[25.1] introducing benzene
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what are the bond requirements for possible structures of a hydrocarbon with the formula C₆H₆?
• 4 bonds per carbon • 1 bond per hydrogen
draw athe skeletal and displayed structures what is kekulé’s structure of benzene?
• six-membered ring of carbon atoms • alternating single and double bonds between carbon atoms • planar
describe the accepted delocalised model of benzene
• each carbon uses 3 of its available carbom atoms in bonding to 2 other carbon atoms and one hydrogen atom • each carbon atom has 1 electron in a p-orbital at right angles to the carbon ring • adjacent p-orbital electrons overlap sideways, above and below the plane of the carbon ring • this generates a delocalised pi-bond ring system • the spreading of electrons stabilises the molecule
problems with kekulé’s structure: reactivity
• expected reaction with orange bromine water: bromine water to decolourise, as the bromine adds across the double bonds • observed: no colour change • conclusion: benzene does not react like a normal alkene
problems with kekulé’s structure: thermodynamic stability
• expected enthalpy of hydrogenation: 3 × Δ hydrogenation of cyclohexene • observed: less than 3 × Δ hydrogenation of cyclohexene • conclusion: benzene is more stable than if it contained 3 C=C bonds
problems with kekulé’s structure: bond lengths
• expected shape as C-C bonds are longer than C=C bonds: distorted hexagon due to difference in bond lengths between C atoms • observed: undistorted hexagon with all bonds between carbon atoms measuring 0.140nm • conclusion: all carbon-carbon bonds are of equal length, and the bond length is between a double bond and a single bond
summary of evidence that does not support kekule’s model
• benzene does not undergo addition of halogens, as would be expected from a molecule containing double bonds • benzene is more stable than kekule’s model would predict • the carbon-carbon bonds are of equal length and are between the lengths of a single and double bond
how to name aromatic compounds
• use prefixes for alkyl, halogens and nitro (also benzoic acid) • if 2 or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers • this should be done to give the lowest possible numbers to the substituents • when two or more different substituents are present, their are listed in alphabetical order • when a benzene ring is attached to an alkyl group with more than 6 Cs or a functional group (not halide, alklyl or nitro) then benzene is the substituent so use the prefix phenyl (eg. phenylethene)