Aromatic Compounds Flashcards
What’s the simplest aromatic hydrocarbon?
Benzene
Describe basic properties of benzene
- colourless
- sweet smelling
- highly flammable
- found in crude oil
- carcinogen (cancer causing)
Evidence that disproves Kekulé model
- X-ray diffraction = bond length in between single and double bond
- hydrogenation is less exo than expected (comparing to cyclohexene)
- only reacts w/Br2 w/ high temp + halogen carrier catalyst
Nitration of benzene
- H2SO4 + 50 degrees
- H2O bath = maintain steady temp
- electrophile = NO2+
- intermediate = unstable
- electrophilic substitution
What happens if reaction > 50 degrees in nitration of benzene
Further substitution
Uses of nitrobenzene
- dyes
- pharmaceuticals
- pesticides
- paracetamol
Bromination of benzene
• room pressure + temp + halogen carrier catalyst
Why is a halogen carrier catalyst necessary?
Benzene = too stable to react w/ non-polar bromine molecule
Alkylation reaction
• benzene + haloalkane + AlCl3
Acylation reaction w/ benzene
- acyl chloride + benzene + AlCl3
* product = aromatic ketone
Compare reactivity of alkenes and bromine
- B = pi bonds = delocalised ∴ lower electron density
- A = pi bonds = localised around 2 carbons ∴ higher e density
- B = polarises Br2 less -the more E required
Why is phenol a weak acid?
Non-polar benzene ring = partially dissociates -> phenoxide ion + proton
How do you distinguish between phenol and a COOH
Only COOH will react w/Na2CO3
Phenol + NaOH
Sodium phenoxide salt + H2O
Bromination of phenol
White precipitate of 2,4,6-tribromophenol + HBr
Solution orange -> colourless
RTP
Nitration of phenol (dilute HNO3)
Mixture of 2-nitrophenol and 4-nitrophenol is produce + H2O
RTP
Why does phenol only need dilute acid to react compared to benzene?
Lone pair of electrons from the O p-orbital of -OH = delocalised into the pi system -> electron density of ring increases = attracts electrophiles more strongly + more susceptible to attack + able to polarise bromine molecules more easily
What does delocalised e lead to?
Stability
How to make an aromatic amine
Nitrobenzene + 6[H] + Sn/c.HCl + reflux -> ammonium salt
+ NaOH -> amine
Compare acidity of alcohol, phenols and COOH
Ethanol = weakest + doesn’t react with any strong/weak base Phenol = only reacts w/strong bases COOH = strong enough to react with all bases
Name the 2,4 activating groups
1) NH2/NHR
2) OH
3) OR
4) R
5) halogens have 2,4 directing but are not activating groups
Why are many reactions different between alcohol and phenols
Because the proximity of the delocalised ring influences the OH group
Why is an acyl chloride used to make an Ester with phenol?
Carboxylic acid is reactive enough to form esters w/ phenol
