F324 MOD1: Arenes
- Created by: Anju
- Created on: 28-03-17 10:53
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- F324 MODULE 1: Arenes
- Benzene
- Structure & Bonding
- Delocalised Model
- *sideways double overlap of p-orbitals between adjacent carbon atoms both above and below the plane of the ring
- *results in one pi bonding region with delocalised (lowers e- density) pi electrons.
- *sigma bonds between C-H
- Kekule model/ resonance structure
- Localised electrons between c=c ( pi bond)
- sideways double overlap of p-orbitals between two carbon atoms
- Evidence against kekule
- X-ray diffraction shows that all bonds in benzene have an equal length
- Benzene does not decolourise bromine water at rm temp,it can only react with Br2 in the presence of a halogen carrier and a high temperature.
- enthalpy of hydrogenation ( -152kjmol-1 delocalisation enthalpy) is less exothermic than expected, when compared to cyclohexene, benzene is more stable
- Benzene does not decolourise bromine water at rm temp,it can only react with Br2 in the presence of a halogen carrier and a high temperature.
- X-ray diffraction shows that all bonds in benzene have an equal length
- Delocalised Model
- Reactions: Electrophilic substitution
- Nitration
- electrophile generation: H2SO4 + HNO3 -> HSO4- + NO2+ + H2O
- Overall eq.: C6H6 + NO2+ ->C6H5NO2 + H2O
- Catalyst regenerationHSO4- + H+-> H2SO4
- Halogenation
- Electrophile generation : FeBr3 + Br2 -> FeBr4- + Br+
- Overall eq. : C6H6 + Br2 -> C6H5Br + HBr
- Catalyst regeneration : FeBr4- + H+ -> FeBr3 + HBr
- Nitration
- Structure & Bonding
- Cyclohexene
- Reactions
- Bromine
- pi bond contains localised electrons above and below the C=C
- producesa region of high electron density; allowing cyclohexene to react with a non-polar molecule such as bromine
- electrons in pi bond repels those in Br-Br bond; inducing adipole in the Br2 molecule
- pielectron pair in C=C is attracted to slightly positive Br; double bond breaksto form a new bond with Br, creating a positive carbocation
- Br- formed from the heterolytic fission of Br2, is attracted to the carbocation and forms a covalent bond
- pielectron pair in C=C is attracted to slightly positive Br; double bond breaksto form a new bond with Br, creating a positive carbocation
- electrons in pi bond repels those in Br-Br bond; inducing adipole in the Br2 molecule
- producesa region of high electron density; allowing cyclohexene to react with a non-polar molecule such as bromine
- pi bond contains localised electrons above and below the C=C
- Comparison with Benzene
- benzene has 6 delocalised pi electrons spread over 6 carbons, whereas cyclohexene has localised pi electrons in C=C meaning, it has a higher e- density than benzene
- benezene's insufficient pi electron density means it cannot polarise a non-polar molecule such as Br2 without the aid of a halogen carrier to turn the bromine molecule a more powerful electrophile.
- benzene has 6 delocalised pi electrons spread over 6 carbons, whereas cyclohexene has localised pi electrons in C=C meaning, it has a higher e- density than benzene
- Bromine
- Reactions
- Phenol
- Structure & Bonding
- pink crystalline solid at rm. temp
- weak acid in which the hydroxyl group is attached directly to the benzene ring
- soluble in H2O, as OH froms hydrogen bonds with water molecules
- weak acid in which the hydroxyl group is attached directly to the benzene ring
- pink crystalline solid at rm. temp
- Reactions
- Bromination
- phenol being a weak acid; partially dissociates and is reactive in positions: 2,4,6
- forms 2,4,6 tribromophenol; white ppt
- phenol being a weak acid; partially dissociates and is reactive in positions: 2,4,6
- Bases
- bases such as NaOH, Na causes a neutralisation reaction to form a salt e.g. C6H5O-Na+
- the reaction can be reversed upon the addition of a strong acid (proton donor) e.g. HCL
- bases such as NaOH, Na causes a neutralisation reaction to form a salt e.g. C6H5O-Na+
- Bromination
- Uses
- alkyl phenols: detergents
- chloro phenols: antiseptics
- Salicyclic acid, a derivative of phenol, is used in the preparation of Aspirin
- chloro phenols: antiseptics
- alkyl phenols: detergents
- Reactivity
- lone pair of electrons occupying the p-orbital on oxygen are partially dolcalised into the pi region of the benzene ring
- this increases the electron density, therefore the ring is activated and becomes more nucleophilic
- as a result phenol is able to polarise the electrophile and attract it more readily
- this increases the electron density, therefore the ring is activated and becomes more nucleophilic
- lone pair of electrons occupying the p-orbital on oxygen are partially dolcalised into the pi region of the benzene ring
- Structure & Bonding
- Benzene
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