benzene, phenols and carbonyls

structure and bonding in benzene

• C-C bonds of equal length

• sigma bonds between C-H and C-C

shape and angle of benzene

120, trigonal planar

pi system in benzene

6 pi electrons become delocalised

p-orbitals

• half filled

• sit above and below the plane

• overlap sideways

hydrogenation of cyclohexene

• benzene has an enthalpy of -208 instead of -360

• benzene is less exothermic

• benzene is more stable

why is benzene more stable than expected

• bond breaking requires more energy

• more endothermic

• pi system = stability

regular hexagon shape in benzene

• alternate C-C and C=C bonds would NOT have a regular hexagon shape

• benzene is not skewed

• bond length between C-C and C=C

test for saturation

• Bromine water

• occurs at room temp

• orange to colourless

why is phenol more reactive than benzene

• one of the lone pairs of electrons on the oxygen atom is delocalised with the pi-electron charge cloud of the arene ring

• ring is activated towards electrophilic attack

• increased electron density

carbonyls

C=O

can ketones be oxidised

no

oxidation of aldehydes colour change

orange to green