6.1.1 Aromatic Compounds
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what is the formula of benzene
C6H6
what are characteristics of benzene
• colourless, sweet smelling and highly flammable • found in crude oil, cigarette smoke and petrol • classified as a carcinogen
what is the structure of benzene
• hexagon with circle in middle • hexagon with double bond on ever other bond
what is the Kekule model of benzene
• hexagon, with alternating double and single bonds
what are the 3 evidence points disproving the Kekule model
1) lack of reactivity 2) length of the C=C bond 3) hydrogenation enthalpies
how does lack of reactivity disprove Kekule’s model
• if benzene did contain C=C • it would discolour bromine HOWEVER • benzene does not undergo electrophilic addition • so it does not discolour bromine under normal conditions • so must not contain C=C
how does the C=C bond length disprove Kekule
you can measure bond lengths using x-ray diffraction: - length of C-C is 1.53 nm - length of C=C is 1.34 nm HOWEVER - bromine contains only one bond length - equal to 1.39 nm in between both - so must not contain those two bonds - (evidence examined by crystallographer Kathleen Lonsdale)
how does hydrogenation enthalpies disapprove Kekules theory
• Kekule’s benzene would be called cyclohexatriene • so would expect its enthalpy to be three times that of cyclohexene HOWEVER: 1) for cyclohexene, enthalpy change is -120kJmol 2) for cyclohexadiene, its -240kJmol, as expected 3) HOWEVER, for benzene it is -208kJmol, which is 152 kJ mol less energy than expected • so actual structure of benzene must be more stable than expected
what is the actual model of benzene called
the delocalised model
what is the structure of the delocalised model of benzene
• two rings connecting the top and bottom p-orbitals of all the C atoms in benzene • called the pi-bond system
explain the delocalised model of benzene
• each C-atom uses 3/4 of its available electrons • each carbon has 1 electron left in a p-orbital perpendicular to the plane of carbon and hydrogen atoms • adjacent p-orbital electrons overlap sideways in both directions above and below plane of C-atoms • forms a ring of e-density, creating a pi-bond system spread across all C-atoms • electrons in pi-bond are delocalised • DELOCALISED RING OF E- DENSITY ABOVE AND BELOW PLANE OF CARBON ATOMS
how do you name aromatic compounds
• need to use benzene • and then add prefixes 1) for short alkyl chains (1,4-dimethylbenzene) 2) halogens (bromobenzene) 3) nitro groups NO2 (1,3-dinitrobenzene) • all mono-substituted
when is benzene not the main element of a compound
• when attached to an alkyl chain with a functional group • when attached to an alkyl group with more than 7 carbon atoms
what is the prefix used for benzene
phenyl-
what is benzene attached to COOH called
benzoic acid
what is benzene attached to NH2 called
phenylamine
what is benzene attached to CHO called
benzaldehyde
what is benzene attached to HC=CH2
phenylethene
what is benzene attached to OH called
phenol
what is benzene attached to CH2COCH3 called
phenylpropanone
what do you need to remember when drawing out anything with benzene
which molecule is attached to the benzene - eg for phenol IT MUST BE THE O ATTACHED TO THE LINE, AND THEN H AFTER, NOT JUST IN THE MIDDLE
what mechanism does benzene undergo
electrophilic substitution
what is the typical overall equation of electrophilic substitution of benzene
benzene + E+ ===> Ebenzene + H+ • where E+ represents an electrophile
what is the typical mechanism for electrophilic substitution of benzene
1) the E+ attracts an electron pair from the dense pi-bond (shown via curly arrow going from circle in benzene to the E+) 2) the H bonded to the carbon that has gained the E gives its e- pair to repair the pi-bond (shown by curly arrow going from C-H bond to the broken horseshoe bond in benzene, with a + towards the bottom of it) 3) the H+ leaves the molecule and the pi-bond is repaired, leaving the product