Aromatics. Flashcards
Benzene properties and formula.
C6H6
Colourless, sweet-smelling, high flammable liquid
In crude oil
Carcinogen
Hexagonal ring of 6 carbons
Each carbon atom attached to 2 other carbons and 1 hydrogen
Aromatic hydrocarbon/arene.
Kekule model description.
A 6 membered ring of carbon atoms
Joined by alternate single and double bonds.
Evidence to disprove Kekule’s model.
1)Lack of reactivity:
-Should decolourise bromine in electrophilic addition due to C=C
-Benzene does not undergo electrophilic addition or decolourise Br.
2)The lengths of the C-C bonds:
-X-ray diffraction measured bond lengths
-Benzene bonds=0.139nm
-Is between single bond(0.153) and double(0.134).
3)Hydrogenation enthalpies:
-With kekule structure expected enthalpy change of hydrogenation would be 3x cyclohexene’s
-Cyclohexene=-120(from 1 C=C reacting w/ H)
-Benzene should then be -360
-Actual=-208
-Actual structure must be more stable than Kekule’s then.
The delocalised model of Benzene.
-Planar,cyclic, hexagonal hydrocarbon
-C6H6
-Each Carbon uses 3/4 electrons in bonding to 2 other Carbons and 1 Hydrogen
-Each Carbon has 1 electron in p-orbital (right angle to plane of bonded C and H)
-Adjacent p-orbital electrons overlap sideways (both directions) above and below plane=ring of electron density
-Creates Pi bond system
-6 electrons in Pi bonds=delocalised.
Naming aromatics w/ one substituent group.
Benzene ring=parent chain
Alkyl, halogen, nitro=prefixes
Benzene=substituent when:
-attached to an alkyl chain w/ a functional group
-or to an alkyl chain w/ 7(+) carbons
-Phenyl used as the prefix instead.
Naming aromatics w/ more than one substituent group.
-Ring becomes numbered
-Starting w/ one of the substituent groups
-Substituent groups listed in alphabetical order.
Nitration of benzene.
-Slow reaction
-Forms nitrobenzene
-Catalysed by sulfuric acid
-heated to 50 degrees
-One hydrogen atom on ring replaced by -NO2 group
-Higher temps=further subs=dinitrobenzene.
Benzene nitration mechanism explanation.
1) Nitronium ion(NO2+)(electrophile) produced by reaction of conc nitric acid and conc sulfuric acid.
2) Electrophile accepts electron pair from benzene ring=dative cov bond
Organic intermediate made=unstable, so breaks down to make nitrobenzene and H+ ion(stable benzen ring made)
3) H+ reacts w/ H2SO4- ion to regenerate sulfuric acid catalyst.
Halogenation of benzene.
Do not react unless halogen carrier present
AlCl3, FeCl3, AlBr3, FeBr3
Generated in situ from metal and halogen.
Bromination of benzene.
Room temperature, pressure and presence of halogen carrier
Electrophilic substitution
!) Bromonium (Br+) generated as the electrophile when halogen carrier catalyst reacts w/ Br
2) Bromonium ion accepts pair of electrons from benzene ring= dative covalent bond
Organic intermediate= unstable
Breaks down and forms organic product, bromobenzene & H+ ion.
3) H+ reacts w/ FeBr4(-) ion to regenerate FeBr3 catalyst.
Chlorination of benzene.
Carrier used= FeCl3, AlCl3 or iron metal = chlorine (FeCl3)
Same mechanism as the other 2 halogenation reactions w/ benzene.
Alkylation reactions.
Substitution of H atom in the benzene ring by an alkyl group
benzene+ haloalkane (w/ AlCl3)
Forms C-C bonds (more carbon atoms)
Also called Friedel-Crafts.
Acylation reactions.
Benzene+acyl chloride (AlCl3) –> aromatic ketone
Electrophilic substitution
Forms C-C bonds.
Reaction of cyclohexene and bromine.
1) Pi bond in alkene has localised electrons above and below plane of the 2 C in the double bond= area of high electron density
2) Localised electrons induce a dipole on non-polar Br molecule= 1 Br of Br2 molecule delta+ and the other delta-
3) Br delta+ enable Br molecule to act as an electrophile.
Alkenes vs arenes reactivity.
Benzene only reacts w/ Br w/ a halogen carrier catalyst
Due to delocalised Pi electrons
Electron density around any 2 C atoms is less than any 2 C=C in an alkene
Insufficient Pi electron density to polarise bromine= no reaction can take place.
