6.1.1 - Aromatic Compounds

a) What are the two different models of Benzene?

Kekule

Delocalised Pi System

Brief description of Benzene?

C6H6

Cyclic

Planar

Lone electrons from p orbitals form a pi system = delocalised ring

Which is able to form due to C-C being all the same bond length

b) What are the differences between Kekule model and delocalised system?

Kekule:

• Alternating single and double bonds

• Pi bonds are localised

Delocalised:

• Pi bonds are delocalised

• P orbitals overlap

• Carbon - Carbon bond lengths are the same

• Enthalpy change of hydrogenation is less exothermic tha regular alkenes

Explain why Benzene is resistant to reactions?

• Too stable

• Requires catalysts

c) What is the name of these benzenes if they have an additional NO2/ OH/ BR/ 2 CH3/ NH2 groups?

1. Nitrobenzene

2. Phenol

3. Bromobeneze

4. 1,2 - dimethylbenzene

5. Phenylamine

d) Why doesn’t Benzene undergo electrophilic addition what happens instead?

EA will disrupt the ring so it undergoes electrophilic substitution

What is the Friedel Craft Acylation?

Adds an acyl group to allow benzene to be weaker for substitution

Requires halogen carrier to act as a catalyst

What does a halogen carrier and acyl chloride form together?

Carbocation

What is the general mechanism for Acylation

What conditions does nitration occur?

• 55 degrees celcuis

• Must react sulfuric acid and nitric aid to form electrophile

Draw the mechanism of nitration of benzene?

h) What is a phenol?

• Weak acid

Why is phenol more reactive benzene?

• Phenol has a higher electron density

• Due to hydroxyl group (OH)

• O contains lone pair of electrons in p orbital - overlaps with delocalised ring

• So electron density increasing making it more susceptible to attack from electrophile
  

What are electron withdrawing groups?

• Withdraw electron density from carbon 3 and 5

• Where electrophiles are more likely to attack

• Examples: NO2

What is an electron donating groups?

• Donate electrons increasing electron density at carbon 2,4,6

• Example: NH2,OH

Example: Predict reaction between nitrobenzene and chloroethane using AlCl2 as a catalyst?

When phenols react with alkalis what do they form?

• Salt + Water