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A2 chemistry 5.4

Edexcel chemistry - arenes

QuestionAnswer
Describe the bonding in benzene Each C atom has 3 σ bonds, 1 electron in Pz orbital at 90* to plane. Pz orbital of one C atom overlaps equally with Pz orbitals of both adjacent C atoms, forms delocalised system of π bonds above/ below plane.Hybrid of 2 theoretical resonant structures
Give some evidence for resonance in benzene X-ray diffraction: C-C same length, shorter than aliphatic single/ longer than double. Electron density map: e- above/below ring. X electrophilic addition. C=C/C-H IR stretching vibrations. ΔH hydrogenation only -207kJ/mol
What is the resonance stabilisation energy? ΔH hydrogenation(benzene) is -207kJ/mol, ΔH hydrogenation(theoretical cyclohexatriene) s -357 kJ/mol. Difference is -150kJ/mol, the extent to which delocalisation of π electrons stabilises benzene.
Write the equation for complete combustion of benzene, and use this to explain why benzene normally burns with a smoky flame C6H6 + 15/2 O2 -> 6CO2 + 3H2O. 7.5 moles of oxygen required for complete combustion of 1 mol of benzene, not always available -> incomplete combustion instead, smoky flame. High proportion of carbon relative to hydrogen
Describe hydrogenation of benzene Addition reaction, H2, nickel catalyst, 500K, forms cyclohexane
Why does benzene undergo electrophilic substitution reactions? Benzene ring is electron-rich, pi bonds above and below plane are SUSCEPTIBLE to electrophilic attack. Substitution = addition + elimination, elimination of H+ in 2nd step regains RESONANCE so product is stable, ENERGETICALLY FAVOURABLE.
Describe the general process of electrophilic substitution 1) formation of electrophile - strong, normally full positive charge. 2) electrophile adds to benzene ring, carbocation, ring of delocalisation is partially lost, endothermic, high Ea. 3) elimination of H+, often removed by a base. Catalyst!
Describe acylation, draw mechanism for benzene + ethanoyl chloride Friedel-crafts, catalyst AlCl3, electrophile is RC+=O, formation: AlCl3 + RCOCl -> AlCl4- (base) + RC+=O. Forms phenyl aldehyde/ phenyl ketone + HCl. Ether solvent, anhydrous. E.g: phenylethanone
Describe nitration, draw mechanism Conc. HNO3, conc. H2SO4 catalyst, electrophile is NO2+, formation: HNO3 + H2SO4 -> H2NO3+ + HSO4-, H2NO3+ -> H2O + NO3+. forms nitrobenzene + H2O. 60*c - too high means several substitutions
Describe bromination, draw mechanism Liquid Br2, iron filings, forms FeBr3 catalyst in situ. Electrophile is Br+. Formation: FeBr3+ Br2 -> FeBr4- (base) + Br+. forms bromobenzene + HBr. room temperature, ether solvent, anhydrous
Describe sulphonation, draw mechanism Conc H2SO4, electrophile is SO3: H2SO4 <-> SO3 + H2O. Forms benzene sulphonic acid + H2O, heat under reflux
Describe alkylation, draw mechanism for chlorobutane + benzene Friedel-crafts, AlCl3 catalyst, electrophile is R+, formation: AlCl3 + RCl -> AlCl4- + R+. forms alkyl benzene + HX, ether solvent, anhydrous
What are the effects of different side groups on benzene ring? Electron releasing (OH, alkyl groups) increase e- density of ring, ACTIVATE ring, further substitution likely, 2/4/6 positions. Electron withdrawing (NO2) decreases electron density of ring, DEACTIVATES ring, further substitution less likely, 3/5
Describe the bonding in phenol Each C has 3 σ bonds, 1 electron in Pz orbital at 90* to plane. Pz orbital of 1 C overlaps equally with Pz orbitals of both adjacent Cs, delocalised system of π bonds above/ below plane. Lone pair on O interact with delocalised π e-s in ring, activates
Why do the properties of benzene and phenol differ? Lone pair of electrons on oxygen interact with delocalised π electrons in benzene ring, increasing electron density in ring, activates ring, more susceptible to electrophilic attack, further substitution more lilely, 2/4/6 positions
Give evidence for the activation of the benzene ring in phenol Nitration: phenol + dilute HNO3 -> 2nitrophenol, 4-nitrophenol H2O. Bromination: phenol + Br2 water -> 2,4,6-tribromophenol + HBr. yellow -> colourless, white ppt, antiseptic smell, electrophile is δ+ Br in HOBr
Phenol + sodium equation (+ observations) C6H5OH + Na -> C6H5O-Na+ + 0.5 O2. Sodium dissolves, white solid forms, effervescence
How does phenol's chemical properties differ to aliphatic alcohols? More acidic: H is more δ+ so H+ ion is more readily lost, phenate ion is stabilised by resonance e.g. NaOH + C6H5OH -> C6H5O-Na+ + H2O. Weaker nucleophile: O is less δ- as electrons are drawn into ring e.g. X RCOOH, reacts slowly with RCOCl
How does phenol's chemical properties differ to carboxylic acids? Less acidic: in carboxylate ion the negative charge is shared between 2 oxygen atoms. In phenate ion, there is less resonance so O is more negatively charged. Doesn't form CO2 with NaCO3/ NaHCO3
How can the rate of reaction between phenol and acyl chlorides be increased? Add NaOH, phenol forms phenate ion which is a stronger nucleophile, more likely to form phenyl ester
Describe the solubility of phenol Insoluble in water (enthalpy of hydration from hydrogen bonding can't compensate for lack of attraction between non polar benzene ring+water, accounts for large proportion, bulky so disrupts hydrogen bonding between water molecules), dissolves in organics
Describe the hazards associated with benzene Flammable, carcinogen
Created by: 11043