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Fichas del conjunto (48)
- 1Kekule modelAlternate double and single bonds
- 2Evidence for this modelOnly one isomer for the monoderivative
- 3So?All carbons must be the same
- 4More evidence3 isomers of the di-derivative
- 5Why was Kekule wrong?All bonds were the same length
- 6What lengthIn between a C=C and C-C
- 7Second problem with Kekule's theoryBenzene not as reactive as you would expect if it had C=C
- 8Third reason why Kekule was wrongEnthalpy change of hydrogenation is less exothermic than expected for 3 C=C bonds
- 9So what is the other model?Delocalised ring of electrons
- 10Formed bySideways overlap of p orbitals
- 11To formpi bonds which form a ring of charge
- 12WhereAbove and below the plane of the ring of carbons
- 13What type of reactions do arenes undergoElectrophilic substitution
- 14For example nitration. This needs aNitrating mixture
- 15What is thatSulfuric & nitric acids (reagents)
- 16ConditionsConcentrated acids and 50 C
- 17Overall equationC6H6 + HNO3 --> C6H5NO2 + H2O
- 18Mechanism – step 1 makes aNitronium ion NO2+
- 19HowH2SO4 + HNO3 NO2+ + HSO4- + H2O
- 20Draw the mechanismcheck notes
- 21How comes the H2SO4 is a catalyst?It is remade in a last step
- 22What step?H+ + HSO4- --> H2SO4
- 23Halogenation needs a halogen carrier Why?Because the electron density in benzene is lower than in alkenes
- 24Why?Because the 6 electrons are spread out between 6 carbons in benzene pi bond
- 25Whereas in alkenesThere are 2 electrons concentrated between the 2 carbons
- 26What are halogen carriers?Fe, FeBr3 or AlCl3
- 27e.g for Cl2 we need to makeCl+
- 28HowAlCl3 + Cl2 --> AlCl4- + Cl+
- 29Mechanism for reaction of Br2 with benzene. FirstFeBr3 + Br2 --> FeBr4- + Br+
- 30Br+ is theelectrophile
- 31Then (DIY) mechanismcheck notes
- 32Overall equationC6H6 + Br2 --> C6H5Br + HBr
- 33relative resistance to bromination of benzenedelocalised electron density of the π bonds in benzene is less than with the localised electron density of the C=C bond in alkenes
- 34Phenol isPhenol is C6H5OH
- 35Reaction with aqueous alkalisMakes a salt and water as it acts as an acid
- 36equationC6H5OH + NaOH --> C6H5O-Na+ + H2O
- 37With NaC6H5OH + Na --> C6H5O-Na+ + ½ H2
- 38What is the name of C6H5O-Na+Sodium phenoxide
- 39With Br2 we getA white ppt and the Br2 decolourises
- 40EquationC6H5OH + 3 Br2 --> C6H3OBr3 + 3HBr
- 41What is C6H3OBr32,4,6-tribromophenol (white ppt)
- 42HBr is aSteamy gas
- 43explain the relative ease of bromination of phenol compared with benzene,Does not need a halogen carrier
- 44Why?The electron pair on the p orbital of oxygen delocalises onto the benzene ring
- 45WhichIncreases the electron density of the ring
- 46So?Electrophiles are more attracted to the ring
- 47This is calledRing activation
- 48Use of phenolplastics, antiseptics, disinfectants & resins for paints.
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- A-Levels Chemistry Fichas sobre Arenes, creado por jasmin.sahota el 25/09/2013.
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- jasmin.sahota
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