Reactions Of Arenes
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How do arenes burn in air?
With visible smoky flames
Why do arenes produce smoky flames?
They have a high carbon: hydrogen ratio
What forms during the combustion of arenes?
Unburnt carbon (soot)
Why is soot formed when arenes burn? (inc. comb. - c. part.)
Incomplete combustion leaves carbon particles
What does a smoky flame suggest in a chemical test?
The presence of an aromatic compound
Why doesn’t benzene react like an alkene, even though it is unsaturated? (typ. alk. reac. - break - d. arom. sys. - engy. need)
Typical reactions of alkenes would require benzene to break the delocalised aromatic system, which requires energy
Compare benzene and typical alkenes, concerning their reactions with bromine (alk. - rdy. rct. w/ Br - room temp/benz. no rct - halo. carr. ctlyst.) (benz - elec. - delo, alk. - elec. - loc., benz - pola. B< + ind. wkr dip. B<alk.)
Alkenes react with bromine readily at room temperature, whereas benzene does not react with bromine without a halogen carrier catalyst In benzene its electrons in π-bonds are delocalised, whereas alkenes have their π electrons localised between two carbon atoms Therefore benzene has a lower electron density than C=C so it polarises bromine less and induces a weaker dipole in bromine than alkene would
What is the typical reaction of benzene?
Electrophilic substitution
Why does benzene undergo substitution rather than addition? (dsty. arom. sys. + delo. engy.)
Addition would destroy the aromatic system and require delocalisation energy
What happens to the aromatic system during electrophilic substitution?
It is left unchanged overall
Why does the benzene ring attract electrophiles?
It has a high electron density due to delocalised π electrons
What happens to the π electrons when an electrophile approaches?
They are attracted towards the electrophile
Where does the electrophile form a bond, and where do the electrons for this bond come from? (C-E + elec. from delo. sys.)
The electrophile forms a bond with one of the carbon atom from benzene, the carbon uses one of the electrons the delocalised π system
Initially, what happens to the aromatic system when the C-E bond forms?
The system is temporarily destroyed
Why is the aromatic system unstable when the carbon takes the electron, and how is the stability restored? (ring - lst. - delo. stblty. - stblsd. - C los. H)
The ring has lost delocalisation stability, it is stabilised through the same carbon atom losing a H+ ion
What happens to the electrons in the C-H bond?
They return to the delocalised π system
What is the overall result of electrophilic substitution?
H+ is substituted by the electrophile
Why is electrophilic substitution possible for benzene, despite it being unreactive compared to typical alkenes? (ring - high elec. dens. - ovral. pres. - delo. arom. sys.)
Benzene has a ring of high electron density, and the overall reaction preserves the delocalised aromatic system
Draw the general mechanisms of benzene reacting with electrophiles
How do electron-releasing groups affect electrophilic substitution? (inc. elec. dens. - ring> sus. + reac. fstr.)
They increase electron density, making the ring more susceptible and reactions faster
How do electron-withdrawing groups affect electrophilic substitution? (red. elec. dens. - ring<sus. - subs.)
They reduce electron density, making the ring less susceptible to substitution