Arenes Flashcards
Give evidence for the actual structure of benzene that proves Kekule’s structure wrong.
- X-ray diffraction shows that all the bonds are the same length
- Enthalpy of hydrogenation: value for benzene is -208, not -360 kjmol-1 so benzene is more stable by 150kjmol-1.
There are not 3 double bonds. - Infrared data shows differences in absorption of C=C and C-H bonds.
- Electrophilic substitution rather than elimination - doesn’t readily react
Will benzene decolorise bromine water?
Write the equation for the formation of the electrophile in the reaction of benzene with bromine.
Benzene will not decolorise bromine water because it requires too much energy to break the delocalised ring.
Br2 + FeBr3 -> Br+ + FeBr4-
a) What happens if the temperature is raised above 55 degrees C in the nitration of benzene?
b) Why is benzene slowly added dropwise to the mixture?
Write the equation for the formation of the electrophile in the reaction.
a) above 55C, multiple substitution occurs and 1,3-dinitrobenzene forms
b) benzene is added slowly because the acids react and give out heat (as does the addition of benzene)
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
What reagent is required for the sulfonation of benzene and what is the name of the product formed?
FUMING sulfuric acid (at 40C)
Benzenesulfonic acid is formed.
What is the name of reactions involving alkylation and acylation?
Write the equation for the formation of the electrophile in:
a) alkylation
b) acylation
Friedel-Crafts reactions and catalyst
a) RCl + AlCl3 -> R+ + AlCl4-
b) RCOCl + AlCl3 -> RCO + AlCl4-
Why are methylbenzene and methoxybenzene used instead of benzene?
Benzene is stable so reactions are slow Methyl and methoxy groups donate electrons and increase the electron density of the delocalised ring.
This makes the ring more reactive and more susceptible to electrophilic attack
Why does phenol react faster than benzene? (Why is it easier to nitrate phenol and why does phenol decolorise bromine water?)
Phenol has an OH group which has an activating effect on the benzene ring. The oxygen donates a lone pair of electrons to the delocalised ring, increasing the electron density. This makes phenol more reactive.
Why is phenol a stronger acid than ethanol, but still a weaker acid than carboxylic acids?
The lone pair of electrons from the oxygen atom is incorporated (withdrawn) into the benzene ring, making the O more electronegative and H more electropositive. The hydrogen atom is more easily lost, as a result.
State the physical properties of phenol.
- A toxic, white solid
- Polar molecule - can form h bonds with molecules (delta+ O, delta- H)
- Higher melting temp than methyl
- Soluble in water at room T (2 layers) high T makes it completely miscible
What form when phenol reacts with:
a) Bromine water
b) Water
a) 2,4,6-tribromophenol: it is insoluble in water so precipitates as a white solid
b) Phenate ion: stabilised by resonance
Describe and explain the structure of benzene.
Planar molecule (120 degrees) Each carbon atom has a p orbital (each has 1 electron). The p orbitals overlap above and below the plane (forming extended pi bonds), creating a delocalised ring of pi electrons. This makes benzene very stable.
a) What is formed when methylbenzene reacts with KMnO4 and dilute H2SO4?
b) What forms when phenol reacts with
(i) nitric acid
(ii) sodium carbonate
(iii) alkali (e.g. sodium hydroxide)
a) Benzoic acid
b) (i) 2,4,6-trinitrophenol
(ii) no reaction
(iii) a salt (e.g. Sodium phenate ion)
Why does the lateral overlap that forms a pi bond result in a barrier to free rotation around a C=C double bond?
Rotation around the pi bond would shift it to an energetically less favourable position (which would break the bond). Restricted rotation maintains the pi bond.
Why does benzene undergo substitution and not addition in reactions?
Benzene has a stable delocalised ring of pi electrons so addition would disrupt this delocalisation and reduce stability. Substitution reactions only involve temporary disruption so stability is maintained.
