describe the thermochemical evidence for the structure of benzene
enthalpy change of hydrogenation.
▫️ enthalpy change when hydrogen is added to cyclohexene = -120kjmol-1
- C=C and C-H bonds broken, taking in energy, new bonds form, releasing energy
- bonds made stronger, more energy released
▫️ Enthalpy change for Benzene would be expected to be 3x more yet -208kjmol-1
▫️ Real benzene at a much lower energy level than kekulé predicted –> more energetically stable
▫️ increase in stability = D-energy of benzene
Describe how x-Ray diffraction is used as evidence for the structure and stability of benzene ring
cyclohexene:
C=C BL = 0.155nm
C-H BL = 0.133nm
▫️ x-Ray diffraction gives only a single value for c-c bonds, two diff BL expects for Kekulé structure
▫️suggest all bonds are the same in nature
▫️ All evidence pointed to a symmetrical molecule with 6 carbon atoms with identical BL
Describe how IR data is used as evidence for the structure and stability of benzene
▫️Showed that none of the C-C bonds in benzene are normal double or single bonds
▫️ Bonds absorbs energy at different frequencies
describe the resonance structure of benzene
promotion ?
hybridisation ?
- Promotion of electrons
▫️ 1S1, 2Px, 2Py, 2Pz
- each carbon atom has to join to 3 other carbon atoms so needs another unpaired e- so electron from 2s sub shell is promoted - Hybridisation
▫️ 3 e- instead of four are hybridised
▫️P orbitals e- on each carbon atom is overlapping
- sideways overlap produces a system of pi bonds which spread over the whole carbon ring
- forms a D- pi system above and below molecule
describe how the properties of benzene coincide with its structure
identical bonds
energetic stability
reluctance off addition reactions
- Identical bonds:
▫️ shape is a planar regular hexagon, 120 degrees bond angle
▫️ All bonds identical so regular hexagon
▫️Delocalised of e- means that there aren’t alternating double and single bonds
▫️ Planar b/c that is the only way P orbitals can overlap sideways to give a delocalised pi system - Energetic stability:
▫️ The more you can spread e- around - more delocalisation - the more stable the molecule is - Reluctancy:
▫️ if atoms are added to benzene, e- from delocalise system have to join to new atoms
- system is disrupted
- molecule will become less stable
describe the combustion of benzene to form a smoky flame
2C6H6 + 15O2 —–> 12CO2 + 6H2O
▫️ Complete combustion of benzene requires a large vol of O2
▫️ may be insufficient O2 when burnt, u burned carbon will remain
- flames becomes yellow in colour and smoky
Describe the evidence for the structure of benzene ➡️➡️case against the kekulé model
double bonds
enthalpy of formation
- If structure contained 3 double bonds, would be expected for bonds to show similar reactivity and tendency to undergo add reactions ➡️ Alkenes
- most reactions are sub - Enthalpy change for formation of gaseous benzene from its elements
- theoretical= +252 kjmol-1
- on practise = +49 kjmol-1 (lower energy lvl)
➡️ actual structure is considerably more energetically stable than the Kekulé model
Addition reactions of Benzene - Hydrogen
Reagents?
products?
▫️ Reagents:
- benzene
- hydrogen
- presence of Ni catalyst (Raney nickel catalyst)
- temp 150
▫️product ➡️ cyclohexane
Reaction of benzene w/ halogen
▫️ rapid addition reaction in the presence of UV light
▫️the rapidity of the reaction and the requirement for light to overcome the activation energy suggest a free radical reaction
▫️ products: hexabromohexane
Nitration of benzene:
Reagents?
conditions?
Formation of electrophile?
- Conc nitric acid and conc sulphuric acid
- Temp must not exceed 50 degrees ➡️ multiple substitution of NO2 groups
- Nitric acid and sulphuric acid to make NO2+ electrophile
HNO3 + 2H2SO4 —-> NO2+ (+) 2HSO4- (+) H3O+
describe the nitration of benzene reaction
▫️2 e- in delocalised system are attracted to the electrophile, NO2+
▫️ They bond with the electrophile, partly breaking the d-system to form a pos ion
▫️lone pair of e- on HSO4- forms a bond with hydrogen
▫️ pair of e- joining hydrogen onto ring aren’t needed anymore, the e- move to the delocalised ring of e-
Sulphonation of benzene:
reagents?
conditions ?
formation of electrophile, SO3 (slight dissociation of the acid)
- Regent:
- benzene
- fuming sulphuric acid - Conditions:
- reflux - Dissociation of sulphuric acid
H2SO4 —> H2O + SO3
▫️SO3 is highly polar molecule w/ a fair amount of positive charge on the sulphur atoms when ch attaches to ring of e-
Describe the Sulphonation of benzene
▫️2e- from d-ring used to form a bond with the sulphur atom
▫️ Delocalisation partly broken
▫️ 2e- joining sulphur to one oxygen are forced out –> oxygen gains neg charge
▫️Hydrogen on ring is removed by a lone pair on the neg oxygen atom to form a bond
▫️e- joining hydrogen to ring are released to establish delocalisation
Describe the halogenation of benzene
Reagents ?
