Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
A2 chemistry 5.4
Edexcel chemistry - arenes
| Question | Answer |
|---|---|
| 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
Popular Chemistry sets