aromatics

C6H6

planar molecule its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom

the ring contains three C=C double bonds and three C-C single bonds thsi molecule would be called a triene (cyclohexa-1,3,5-triene) with three C=C double bonds rather than a delocalised ring system

each C atom is bonded to two other C atoms and one H atom by single covalent sigma bonds this leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring

each p orbital overlaps with the neighbouring p orbitals to form a pi bond, this results in a ring of negative charge (electron cloud) above and below the plane of the ring

they don’t belong to any particular carbon atom (or to a bond between two C atoms), they are free to move throughout the whole pi system, ie they are DELOCALISED due to this delocalisation, the structure of benzene is represented with a circle in the middle of the structure

there will be less electron-electron repulsion which makes the molecule more stable

1) C-C bond length 2) addition reactions 3) enthalpy of hydrogenation

all C=C bonds have a defined length of 135pm, and all single C-C bonds have a defined length of 147pm so if we analysed benzene, we would expect 3x 135 and 3x 147 because there is a mixture of single and double bonds in Kekule’s structure however when we actually analyse benzene, all C-C bonds are exactly the same length at 140ppm which supports delocalised structure

when cyclohexene reacts with H2 to make cyclohexane, the enthalpy change is -120kJ/mol when cyclohex-1,3-diene reacts with 2H2 to make cyclohexene, the enthalpy change is -240kJ/mol we might expect a triene to react with 3H2 to make cyclohexene with an enthalpy change of -360kJ/mol however the enthalpy change of benzene is -208kJ/mol, 152kJ/mol lower than we might expect this means benzene is 152kJ/mol more stable than the hypothetical triene molecule this extra stability is due to the delocalisation of electrons and is known as the delocalisation stability

benzene does not readily undergo addition reactions (eg doesn’t decolourise bromine water) if benzene was a triene we would expect it to undergo these addition reactions.

if your molecule is really complicated, treat benzene as a side chain only use benzene as the main namin part for the following examples: -methyl benzene (toluene) -ethyl benzene -bromo/chloro/iodo benzene (halogen benzenes) -nitrobenzene

phenol benzenecarbocylic acid (benzoic acid) benzenecarbaldehyde (benzaldehyde)

phenyl group included in name eg phenylethanone

electrophilic substitution reactions his is because the benzene ring in aromatic compounds is very electron rich due to the cloud of electrons above and below the ring so the benzene ring is attacked by electrophiles (lone pair acceptors) they undergo substitution mechanisms where H atoms on the ring are replaced they do not readily undergo addition reactions as they would lose their delocalisation and so extra stability in the process.

reagents- conc HNO3 and conc H2SO4 conditions- 50 degrees C

H atom on ring is replaced by NO2 (nitro) group

aromatic nitro compounds which are used to make aromatic amines or explosives (eg TNT)

NO2+ (nitronium ion) generation equation: HNO3 + 2H2SO4 —–> NO2+ + 2HSO4- + H3O+

reagent- acyl chloride or acid anhydride and anhydrous AlCl3 catalyst conidtions- heat under reflux (50 degrees)

H atom on ring replaced by RCO (acyl) group

aromatic ketones- this reaction is extremely useful for adding C atoms to aromatic rings and any reaction that adds C atoms onto the aromatic ring is very valuable in organic synthesis also introduces a reactive functional group on the benzene ring

RCO+ (acylium ion) AlCl3 + CH3COCl —> [CH3CO]+ + [AlCl4]-