RPA - Arenes Flashcards
What three pieces of evidence suggested the Kekule structure was incorrect?
Enthalpy change of hydrogenation was not 3x that of cyclohexene, so not just three double bonds
Does not decolourise bromine, so its electron density can be presumed to be lower
All the bond lengths are the same, which is not to be expected of alternating double/single bonds
What are the conditions for the nitration of benzene?
Less than 50, otherwise multiple substitutions occur
Reflux
Acid catalyst to produce the nitronium electrophile
What are the conditions for halogenation of benzene, take bromination in this case.
FeBr3 catalyst to produce the Br+ electrophile,
What is the reaction to produce nitronium ions?
What is the final step to recover the catalyst?
HNO3 + H2SO4 — > NO2+ + HSO4- + H2O
HSO4- + H+ – > H2SO4
Give the overall reaction for the nitration of benzene.
C6H6 + HNO3 – > C6H5NO2 + H2O
Give the overall reaction for the bromination of benzene
C6H6 + Br2 – > C6H5Br + HBr
Give the equation for the production of the electrophile in a bromoination reaction.
Give the equation for the reformation of the halogen carrier after bromination has occured
Br2 + FeBr3 – > FeBr4- + Br+
FeBr4- + H+ – > FeBr3 + HBr
Give the skeleton of a long answer question about differences between bonding in benzeneand cyclohexene
Benzene has delocalised electrons from region of electron density above/below plane of molecule creating a pi SYSTEM
Cyclohexene has localised electrons above and below the plane of the sigma bond, creating a pi bond. The electron density is greater than in benzene
THEREFORE CYCLOHEXENE IS MORE REACTIVE AS IT CAN POLARISE BROMINE
Benzene therefore requires halogen carrier catalyst in order to react, whereas cyclohexene reacts spontaneously
Benzene less susceptible to electrophilic attack.
What are the three types of inorganic compounds which phenols can react with? Give reactions for all three
Reactive metals e.g. Na
2C6H5OH + 2Na –> 2C5H5O-Na+ + H2
Bases e.g. NaOH
C6H5OH + NaOH — > C6H5O-Na+ + H2O
Displacement of salts
C6H5O-Na+ + HCl – > C6H5OH + NaCl
What is the observation when bromine reacts with phenol?
A white precipitate is formed
What is the observation when a salt such as C6H5O-Na+ reacts with HCl?
phenol is precipitated out as a white solid
By what mechanism does phenol react with bromine? What are the conditions, and what is the reason for the difference between the reactivity of benzene and phenol?
Electrophilic substitution,
no catalyst
oxygen’s p-orbital overlaps with the delocalised pi system of the ring and causes it to be activated by increasing electron density
What colour is phenol at room temperature?
Pink solid.
What two visible changes occur when phenol reacts with bromine?
white precipitate forms
bromine decolourises
What causes phenol to be more reactive with Br2 than benzene? Key word?
Phenol has OH bonded directly to ring. Lone pair from oxygen overlaps with pi system and the electrons in that p-orbital are delocalised into the ring, activating it.
