aromatic chem ...

draw Kekule’s structure of benzene:

give 3 issues with Kekule’s structure of benzene:

• C-C bonds in benzene are all the same length (C-C and C=C bonds are different lengths)

• benzene does not undergo addition reactions readily

• ΔH hydrogenation about 152 kJ mol^-1 less than expected

describe the (actual) structure of benzene:

• each C has 3 covalent bonds

• spare e^- in a p orbital overlap to form a π cloud and are delocalised

• planar

• 6 C ring with 120^o bond angle

• C-C bonds = in length, length in between lengths of typical C-C and C=C bond

why is benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene?

benzene has delocalisation of p e^-

how can we use thermochemical evidence from enthalpies of hydrogenation to account for the extra stability of benzene?

• enthalpy of hydrogenation of cyclohexene + H₂ → cyclohexane = -120 kJ mol^-1

• so if the structure of the hypothetical triene was accurate, we would expect triene + 3H₂ → cyclohexane = -360 kJ mol^-1

• but the real value for the enthalpy of hydrogenation is 208 kJ mol^-1 which is 152 kJ mol^-1 less exothermic than we might expect ∴ benzene is 152 kJ mol^-1 more stable than the hypothetical triene molecule

• this extra stability is due to the delocalisation of p e^- and is known as the delocalisation stability

what are the prefixes/suffixes used to name aromatic compounds?

• benzene (suffix) - e.g. methylbenzene

• phenyl (prefix) - e.g. phenylethanone