Aromatic compounds - 6
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What are the 3 features of benzene that don’t support kekules model
Benzene is resistant to addition reactions Enthalpy change of hydrogenation if less exothermic than expected All carbon bond lengths are the same
What happens to the 4th electron in the p-orbital of each carbon atom in benzene
It delocalises to form rings of electron density above and below the hexagon, f
Describe the current modl of bonding in benzene
Each carbon creates 3 covalent sigma bonds Remaining electrons is found in a p orbital above and below the structure P-orbitals sideways overlap evenly create a ring of delocalised pi-electrons above and below the structure
Describe in terms of overlap, explain the similarities and differences between the bonding in the kekule model and the delocalised model
Similarities: Sideways overlap of p-orbitals Pi ring above and below the molecule Differences: Kekule has 3 alternating Pi bonds each with 2 delocalised Delocalised model has a single Pi ring system containing 6 delocalised electrons
What type of reaction is the nitration of benzene and what are conditions/reagents needed
Electrophilic substitution [Nitric acid] + [Sulphuric acid] 50-55 degree c
How is No2+ ion formed
H2SO4 + HN03 —-. H2NO3 + HSO4 H2NO3 —> H20 + NO2
How is the catalyst reformed in the nitration of benzene
HS04- + H+ —> H2SO4
What type of reaction is the halogenation of benzene and what are conditions/reagents needed
Electrophilic substitution Reflux under heat Halogen + Halogen carrier
How is the halogen carrier formed in the halogenation of benzene
AlX3 + x2 —> AlX4- + x+
How is the halogen carrier reformed
AlX4- + H+ —> HX + AlX3
What type of reaction is the alkylation of benzene and what are conditions/reagents needed
Electrophilic substitution A halogenalkane and anhydrous AlCl3 RTP. dry inert solvent
How is the electrophile formed in the alkylation of benzene
RX + AlX3 —> R+ + AlX4
What type of reaction is the acylation of benzene and what are conditions/reagents needed
Electrophilic substitution Acyl chloride, AlCl3 Reflux, 50 degree c
How is the electrophile formed in the alkylation of benzene
RCOCl + AlCl3 —> RCO+ + AlCl4-
What is the relative ease of electrophilic substitution of phenol compared with benzene, why?
It is easier for electrophilic addition to occur in phenol because: The OH group are partially delocalised into the ring therefore, increasing electron density of the ring Making phenol more susceptible to electrophile attacks
Why do alkanes have a different reactivity to benzene and phenols
The Pi bond in alkanes has localised electrons which provide a high electron density above and below the molecule This can induce dipoles in other molecules
Why does benzene have a low reactivity compared to alkanes and phenol
the Pi electrons are spread over the ring structure, forming an area of delocalised electrons, lowering electron density to increase stability Cant induce dipoles in molecules, so requires a halogen carrier