aromatic chemisty Flashcards
Is benzene an alkene- reactivity with Br2
Alkenes undergo electrophilic addition where bromine water would turn from orange to colourless
But Benzene actually undergoes substitution reactions not addition reactions
Is benzene an alkene
Enthalpy of hydrogenation
Hydrogenation of cyclohexane-1,3,5-triene is -360
Hydrogenation of benzene is -208 which is -152 less exothermic then excepted
Benzene is 152Kjmol-1 more stable this is called delocalisation
Is benzene an alkene- bond lengths
X-rat crystallography allows us to measure bond lengths
Benzene has bond length that are all the same they are an intermediate between single and double
Is benzene an alkene summary
The evidence that led scientists to doubt the model of benzene proposed by Kekule is
Bond lengths are an intermediate between short C=C and long C-C
^H hydrogenation less exothermic than excepted because benzene is more stable
Benzene is less reactive than an alkene it doesn’t undergo an addition reaction to decolurise bromine
Benzene undergoes substition reaction in order to maintain the ring of delocalised electrons
Delocalised model of benzene
Six “unused” electrons in six p orbitals
When 2 p orbitals overlap side by side a pi bond is formed. In benzene all the p orbitals can overlap to form an extended pi electron system
Bonding, shape and stability in benzene (6marks)
Bonding:
each C has 3 covalent bonds
Spare electrons in a p orbital overlap (to form a pi cloud)
Delocalisation
Shape:
Planar
Hexagonal/120° bond angle
C-C bonds equal in length intermediate between single and double bonds
Stability:
Excepted ^H hydrogenation of cyclohexane-1,3,5-triene is -360Kjmol-1
Hydrogenation of benzene is less exothermic by 152Kjmol-1
Benzene is lower in energy then cyclohexane-1,3,5-triene
Physical properties of Arenes
Benzene is a colourless liquid at room temperature
Benzene has a Bp= 353K similar to hexane
Benzene has a Mp=279K a lot higher then hexane (178K) because benzene is planar and can stack well in solid state making van der waals intermolecular forces harder to break
Reactivity of aromatic compounds
The ring is an area of high electron density due to the delocalisation this means arenes react with electrophile
Aromatic ring is very stable energy has to be put in to break the ring system. This means almost all reactions the ring remains intact at the end of the reaction
Combustion of arenes
Arenes are rarely burnt because:
Arenes are extremely useful substance in pharmaceutical industry and so burning them is wasteful
Unsaturated so tend to undergo incomplete combustion
(Don’t get as much energy out)
(Burn with very smoky flame as solid carbon molecules form)
Equation for how electrophile is generated in nitration
H2SO4 + HNO3 -> H2O + NO2+ + HSO4-
