrier - iron bromide /aluminium chloride (CATALYST)
Overall :
C6H6 + Br2 → C6H5Br + HBr
Halogen atom substituted in place of H atom - eg of electrophilic substitution
Purpose of metal halide carrier?
- make stronger electrophiles
Halogen carrier accepts lone pair of electrons from halogen atom on electrophile
-as lone pair of electrons pulled away, polarisation in Molecule increases + carbocation sometimes forms — STRONGER ELECTROPHILE
Nitration of benzene ? Conditions , overall reaction?
Nitro (-NO2) group replaces hydrogen atom on Arene - form NITROBENZENE
CONDITIONS :
- use conc nitric acid (HNO3)
- conc sulfuric acid (H2SO4) catalyst
- temp <55 degrees for mononitration
Benzene + HNO3 —> nitrobenzene + H2O
How does sulfuric acid help make nitronium ion (NO2+) electrophile and how is catalyst reformed?
HNO3 + H2SO4 —> H2NO3+ + HSO4-
H2NO3 + —> NO2+ + H20
Reforming catalyst:
HSO4 - + H+ —> H2SO4
Conditions and reaction of Friedel-Crafts Alkylation
Conditions : chloroalkane in presence of anhydrous aluminium chloride catalyst
- heat under reflux
Reaction :
C6H6 + chloroalkane (R-Cl) —> C6H5R + HCl
R= alkyl group
Electrophile is R+
FORMS ALKYLBENZENE
Conditions and reaction of Friedel crafts ACYLATION reaction of benzene?
C6H6 + RCOCl —> C6H5COR + HCl
R = alkyl group e.g -CH3
FORMS PHENYLKETONE (phenylethanone)
Electrophile is RCO+
Conditions: acyl chloride in presence of anhydrous aluminium chloride catalyst
Heat under reflux - 50 degrees
Forming electrophile and reforming catalyst for Halogenation?
Formation of electrophile :
AlCl3 + Cl2 —> Cl+ + AlCl4- (Cl+ is electrophile)
FeBr3 + Br2 —> FeBr4- + Br+ (Br+ is electrophile)
Reforming catalyst:
AlCl4- + H+ —> AlCl3 + HCl (AlCl3 is catalyst)
Forming electrophile and reforming catalyst for alkylation ?
Forming electrophile : AlCl3 + CH3CH2Cl —> CH3CH2+ + AlCl4-
Reforming : H+ + AlCl4- —> AlCl3 + HCl
Forming electrophile for acylation and reforming catalyst?
Forming electrophile:
AlCl3 + CH3COCl —> CH3CO+ + AlCl4-
Reforming catalyst :
H+ + AlCl4- —> AlCl3 + HCl