Formation of electrophile?
▫️Reagents:
- halogen
- benzene
- catalyst: Al(Cl3)… OR Iron
➡️ iron isn’t a catalyst b/c it gets permanently changed during reaction.
➡️iron reacts with some of the chlorine, bromine etc, to form iron (III) halide - which acts as the catalyst
▫️halogen molecule becomes polarised
- bonding e- are repelled by the nearby delocalised e- on benzene ring
▫️delta pos side acts as electrophile
describe the two stages involved in the halogenation of benzene using bromine
Stage 1:
▫️ Br2 bond breaks, transferring Br- ion to AlBr3
▫️ 2 e- from d- system used to form bond with delta pos Br atom
▫️ D-system is partly broken, ring gains pos charge
Stage 2:
▫️one of aluminium bromide bond break, two e- from the bond used to join to hydrogen
- hydrogen is removed from ring to form HBr (regenerating catalyst)
▫️e- that were used to join hydrogen to the ring are used to reform d-system
Describe the friedel-crafts acylation of benzene
Reagents?
formation of electrophile? CH3CO +
products ?
▫️ benzene and ethanoyl chloride
- presence of an aluminium chloride catalyst
▫️ ethanoyl chloride + AlCl3 —> CH3CO+ (+) AlCl4-
▫️Substitution of an acyl group into benzene ring
▫️2e- from d-system used to form new bond w/ COCH3 + ion
- d system partly broken, ring gains pos charge
▫️one of the aluminium chloride bonds break and both e- from it are used to join to hydrogen on ring
▫️Pair of e- joining H to ring no longer needed, so moved to D-system in
Describe the alkylation of benzene - sub of an alkyl group into a benzene ring
Reagents?
formation of electrophile?
▫️ Reagents
- Benzene
- Chloromethane
- AlCl3 as a catalyst
▫️ Haloalkane added to AlCl3 —> CH3+ (+) AlCl4-
▫️Methylbenzene formed is more reactive than the original benzene - further methyl groups substituted around the ring
▫️2e- from d-system used to form bond with CH3+ electrophile —> d-system partly broken, gains pos charge
- one of AlCl3 bond breaks and both e- from it are used to join hydrogen atom ring to Cl
- pair of e- joining H to ting no longer needed moves to d-system
Forms: methylbenzene, HCl and AlCl3
Describe the alkylation of benzene in industry
Reagent?
Why isn’t chloroalkane not used ?
Formation of electrophile?
▫️Reagents:
- Alkene
- mixture of AlCl3
- HCl as catalyst
▫️ Cheaper method because it saves having to make the chloroalkane
▫️Electrophile formation:
- 2e- from Alkene double bond given to delta pos hydrogen
- H-Cl bond breaks heterolytically
- forms CH2(H)— (+)CH2(H)CH2 and Cl-
- Cl- immediately picked up by AlCl3, prevents it from reacting with the alkane to form Chloroethane
Describe the alkylation of benzene with more complicated Alkenes (propene)
▫️ 2e- from double bond transferred to delta pos hydrogen
- H-Cl bond breaks heterolytically
▫️secondary carbocation is formed b/c it is more stable than 1o one
- pos charge is on centre carbon, so will become attached to the ring
Describe the nitration of methylbenzene (already subd benzene rings)
- methyl group is 2,4 directing —>seems to push incoming groups into those positions
Reagents?
products?
▫️reagents:
- methylbenzene
- conc HNO3
- conc H2SO4
▫️products:
- 2-nitromethylbenzene
- 4-nitromethylbenzene
partial delocalisation doesn’t cov the carbons that the -NO2 is attached to
Describe the activation of the ring by phenol
▫️ -Oh group attached to benzene ring in phenol has effect of making the ring more reactive
▫️One of the lone pairs on oxygen atom in -Oh group overlaps w/ D-ring e- system
▫️ donation of the oxygen’s lone pair into ring system increases the e- density around the ring
▫️phenols undergo electrophilic sub more readily
▫️Hydrogen is more readily lost
▫️ -OH group has a 2,4 directing effect
Describe the reaction of phenol with bromine
▫️Reagents:
- bromine water @ 20 degrees Celsius
▫️Forms 2,4,6-tribromophenol + 3HBr
Describe the reaction of phenol with nitric acid
▫️ DILUTE HNO3 at RTP ▫️ Phenol is easily oxidised to give complex tarry products ▫️ products: - 2-nitrophenol - 4-nitrophenol
▫️forms 2,4,6- trinitrophenol with conc nitric acid
