Aromatic Compounds Flashcards
What is the bond angle of benzene?
120degrees
What are the 3 features of benzene that don’t support kekule’s model?
Benzene is resistant to addition reactions
Enthalpy Change of hydrogenation of benzene is more stable that predicted
All the carbon bonds are the same length
What technique was used to find the bond lengths of benzene
X ray diffraction
What happens to the 4th electron in the p-orbital of each a carbon atom in benzene?
it delocalises above and below the plane of the 6 carbon atoms forming delocalised rings of electron density above and below the plane of the 6 carbon atoms
How do the rings of electron density affect the stability of benzene?
Makes benzene very stable, even though it is unsaturated (aromatic stability)
Is benzene soluble in water?
No because its non-polar
Why is benzene attacked by electrophiles?
High electron density above and below ring due to delocalised electrons
What typer of reaction is the nitration of benzene?
Electrophilic addition reaction
What are the reagents and conditions in the nitration of benzene?
Reagent : Conc HNO3
Catalyst : conc H2SO4
Conditions : below 50 degrees Celsius
Draw a mechanism and write the overall equation for the nitration of benzene.
How is the NO2+ ion generated? And how is the catalyst regenerated at the end? Give equations
H2SO4 + HNO3 = H2NO3+ + HSO4-
H2NO3+ = H20 + NO2+
Overall : H2SO4 + HNO3 = HSO4- + NO2+ + H2O
HSO4- + H+ = H2SO4 (H+ from benzene ring)
Check notes for reaction mechanism
What are the reagents and conditions for a Friedel-Crafts Alkylation reaction
Benzene to methylbenzene
Catalyst : Halogen carrier e.g. AlCl3
RTP anhydrous
Reagents to form methylbenzene : CH3Cl
Why does benzene not react directly with halogens?
The aromatic ring is too stable
What is happening when AlCl4- is formed in terms of electrons?
The lone pair of electrons on the chlorine atom is forming a coordinate bond to Al.
Draw a complete reaction mechanism to get from benzene to methylbenzene
Look it up
What are the reagents and conditions for the Friedel-Crafts acylation of benzene to get to phenylethanone and then phenylethanol
Catalyst : AlCl3
RTP anhydrous
reagents : CH3COCl (acyl Chloride)
Then to get to phenylethanol
Warm with NaBH4.
Explain why benzene is resistant to bromination, comparing with alkenes.
Alkenes react with bromine water a RTP through an electrophilic addition reaction, as the bromine atoms are added across the double bond. In alkenes the C double bond is an area of localised high electron density which strongly attract electrophiles.
The Pi-system in benzene - the delocalised electron rings above and below the plane of the carbon atoms, they spread the negative charge and make the benzene ring very stable. So benzene is unwilling to undergo the addition reactions. SO benzene would prefer to react through electrophilic substitution.
What reactions can be carried out to show the weak acidity of phenol?
A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates (weaker bases)
Write the displayed equation between phenol with bromine to 2,4,6-tribromophenol.
Check pmt
Why is phenol more reactive compared to benzene?
OH group mean phenol is more likely to undergo electrophilic substitution compared to benzene
Lone pair of electrons in a p-orbital of the oxygen atom overlaps with the delocalised ring of electrons in the benzene ring.
So the lone pair of electrons from the oxygen atom is partially dissociated into the pi-system.
This increasing the electron density of the ring, making it more likely to br attacked by electrophiles.
Which electron donating groups direct substitution to carbons 2, 4 and 6?
OH and NH2
They increased the electron density at these locations
Which electron withdrawing groups direct substitution to carbons 3 and 5.
NO2 is an electron withdrawing group. (Withdraws electron density from the ring at 2, 4 , 6) . It doesn’t have any orbits which can overlap with the delocalised ring
Why is nitrating phenol much easier than nitrating benzene?
Also what is the difference in reagents required to undergo nitration?
Difference is due to the activating effect of the OH group, which makes phenol more reactive.
Nitrating phenol - dilute nitric acid
Nitrating benzene - conc nitric acid and sulfuric acid catalyst
Why doesn’t phenol react with sodium carbonate solution?
Phenol is not a strong enough acid.
How is the weak acidity of phenols shown?
Shown by the neutralisation reaction with NaOH but absence of reaction with carbonates
