Edexcel chemistry - arenes
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| 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
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| 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
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| 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.
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| 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
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| Describe hydrogenation of benzene | Addition reaction, H2, nickel catalyst, 500K, forms cyclohexane
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| 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.
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| 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!
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| 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
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| 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
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| 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
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| Describe sulphonation, draw mechanism | Conc H2SO4, electrophile is SO3: H2SO4 <-> SO3 + H2O. Forms benzene sulphonic acid + H2O, heat under reflux
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| 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
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| 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
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| 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
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| 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
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| 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
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| Phenol + sodium equation (+ observations) | C6H5OH + Na -> C6H5O-Na+ + 0.5 O2. Sodium dissolves, white solid forms, effervescence
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| 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
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| 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
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| 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
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| 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
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| Describe the hazards associated with benzene | Flammable, carcinogen
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